cmd/compile/internal/inline: score call sites exposed by inlines

[gostls13.git] / src / runtime / traceback.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package runtime

import (
        "internal/abi"
        "internal/bytealg"
        "internal/goarch"
        "runtime/internal/sys"
        "unsafe"
)

// The code in this file implements stack trace walking for all architectures.
// The most important fact about a given architecture is whether it uses a link register.
// On systems with link registers, the prologue for a non-leaf function stores the
// incoming value of LR at the bottom of the newly allocated stack frame.
// On systems without link registers (x86), the architecture pushes a return PC during
// the call instruction, so the return PC ends up above the stack frame.
// In this file, the return PC is always called LR, no matter how it was found.

const usesLR = sys.MinFrameSize > 0

const (
        // tracebackInnerFrames is the number of innermost frames to print in a
        // stack trace. The total maximum frames is tracebackInnerFrames +
        // tracebackOuterFrames.
        tracebackInnerFrames = 50

        // tracebackOuterFrames is the number of outermost frames to print in a
        // stack trace.
        tracebackOuterFrames = 50
)

// unwindFlags control the behavior of various unwinders.
type unwindFlags uint8

const (
        // unwindPrintErrors indicates that if unwinding encounters an error, it
        // should print a message and stop without throwing. This is used for things
        // like stack printing, where it's better to get incomplete information than
        // to crash. This is also used in situations where everything may not be
        // stopped nicely and the stack walk may not be able to complete, such as
        // during profiling signals or during a crash.
        //
        // If neither unwindPrintErrors or unwindSilentErrors are set, unwinding
        // performs extra consistency checks and throws on any error.
        //
        // Note that there are a small number of fatal situations that will throw
        // regardless of unwindPrintErrors or unwindSilentErrors.
        unwindPrintErrors unwindFlags = 1 << iota

        // unwindSilentErrors silently ignores errors during unwinding.
        unwindSilentErrors

        // unwindTrap indicates that the initial PC and SP are from a trap, not a
        // return PC from a call.
        //
        // The unwindTrap flag is updated during unwinding. If set, frame.pc is the
        // address of a faulting instruction instead of the return address of a
        // call. It also means the liveness at pc may not be known.
        //
        // TODO: Distinguish frame.continpc, which is really the stack map PC, from
        // the actual continuation PC, which is computed differently depending on
        // this flag and a few other things.
        unwindTrap

        // unwindJumpStack indicates that, if the traceback is on a system stack, it
        // should resume tracing at the user stack when the system stack is
        // exhausted.
        unwindJumpStack
)

// An unwinder iterates the physical stack frames of a Go sack.
//
// Typical use of an unwinder looks like:
//
//      var u unwinder
//      for u.init(gp, 0); u.valid(); u.next() {
//              // ... use frame info in u ...
//      }
//
// Implementation note: This is carefully structured to be pointer-free because
// tracebacks happen in places that disallow write barriers (e.g., signals).
// Even if this is stack-allocated, its pointer-receiver methods don't know that
// their receiver is on the stack, so they still emit write barriers. Here we
// address that by carefully avoiding any pointers in this type. Another
// approach would be to split this into a mutable part that's passed by pointer
// but contains no pointers itself and an immutable part that's passed and
// returned by value and can contain pointers. We could potentially hide that
// we're doing that in trivial methods that are inlined into the caller that has
// the stack allocation, but that's fragile.
type unwinder struct {
        // frame is the current physical stack frame, or all 0s if
        // there is no frame.
        frame stkframe

        // g is the G who's stack is being unwound. If the
        // unwindJumpStack flag is set and the unwinder jumps stacks,
        // this will be different from the initial G.
        g guintptr

        // cgoCtxt is the index into g.cgoCtxt of the next frame on the cgo stack.
        // The cgo stack is unwound in tandem with the Go stack as we find marker frames.
        cgoCtxt int

        // calleeFuncID is the function ID of the caller of the current
        // frame.
        calleeFuncID abi.FuncID

        // flags are the flags to this unwind. Some of these are updated as we
        // unwind (see the flags documentation).
        flags unwindFlags
}

// init initializes u to start unwinding gp's stack and positions the
// iterator on gp's innermost frame. gp must not be the current G.
//
// A single unwinder can be reused for multiple unwinds.
func (u *unwinder) init(gp *g, flags unwindFlags) {
        // Implementation note: This starts the iterator on the first frame and we
        // provide a "valid" method. Alternatively, this could start in a "before
        // the first frame" state and "next" could return whether it was able to
        // move to the next frame, but that's both more awkward to use in a "for"
        // loop and is harder to implement because we have to do things differently
        // for the first frame.
        u.initAt(^uintptr(0), ^uintptr(0), ^uintptr(0), gp, flags)
}

func (u *unwinder) initAt(pc0, sp0, lr0 uintptr, gp *g, flags unwindFlags) {
        // Don't call this "g"; it's too easy get "g" and "gp" confused.
        if ourg := getg(); ourg == gp && ourg == ourg.m.curg {
                // The starting sp has been passed in as a uintptr, and the caller may
                // have other uintptr-typed stack references as well.
                // If during one of the calls that got us here or during one of the
                // callbacks below the stack must be grown, all these uintptr references
                // to the stack will not be updated, and traceback will continue
                // to inspect the old stack memory, which may no longer be valid.
                // Even if all the variables were updated correctly, it is not clear that
                // we want to expose a traceback that begins on one stack and ends
                // on another stack. That could confuse callers quite a bit.
                // Instead, we require that initAt and any other function that
                // accepts an sp for the current goroutine (typically obtained by
                // calling getcallersp) must not run on that goroutine's stack but
                // instead on the g0 stack.
                throw("cannot trace user goroutine on its own stack")
        }

        if pc0 == ^uintptr(0) && sp0 == ^uintptr(0) { // Signal to fetch saved values from gp.
                if gp.syscallsp != 0 {
                        pc0 = gp.syscallpc
                        sp0 = gp.syscallsp
                        if usesLR {
                                lr0 = 0
                        }
                } else {
                        pc0 = gp.sched.pc
                        sp0 = gp.sched.sp
                        if usesLR {
                                lr0 = gp.sched.lr
                        }
                }
        }

        var frame stkframe
        frame.pc = pc0
        frame.sp = sp0
        if usesLR {
                frame.lr = lr0
        }

        // If the PC is zero, it's likely a nil function call.
        // Start in the caller's frame.
        if frame.pc == 0 {
                if usesLR {
                        frame.pc = *(*uintptr)(unsafe.Pointer(frame.sp))
                        frame.lr = 0
                } else {
                        frame.pc = *(*uintptr)(unsafe.Pointer(frame.sp))
                        frame.sp += goarch.PtrSize
                }
        }

        // runtime/internal/atomic functions call into kernel helpers on
        // arm < 7. See runtime/internal/atomic/sys_linux_arm.s.
        //
        // Start in the caller's frame.
        if GOARCH == "arm" && goarm < 7 && GOOS == "linux" && frame.pc&0xffff0000 == 0xffff0000 {
                // Note that the calls are simple BL without pushing the return
                // address, so we use LR directly.
                //
                // The kernel helpers are frameless leaf functions, so SP and
                // LR are not touched.
                frame.pc = frame.lr
                frame.lr = 0
        }

        f := findfunc(frame.pc)
        if !f.valid() {
                if flags&unwindSilentErrors == 0 {
                        print("runtime: g ", gp.goid, ": unknown pc ", hex(frame.pc), "\n")
                        tracebackHexdump(gp.stack, &frame, 0)
                }
                if flags&(unwindPrintErrors|unwindSilentErrors) == 0 {
                        throw("unknown pc")
                }
                *u = unwinder{}
                return
        }
        frame.fn = f

        // Populate the unwinder.
        *u = unwinder{
                frame:        frame,
                g:            gp.guintptr(),
                cgoCtxt:      len(gp.cgoCtxt) - 1,
                calleeFuncID: abi.FuncIDNormal,
                flags:        flags,
        }

        isSyscall := frame.pc == pc0 && frame.sp == sp0 && pc0 == gp.syscallpc && sp0 == gp.syscallsp
        u.resolveInternal(true, isSyscall)
}

func (u *unwinder) valid() bool {
        return u.frame.pc != 0
}

// resolveInternal fills in u.frame based on u.frame.fn, pc, and sp.
//
// innermost indicates that this is the first resolve on this stack. If
// innermost is set, isSyscall indicates that the PC/SP was retrieved from
// gp.syscall*; this is otherwise ignored.
//
// On entry, u.frame contains:
//   - fn is the running function.
//   - pc is the PC in the running function.
//   - sp is the stack pointer at that program counter.
//   - For the innermost frame on LR machines, lr is the program counter that called fn.
//
// On return, u.frame contains:
//   - fp is the stack pointer of the caller.
//   - lr is the program counter that called fn.
//   - varp, argp, and continpc are populated for the current frame.
//
// If fn is a stack-jumping function, resolveInternal can change the entire
// frame state to follow that stack jump.
//
// This is internal to unwinder.
func (u *unwinder) resolveInternal(innermost, isSyscall bool) {
        frame := &u.frame
        gp := u.g.ptr()

        f := frame.fn
        if f.pcsp == 0 {
                // No frame information, must be external function, like race support.
                // See golang.org/issue/13568.
                u.finishInternal()
                return
        }

        // Compute function info flags.
        flag := f.flag
        if f.funcID == abi.FuncID_cgocallback {
                // cgocallback does write SP to switch from the g0 to the curg stack,
                // but it carefully arranges that during the transition BOTH stacks
                // have cgocallback frame valid for unwinding through.
                // So we don't need to exclude it with the other SP-writing functions.
                flag &^= abi.FuncFlagSPWrite
        }
        if isSyscall {
                // Some Syscall functions write to SP, but they do so only after
                // saving the entry PC/SP using entersyscall.
                // Since we are using the entry PC/SP, the later SP write doesn't matter.
                flag &^= abi.FuncFlagSPWrite
        }

        // Found an actual function.
        // Derive frame pointer.
        if frame.fp == 0 {
                // Jump over system stack transitions. If we're on g0 and there's a user
                // goroutine, try to jump. Otherwise this is a regular call.
                // We also defensively check that this won't switch M's on us,
                // which could happen at critical points in the scheduler.
                // This ensures gp.m doesn't change from a stack jump.
                if u.flags&unwindJumpStack != 0 && gp == gp.m.g0 && gp.m.curg != nil && gp.m.curg.m == gp.m {
                        switch f.funcID {
                        case abi.FuncID_morestack:
                                // morestack does not return normally -- newstack()
                                // gogo's to curg.sched. Match that.
                                // This keeps morestack() from showing up in the backtrace,
                                // but that makes some sense since it'll never be returned
                                // to.
                                gp = gp.m.curg
                                u.g.set(gp)
                                frame.pc = gp.sched.pc
                                frame.fn = findfunc(frame.pc)
                                f = frame.fn
                                flag = f.flag
                                frame.lr = gp.sched.lr
                                frame.sp = gp.sched.sp
                                u.cgoCtxt = len(gp.cgoCtxt) - 1
                        case abi.FuncID_systemstack:
                                // systemstack returns normally, so just follow the
                                // stack transition.
                                if usesLR && funcspdelta(f, frame.pc) == 0 {
                                        // We're at the function prologue and the stack
                                        // switch hasn't happened, or epilogue where we're
                                        // about to return. Just unwind normally.
                                        // Do this only on LR machines because on x86
                                        // systemstack doesn't have an SP delta (the CALL
                                        // instruction opens the frame), therefore no way
                                        // to check.
                                        flag &^= abi.FuncFlagSPWrite
                                        break
                                }
                                gp = gp.m.curg
                                u.g.set(gp)
                                frame.sp = gp.sched.sp
                                u.cgoCtxt = len(gp.cgoCtxt) - 1
                                flag &^= abi.FuncFlagSPWrite
                        }
                }
                frame.fp = frame.sp + uintptr(funcspdelta(f, frame.pc))
                if !usesLR {
                        // On x86, call instruction pushes return PC before entering new function.
                        frame.fp += goarch.PtrSize
                }
        }

        // Derive link register.
        if flag&abi.FuncFlagTopFrame != 0 {
                // This function marks the top of the stack. Stop the traceback.
                frame.lr = 0
        } else if flag&abi.FuncFlagSPWrite != 0 && (!innermost || u.flags&(unwindPrintErrors|unwindSilentErrors) != 0) {
                // The function we are in does a write to SP that we don't know
                // how to encode in the spdelta table. Examples include context
                // switch routines like runtime.gogo but also any code that switches
                // to the g0 stack to run host C code.
                // We can't reliably unwind the SP (we might not even be on
                // the stack we think we are), so stop the traceback here.
                //
                // The one exception (encoded in the complex condition above) is that
                // we assume if we're doing a precise traceback, and this is the
                // innermost frame, that the SPWRITE function voluntarily preempted itself on entry
                // during the stack growth check. In that case, the function has
                // not yet had a chance to do any writes to SP and is safe to unwind.
                // isAsyncSafePoint does not allow assembly functions to be async preempted,
                // and preemptPark double-checks that SPWRITE functions are not async preempted.
                // So for GC stack traversal, we can safely ignore SPWRITE for the innermost frame,
                // but farther up the stack we'd better not find any.
                // This is somewhat imprecise because we're just guessing that we're in the stack
                // growth check. It would be better if SPWRITE were encoded in the spdelta
                // table so we would know for sure that we were still in safe code.
                //
                // uSE uPE inn | action
                //  T   _   _  | frame.lr = 0
                //  F   T   _  | frame.lr = 0
                //  F   F   F  | print; panic
                //  F   F   T  | ignore SPWrite
                if u.flags&(unwindPrintErrors|unwindSilentErrors) == 0 && !innermost {
                        println("traceback: unexpected SPWRITE function", funcname(f))
                        throw("traceback")
                }
                frame.lr = 0
        } else {
                var lrPtr uintptr
                if usesLR {
                        if innermost && frame.sp < frame.fp || frame.lr == 0 {
                                lrPtr = frame.sp
                                frame.lr = *(*uintptr)(unsafe.Pointer(lrPtr))
                        }
                } else {
                        if frame.lr == 0 {
                                lrPtr = frame.fp - goarch.PtrSize
                                frame.lr = *(*uintptr)(unsafe.Pointer(lrPtr))
                        }
                }
        }

        frame.varp = frame.fp
        if !usesLR {
                // On x86, call instruction pushes return PC before entering new function.
                frame.varp -= goarch.PtrSize
        }

        // For architectures with frame pointers, if there's
        // a frame, then there's a saved frame pointer here.
        //
        // NOTE: This code is not as general as it looks.
        // On x86, the ABI is to save the frame pointer word at the
        // top of the stack frame, so we have to back down over it.
        // On arm64, the frame pointer should be at the bottom of
        // the stack (with R29 (aka FP) = RSP), in which case we would
        // not want to do the subtraction here. But we started out without
        // any frame pointer, and when we wanted to add it, we didn't
        // want to break all the assembly doing direct writes to 8(RSP)
        // to set the first parameter to a called function.
        // So we decided to write the FP link *below* the stack pointer
        // (with R29 = RSP - 8 in Go functions).
        // This is technically ABI-compatible but not standard.
        // And it happens to end up mimicking the x86 layout.
        // Other architectures may make different decisions.
        if frame.varp > frame.sp && framepointer_enabled {
                frame.varp -= goarch.PtrSize
        }

        frame.argp = frame.fp + sys.MinFrameSize

        // Determine frame's 'continuation PC', where it can continue.
        // Normally this is the return address on the stack, but if sigpanic
        // is immediately below this function on the stack, then the frame
        // stopped executing due to a trap, and frame.pc is probably not
        // a safe point for looking up liveness information. In this panicking case,
        // the function either doesn't return at all (if it has no defers or if the
        // defers do not recover) or it returns from one of the calls to
        // deferproc a second time (if the corresponding deferred func recovers).
        // In the latter case, use a deferreturn call site as the continuation pc.
        frame.continpc = frame.pc
        if u.calleeFuncID == abi.FuncID_sigpanic {
                if frame.fn.deferreturn != 0 {
                        frame.continpc = frame.fn.entry() + uintptr(frame.fn.deferreturn) + 1
                        // Note: this may perhaps keep return variables alive longer than
                        // strictly necessary, as we are using "function has a defer statement"
                        // as a proxy for "function actually deferred something". It seems
                        // to be a minor drawback. (We used to actually look through the
                        // gp._defer for a defer corresponding to this function, but that
                        // is hard to do with defer records on the stack during a stack copy.)
                        // Note: the +1 is to offset the -1 that
                        // stack.go:getStackMap does to back up a return
                        // address make sure the pc is in the CALL instruction.
                } else {
                        frame.continpc = 0
                }
        }
}

func (u *unwinder) next() {
        frame := &u.frame
        f := frame.fn
        gp := u.g.ptr()

        // Do not unwind past the bottom of the stack.
        if frame.lr == 0 {
                u.finishInternal()
                return
        }
        flr := findfunc(frame.lr)
        if !flr.valid() {
                // This happens if you get a profiling interrupt at just the wrong time.
                // In that context it is okay to stop early.
                // But if no error flags are set, we're doing a garbage collection and must
                // get everything, so crash loudly.
                fail := u.flags&(unwindPrintErrors|unwindSilentErrors) == 0
                doPrint := u.flags&unwindSilentErrors == 0
                if doPrint && gp.m.incgo && f.funcID == abi.FuncID_sigpanic {
                        // We can inject sigpanic
                        // calls directly into C code,
                        // in which case we'll see a C
                        // return PC. Don't complain.
                        doPrint = false
                }
                if fail || doPrint {
                        print("runtime: g ", gp.goid, ": unexpected return pc for ", funcname(f), " called from ", hex(frame.lr), "\n")
                        tracebackHexdump(gp.stack, frame, 0)
                }
                if fail {
                        throw("unknown caller pc")
                }
                frame.lr = 0
                u.finishInternal()
                return
        }

        if frame.pc == frame.lr && frame.sp == frame.fp {
                // If the next frame is identical to the current frame, we cannot make progress.
                print("runtime: traceback stuck. pc=", hex(frame.pc), " sp=", hex(frame.sp), "\n")
                tracebackHexdump(gp.stack, frame, frame.sp)
                throw("traceback stuck")
        }

        injectedCall := f.funcID == abi.FuncID_sigpanic || f.funcID == abi.FuncID_asyncPreempt || f.funcID == abi.FuncID_debugCallV2
        if injectedCall {
                u.flags |= unwindTrap
        } else {
                u.flags &^= unwindTrap
        }

        // Unwind to next frame.
        u.calleeFuncID = f.funcID
        frame.fn = flr
        frame.pc = frame.lr
        frame.lr = 0
        frame.sp = frame.fp
        frame.fp = 0

        // On link register architectures, sighandler saves the LR on stack
        // before faking a call.
        if usesLR && injectedCall {
                x := *(*uintptr)(unsafe.Pointer(frame.sp))
                frame.sp += alignUp(sys.MinFrameSize, sys.StackAlign)
                f = findfunc(frame.pc)
                frame.fn = f
                if !f.valid() {
                        frame.pc = x
                } else if funcspdelta(f, frame.pc) == 0 {
                        frame.lr = x
                }
        }

        u.resolveInternal(false, false)
}

// finishInternal is an unwinder-internal helper called after the stack has been
// exhausted. It sets the unwinder to an invalid state and checks that it
// successfully unwound the entire stack.
func (u *unwinder) finishInternal() {
        u.frame.pc = 0

        // Note that panic != nil is okay here: there can be leftover panics,
        // because the defers on the panic stack do not nest in frame order as
        // they do on the defer stack. If you have:
        //
        //      frame 1 defers d1
        //      frame 2 defers d2
        //      frame 3 defers d3
        //      frame 4 panics
        //      frame 4's panic starts running defers
        //      frame 5, running d3, defers d4
        //      frame 5 panics
        //      frame 5's panic starts running defers
        //      frame 6, running d4, garbage collects
        //      frame 6, running d2, garbage collects
        //
        // During the execution of d4, the panic stack is d4 -> d3, which
        // is nested properly, and we'll treat frame 3 as resumable, because we
        // can find d3. (And in fact frame 3 is resumable. If d4 recovers
        // and frame 5 continues running, d3, d3 can recover and we'll
        // resume execution in (returning from) frame 3.)
        //
        // During the execution of d2, however, the panic stack is d2 -> d3,
        // which is inverted. The scan will match d2 to frame 2 but having
        // d2 on the stack until then means it will not match d3 to frame 3.
        // This is okay: if we're running d2, then all the defers after d2 have
        // completed and their corresponding frames are dead. Not finding d3
        // for frame 3 means we'll set frame 3's continpc == 0, which is correct
        // (frame 3 is dead). At the end of the walk the panic stack can thus
        // contain defers (d3 in this case) for dead frames. The inversion here
        // always indicates a dead frame, and the effect of the inversion on the
        // scan is to hide those dead frames, so the scan is still okay:
        // what's left on the panic stack are exactly (and only) the dead frames.
        //
        // We require callback != nil here because only when callback != nil
        // do we know that gentraceback is being called in a "must be correct"
        // context as opposed to a "best effort" context. The tracebacks with
        // callbacks only happen when everything is stopped nicely.
        // At other times, such as when gathering a stack for a profiling signal
        // or when printing a traceback during a crash, everything may not be
        // stopped nicely, and the stack walk may not be able to complete.
        gp := u.g.ptr()
        if u.flags&(unwindPrintErrors|unwindSilentErrors) == 0 && u.frame.sp != gp.stktopsp {
                print("runtime: g", gp.goid, ": frame.sp=", hex(u.frame.sp), " top=", hex(gp.stktopsp), "\n")
                print("\tstack=[", hex(gp.stack.lo), "-", hex(gp.stack.hi), "\n")
                throw("traceback did not unwind completely")
        }
}

// symPC returns the PC that should be used for symbolizing the current frame.
// Specifically, this is the PC of the last instruction executed in this frame.
//
// If this frame did a normal call, then frame.pc is a return PC, so this will
// return frame.pc-1, which points into the CALL instruction. If the frame was
// interrupted by a signal (e.g., profiler, segv, etc) then frame.pc is for the
// trapped instruction, so this returns frame.pc. See issue #34123. Finally,
// frame.pc can be at function entry when the frame is initialized without
// actually running code, like in runtime.mstart, in which case this returns
// frame.pc because that's the best we can do.
func (u *unwinder) symPC() uintptr {
        if u.flags&unwindTrap == 0 && u.frame.pc > u.frame.fn.entry() {
                // Regular call.
                return u.frame.pc - 1
        }
        // Trapping instruction or we're at the function entry point.
        return u.frame.pc
}

// cgoCallers populates pcBuf with the cgo callers of the current frame using
// the registered cgo unwinder. It returns the number of PCs written to pcBuf.
// If the current frame is not a cgo frame or if there's no registered cgo
// unwinder, it returns 0.
func (u *unwinder) cgoCallers(pcBuf []uintptr) int {
        if cgoTraceback == nil || u.frame.fn.funcID != abi.FuncID_cgocallback || u.cgoCtxt < 0 {
                // We don't have a cgo unwinder (typical case), or we do but we're not
                // in a cgo frame or we're out of cgo context.
                return 0
        }

        ctxt := u.g.ptr().cgoCtxt[u.cgoCtxt]
        u.cgoCtxt--
        cgoContextPCs(ctxt, pcBuf)
        for i, pc := range pcBuf {
                if pc == 0 {
                        return i
                }
        }
        return len(pcBuf)
}

// tracebackPCs populates pcBuf with the return addresses for each frame from u
// and returns the number of PCs written to pcBuf. The returned PCs correspond
// to "logical frames" rather than "physical frames"; that is if A is inlined
// into B, this will still return a PCs for both A and B. This also includes PCs
// generated by the cgo unwinder, if one is registered.
//
// If skip != 0, this skips this many logical frames.
//
// Callers should set the unwindSilentErrors flag on u.
func tracebackPCs(u *unwinder, skip int, pcBuf []uintptr) int {
        var cgoBuf [32]uintptr
        n := 0
        for ; n < len(pcBuf) && u.valid(); u.next() {
                f := u.frame.fn
                cgoN := u.cgoCallers(cgoBuf[:])

                // TODO: Why does &u.cache cause u to escape? (Same in traceback2)
                for iu, uf := newInlineUnwinder(f, u.symPC()); n < len(pcBuf) && uf.valid(); uf = iu.next(uf) {
                        sf := iu.srcFunc(uf)
                        if sf.funcID == abi.FuncIDWrapper && elideWrapperCalling(u.calleeFuncID) {
                                // ignore wrappers
                        } else if skip > 0 {
                                skip--
                        } else {
                                // Callers expect the pc buffer to contain return addresses
                                // and do the -1 themselves, so we add 1 to the call PC to
                                // create a return PC.
                                pcBuf[n] = uf.pc + 1
                                n++
                        }
                        u.calleeFuncID = sf.funcID
                }
                // Add cgo frames (if we're done skipping over the requested number of
                // Go frames).
                if skip == 0 {
                        n += copy(pcBuf[n:], cgoBuf[:cgoN])
                }
        }
        return n
}

// printArgs prints function arguments in traceback.
func printArgs(f funcInfo, argp unsafe.Pointer, pc uintptr) {
        // The "instruction" of argument printing is encoded in _FUNCDATA_ArgInfo.
        // See cmd/compile/internal/ssagen.emitArgInfo for the description of the
        // encoding.
        // These constants need to be in sync with the compiler.
        const (
                _endSeq         = 0xff
                _startAgg       = 0xfe
                _endAgg         = 0xfd
                _dotdotdot      = 0xfc
                _offsetTooLarge = 0xfb
        )

        const (
                limit    = 10                       // print no more than 10 args/components
                maxDepth = 5                        // no more than 5 layers of nesting
                maxLen   = (maxDepth*3+2)*limit + 1 // max length of _FUNCDATA_ArgInfo (see the compiler side for reasoning)
        )

        p := (*[maxLen]uint8)(funcdata(f, abi.FUNCDATA_ArgInfo))
        if p == nil {
                return
        }

        liveInfo := funcdata(f, abi.FUNCDATA_ArgLiveInfo)
        liveIdx := pcdatavalue(f, abi.PCDATA_ArgLiveIndex, pc)
        startOffset := uint8(0xff) // smallest offset that needs liveness info (slots with a lower offset is always live)
        if liveInfo != nil {
                startOffset = *(*uint8)(liveInfo)
        }

        isLive := func(off, slotIdx uint8) bool {
                if liveInfo == nil || liveIdx <= 0 {
                        return true // no liveness info, always live
                }
                if off < startOffset {
                        return true
                }
                bits := *(*uint8)(add(liveInfo, uintptr(liveIdx)+uintptr(slotIdx/8)))
                return bits&(1<<(slotIdx%8)) != 0
        }

        print1 := func(off, sz, slotIdx uint8) {
                x := readUnaligned64(add(argp, uintptr(off)))
                // mask out irrelevant bits
                if sz < 8 {
                        shift := 64 - sz*8
                        if goarch.BigEndian {
                                x = x >> shift
                        } else {
                                x = x << shift >> shift
                        }
                }
                print(hex(x))
                if !isLive(off, slotIdx) {
                        print("?")
                }
        }

        start := true
        printcomma := func() {
                if !start {
                        print(", ")
                }
        }
        pi := 0
        slotIdx := uint8(0) // register arg spill slot index
printloop:
        for {
                o := p[pi]
                pi++
                switch o {
                case _endSeq:
                        break printloop
                case _startAgg:
                        printcomma()
                        print("{")
                        start = true
                        continue
                case _endAgg:
                        print("}")
                case _dotdotdot:
                        printcomma()
                        print("...")
                case _offsetTooLarge:
                        printcomma()
                        print("_")
                default:
                        printcomma()
                        sz := p[pi]
                        pi++
                        print1(o, sz, slotIdx)
                        if o >= startOffset {
                                slotIdx++
                        }
                }
                start = false
        }
}

// funcNamePiecesForPrint returns the function name for printing to the user.
// It returns three pieces so it doesn't need an allocation for string
// concatenation.
func funcNamePiecesForPrint(name string) (string, string, string) {
        // Replace the shape name in generic function with "...".
        i := bytealg.IndexByteString(name, '[')
        if i < 0 {
                return name, "", ""
        }
        j := len(name) - 1
        for name[j] != ']' {
                j--
        }
        if j <= i {
                return name, "", ""
        }
        return name[:i], "[...]", name[j+1:]
}

// funcNameForPrint returns the function name for printing to the user.
func funcNameForPrint(name string) string {
        a, b, c := funcNamePiecesForPrint(name)
        return a + b + c
}

// printFuncName prints a function name. name is the function name in
// the binary's func data table.
func printFuncName(name string) {
        if name == "runtime.gopanic" {
                print("panic")
                return
        }
        a, b, c := funcNamePiecesForPrint(name)
        print(a, b, c)
}

func printcreatedby(gp *g) {
        // Show what created goroutine, except main goroutine (goid 1).
        pc := gp.gopc
        f := findfunc(pc)
        if f.valid() && showframe(f.srcFunc(), gp, false, abi.FuncIDNormal) && gp.goid != 1 {
                printcreatedby1(f, pc, gp.parentGoid)
        }
}

func printcreatedby1(f funcInfo, pc uintptr, goid uint64) {
        print("created by ")
        printFuncName(funcname(f))
        if goid != 0 {
                print(" in goroutine ", goid)
        }
        print("\n")
        tracepc := pc // back up to CALL instruction for funcline.
        if pc > f.entry() {
                tracepc -= sys.PCQuantum
        }
        file, line := funcline(f, tracepc)
        print("\t", file, ":", line)
        if pc > f.entry() {
                print(" +", hex(pc-f.entry()))
        }
        print("\n")
}

func traceback(pc, sp, lr uintptr, gp *g) {
        traceback1(pc, sp, lr, gp, 0)
}

// tracebacktrap is like traceback but expects that the PC and SP were obtained
// from a trap, not from gp->sched or gp->syscallpc/gp->syscallsp or getcallerpc/getcallersp.
// Because they are from a trap instead of from a saved pair,
// the initial PC must not be rewound to the previous instruction.
// (All the saved pairs record a PC that is a return address, so we
// rewind it into the CALL instruction.)
// If gp.m.libcall{g,pc,sp} information is available, it uses that information in preference to
// the pc/sp/lr passed in.
func tracebacktrap(pc, sp, lr uintptr, gp *g) {
        if gp.m.libcallsp != 0 {
                // We're in C code somewhere, traceback from the saved position.
                traceback1(gp.m.libcallpc, gp.m.libcallsp, 0, gp.m.libcallg.ptr(), 0)
                return
        }
        traceback1(pc, sp, lr, gp, unwindTrap)
}

func traceback1(pc, sp, lr uintptr, gp *g, flags unwindFlags) {
        // If the goroutine is in cgo, and we have a cgo traceback, print that.
        if iscgo && gp.m != nil && gp.m.ncgo > 0 && gp.syscallsp != 0 && gp.m.cgoCallers != nil && gp.m.cgoCallers[0] != 0 {
                // Lock cgoCallers so that a signal handler won't
                // change it, copy the array, reset it, unlock it.
                // We are locked to the thread and are not running
                // concurrently with a signal handler.
                // We just have to stop a signal handler from interrupting
                // in the middle of our copy.
                gp.m.cgoCallersUse.Store(1)
                cgoCallers := *gp.m.cgoCallers
                gp.m.cgoCallers[0] = 0
                gp.m.cgoCallersUse.Store(0)

                printCgoTraceback(&cgoCallers)
        }

        if readgstatus(gp)&^_Gscan == _Gsyscall {
                // Override registers if blocked in system call.
                pc = gp.syscallpc
                sp = gp.syscallsp
                flags &^= unwindTrap
        }
        if gp.m != nil && gp.m.vdsoSP != 0 {
                // Override registers if running in VDSO. This comes after the
                // _Gsyscall check to cover VDSO calls after entersyscall.
                pc = gp.m.vdsoPC
                sp = gp.m.vdsoSP
                flags &^= unwindTrap
        }

        // Print traceback.
        //
        // We print the first tracebackInnerFrames frames, and the last
        // tracebackOuterFrames frames. There are many possible approaches to this.
        // There are various complications to this:
        //
        // - We'd prefer to walk the stack once because in really bad situations
        //   traceback may crash (and we want as much output as possible) or the stack
        //   may be changing.
        //
        // - Each physical frame can represent several logical frames, so we might
        //   have to pause in the middle of a physical frame and pick up in the middle
        //   of a physical frame.
        //
        // - The cgo symbolizer can expand a cgo PC to more than one logical frame,
        //   and involves juggling state on the C side that we don't manage. Since its
        //   expansion state is managed on the C side, we can't capture the expansion
        //   state part way through, and because the output strings are managed on the
        //   C side, we can't capture the output. Thus, our only choice is to replay a
        //   whole expansion, potentially discarding some of it.
        //
        // Rejected approaches:
        //
        // - Do two passes where the first pass just counts and the second pass does
        //   all the printing. This is undesirable if the stack is corrupted or changing
        //   because we won't see a partial stack if we panic.
        //
        // - Keep a ring buffer of the last N logical frames and use this to print
        //   the bottom frames once we reach the end of the stack. This works, but
        //   requires keeping a surprising amount of state on the stack, and we have
        //   to run the cgo symbolizer twice—once to count frames, and a second to
        //   print them—since we can't retain the strings it returns.
        //
        // Instead, we print the outer frames, and if we reach that limit, we clone
        // the unwinder, count the remaining frames, and then skip forward and
        // finish printing from the clone. This makes two passes over the outer part
        // of the stack, but the single pass over the inner part ensures that's
        // printed immediately and not revisited. It keeps minimal state on the
        // stack. And through a combination of skip counts and limits, we can do all
        // of the steps we need with a single traceback printer implementation.
        //
        // We could be more lax about exactly how many frames we print, for example
        // always stopping and resuming on physical frame boundaries, or at least
        // cgo expansion boundaries. It's not clear that's much simpler.
        flags |= unwindPrintErrors
        var u unwinder
        tracebackWithRuntime := func(showRuntime bool) int {
                const maxInt int = 0x7fffffff
                u.initAt(pc, sp, lr, gp, flags)
                n, lastN := traceback2(&u, showRuntime, 0, tracebackInnerFrames)
                if n < tracebackInnerFrames {
                        // We printed the whole stack.
                        return n
                }
                // Clone the unwinder and figure out how many frames are left. This
                // count will include any logical frames already printed for u's current
                // physical frame.
                u2 := u
                remaining, _ := traceback2(&u, showRuntime, maxInt, 0)
                elide := remaining - lastN - tracebackOuterFrames
                if elide > 0 {
                        print("...", elide, " frames elided...\n")
                        traceback2(&u2, showRuntime, lastN+elide, tracebackOuterFrames)
                } else if elide <= 0 {
                        // There are tracebackOuterFrames or fewer frames left to print.
                        // Just print the rest of the stack.
                        traceback2(&u2, showRuntime, lastN, tracebackOuterFrames)
                }
                return n
        }
        // By default, omits runtime frames. If that means we print nothing at all,
        // repeat forcing all frames printed.
        if tracebackWithRuntime(false) == 0 {
                tracebackWithRuntime(true)
        }
        printcreatedby(gp)

        if gp.ancestors == nil {
                return
        }
        for _, ancestor := range *gp.ancestors {
                printAncestorTraceback(ancestor)
        }
}

// traceback2 prints a stack trace starting at u. It skips the first "skip"
// logical frames, after which it prints at most "max" logical frames. It
// returns n, which is the number of logical frames skipped and printed, and
// lastN, which is the number of logical frames skipped or printed just in the
// physical frame that u references.
func traceback2(u *unwinder, showRuntime bool, skip, max int) (n, lastN int) {
        // commitFrame commits to a logical frame and returns whether this frame
        // should be printed and whether iteration should stop.
        commitFrame := func() (pr, stop bool) {
                if skip == 0 && max == 0 {
                        // Stop
                        return false, true
                }
                n++
                lastN++
                if skip > 0 {
                        // Skip
                        skip--
                        return false, false
                }
                // Print
                max--
                return true, false
        }

        gp := u.g.ptr()
        level, _, _ := gotraceback()
        var cgoBuf [32]uintptr
        for ; u.valid(); u.next() {
                lastN = 0
                f := u.frame.fn
                for iu, uf := newInlineUnwinder(f, u.symPC()); uf.valid(); uf = iu.next(uf) {
                        sf := iu.srcFunc(uf)
                        callee := u.calleeFuncID
                        u.calleeFuncID = sf.funcID
                        if !(showRuntime || showframe(sf, gp, n == 0, callee)) {
                                continue
                        }

                        if pr, stop := commitFrame(); stop {
                                return
                        } else if !pr {
                                continue
                        }

                        name := sf.name()
                        file, line := iu.fileLine(uf)
                        // Print during crash.
                        //      main(0x1, 0x2, 0x3)
                        //              /home/rsc/go/src/runtime/x.go:23 +0xf
                        //
                        printFuncName(name)
                        print("(")
                        if iu.isInlined(uf) {
                                print("...")
                        } else {
                                argp := unsafe.Pointer(u.frame.argp)
                                printArgs(f, argp, u.symPC())
                        }
                        print(")\n")
                        print("\t", file, ":", line)
                        if !iu.isInlined(uf) {
                                if u.frame.pc > f.entry() {
                                        print(" +", hex(u.frame.pc-f.entry()))
                                }
                                if gp.m != nil && gp.m.throwing >= throwTypeRuntime && gp == gp.m.curg || level >= 2 {
                                        print(" fp=", hex(u.frame.fp), " sp=", hex(u.frame.sp), " pc=", hex(u.frame.pc))
                                }
                        }
                        print("\n")
                }

                // Print cgo frames.
                if cgoN := u.cgoCallers(cgoBuf[:]); cgoN > 0 {
                        var arg cgoSymbolizerArg
                        anySymbolized := false
                        stop := false
                        for _, pc := range cgoBuf[:cgoN] {
                                if cgoSymbolizer == nil {
                                        if pr, stop := commitFrame(); stop {
                                                break
                                        } else if pr {
                                                print("non-Go function at pc=", hex(pc), "\n")
                                        }
                                } else {
                                        stop = printOneCgoTraceback(pc, commitFrame, &arg)
                                        anySymbolized = true
                                        if stop {
                                                break
                                        }
                                }
                        }
                        if anySymbolized {
                                // Free symbolization state.
                                arg.pc = 0
                                callCgoSymbolizer(&arg)
                        }
                        if stop {
                                return
                        }
                }
        }
        return n, 0
}

// printAncestorTraceback prints the traceback of the given ancestor.
// TODO: Unify this with gentraceback and CallersFrames.
func printAncestorTraceback(ancestor ancestorInfo) {
        print("[originating from goroutine ", ancestor.goid, "]:\n")
        for fidx, pc := range ancestor.pcs {
                f := findfunc(pc) // f previously validated
                if showfuncinfo(f.srcFunc(), fidx == 0, abi.FuncIDNormal) {
                        printAncestorTracebackFuncInfo(f, pc)
                }
        }
        if len(ancestor.pcs) == tracebackInnerFrames {
                print("...additional frames elided...\n")
        }
        // Show what created goroutine, except main goroutine (goid 1).
        f := findfunc(ancestor.gopc)
        if f.valid() && showfuncinfo(f.srcFunc(), false, abi.FuncIDNormal) && ancestor.goid != 1 {
                // In ancestor mode, we'll already print the goroutine ancestor.
                // Pass 0 for the goid parameter so we don't print it again.
                printcreatedby1(f, ancestor.gopc, 0)
        }
}

// printAncestorTracebackFuncInfo prints the given function info at a given pc
// within an ancestor traceback. The precision of this info is reduced
// due to only have access to the pcs at the time of the caller
// goroutine being created.
func printAncestorTracebackFuncInfo(f funcInfo, pc uintptr) {
        u, uf := newInlineUnwinder(f, pc)
        file, line := u.fileLine(uf)
        printFuncName(u.srcFunc(uf).name())
        print("(...)\n")
        print("\t", file, ":", line)
        if pc > f.entry() {
                print(" +", hex(pc-f.entry()))
        }
        print("\n")
}

func callers(skip int, pcbuf []uintptr) int {
        sp := getcallersp()
        pc := getcallerpc()
        gp := getg()
        var n int
        systemstack(func() {
                var u unwinder
                u.initAt(pc, sp, 0, gp, unwindSilentErrors)
                n = tracebackPCs(&u, skip, pcbuf)
        })
        return n
}

func gcallers(gp *g, skip int, pcbuf []uintptr) int {
        var u unwinder
        u.init(gp, unwindSilentErrors)
        return tracebackPCs(&u, skip, pcbuf)
}

// showframe reports whether the frame with the given characteristics should
// be printed during a traceback.
func showframe(sf srcFunc, gp *g, firstFrame bool, calleeID abi.FuncID) bool {
        mp := getg().m
        if mp.throwing >= throwTypeRuntime && gp != nil && (gp == mp.curg || gp == mp.caughtsig.ptr()) {
                return true
        }
        return showfuncinfo(sf, firstFrame, calleeID)
}

// showfuncinfo reports whether a function with the given characteristics should
// be printed during a traceback.
func showfuncinfo(sf srcFunc, firstFrame bool, calleeID abi.FuncID) bool {
        level, _, _ := gotraceback()
        if level > 1 {
                // Show all frames.
                return true
        }

        if sf.funcID == abi.FuncIDWrapper && elideWrapperCalling(calleeID) {
                return false
        }

        name := sf.name()

        // Special case: always show runtime.gopanic frame
        // in the middle of a stack trace, so that we can
        // see the boundary between ordinary code and
        // panic-induced deferred code.
        // See golang.org/issue/5832.
        if name == "runtime.gopanic" && !firstFrame {
                return true
        }

        return bytealg.IndexByteString(name, '.') >= 0 && (!hasPrefix(name, "runtime.") || isExportedRuntime(name))
}

// isExportedRuntime reports whether name is an exported runtime function.
// It is only for runtime functions, so ASCII A-Z is fine.
// TODO: this handles exported functions but not exported methods.
func isExportedRuntime(name string) bool {
        const n = len("runtime.")
        return len(name) > n && name[:n] == "runtime." && 'A' <= name[n] && name[n] <= 'Z'
}

// elideWrapperCalling reports whether a wrapper function that called
// function id should be elided from stack traces.
func elideWrapperCalling(id abi.FuncID) bool {
        // If the wrapper called a panic function instead of the
        // wrapped function, we want to include it in stacks.
        return !(id == abi.FuncID_gopanic || id == abi.FuncID_sigpanic || id == abi.FuncID_panicwrap)
}

var gStatusStrings = [...]string{
        _Gidle:      "idle",
        _Grunnable:  "runnable",
        _Grunning:   "running",
        _Gsyscall:   "syscall",
        _Gwaiting:   "waiting",
        _Gdead:      "dead",
        _Gcopystack: "copystack",
        _Gpreempted: "preempted",
}

func goroutineheader(gp *g) {
        gpstatus := readgstatus(gp)

        isScan := gpstatus&_Gscan != 0
        gpstatus &^= _Gscan // drop the scan bit

        // Basic string status
        var status string
        if 0 <= gpstatus && gpstatus < uint32(len(gStatusStrings)) {
                status = gStatusStrings[gpstatus]
        } else {
                status = "???"
        }

        // Override.
        if gpstatus == _Gwaiting && gp.waitreason != waitReasonZero {
                status = gp.waitreason.String()
        }

        // approx time the G is blocked, in minutes
        var waitfor int64
        if (gpstatus == _Gwaiting || gpstatus == _Gsyscall) && gp.waitsince != 0 {
                waitfor = (nanotime() - gp.waitsince) / 60e9
        }
        print("goroutine ", gp.goid, " [", status)
        if isScan {
                print(" (scan)")
        }
        if waitfor >= 1 {
                print(", ", waitfor, " minutes")
        }
        if gp.lockedm != 0 {
                print(", locked to thread")
        }
        print("]:\n")
}

func tracebackothers(me *g) {
        level, _, _ := gotraceback()

        // Show the current goroutine first, if we haven't already.
        curgp := getg().m.curg
        if curgp != nil && curgp != me {
                print("\n")
                goroutineheader(curgp)
                traceback(^uintptr(0), ^uintptr(0), 0, curgp)
        }

        // We can't call locking forEachG here because this may be during fatal
        // throw/panic, where locking could be out-of-order or a direct
        // deadlock.
        //
        // Instead, use forEachGRace, which requires no locking. We don't lock
        // against concurrent creation of new Gs, but even with allglock we may
        // miss Gs created after this loop.
        forEachGRace(func(gp *g) {
                if gp == me || gp == curgp || readgstatus(gp) == _Gdead || isSystemGoroutine(gp, false) && level < 2 {
                        return
                }
                print("\n")
                goroutineheader(gp)
                // Note: gp.m == getg().m occurs when tracebackothers is called
                // from a signal handler initiated during a systemstack call.
                // The original G is still in the running state, and we want to
                // print its stack.
                if gp.m != getg().m && readgstatus(gp)&^_Gscan == _Grunning {
                        print("\tgoroutine running on other thread; stack unavailable\n")
                        printcreatedby(gp)
                } else {
                        traceback(^uintptr(0), ^uintptr(0), 0, gp)
                }
        })
}

// tracebackHexdump hexdumps part of stk around frame.sp and frame.fp
// for debugging purposes. If the address bad is included in the
// hexdumped range, it will mark it as well.
func tracebackHexdump(stk stack, frame *stkframe, bad uintptr) {
        const expand = 32 * goarch.PtrSize
        const maxExpand = 256 * goarch.PtrSize
        // Start around frame.sp.
        lo, hi := frame.sp, frame.sp
        // Expand to include frame.fp.
        if frame.fp != 0 && frame.fp < lo {
                lo = frame.fp
        }
        if frame.fp != 0 && frame.fp > hi {
                hi = frame.fp
        }
        // Expand a bit more.
        lo, hi = lo-expand, hi+expand
        // But don't go too far from frame.sp.
        if lo < frame.sp-maxExpand {
                lo = frame.sp - maxExpand
        }
        if hi > frame.sp+maxExpand {
                hi = frame.sp + maxExpand
        }
        // And don't go outside the stack bounds.
        if lo < stk.lo {
                lo = stk.lo
        }
        if hi > stk.hi {
                hi = stk.hi
        }

        // Print the hex dump.
        print("stack: frame={sp:", hex(frame.sp), ", fp:", hex(frame.fp), "} stack=[", hex(stk.lo), ",", hex(stk.hi), ")\n")
        hexdumpWords(lo, hi, func(p uintptr) byte {
                switch p {
                case frame.fp:
                        return '>'
                case frame.sp:
                        return '<'
                case bad:
                        return '!'
                }
                return 0
        })
}

// isSystemGoroutine reports whether the goroutine g must be omitted
// in stack dumps and deadlock detector. This is any goroutine that
// starts at a runtime.* entry point, except for runtime.main,
// runtime.handleAsyncEvent (wasm only) and sometimes runtime.runfinq.
//
// If fixed is true, any goroutine that can vary between user and
// system (that is, the finalizer goroutine) is considered a user
// goroutine.
func isSystemGoroutine(gp *g, fixed bool) bool {
        // Keep this in sync with internal/trace.IsSystemGoroutine.
        f := findfunc(gp.startpc)
        if !f.valid() {
                return false
        }
        if f.funcID == abi.FuncID_runtime_main || f.funcID == abi.FuncID_handleAsyncEvent {
                return false
        }
        if f.funcID == abi.FuncID_runfinq {
                // We include the finalizer goroutine if it's calling
                // back into user code.
                if fixed {
                        // This goroutine can vary. In fixed mode,
                        // always consider it a user goroutine.
                        return false
                }
                return fingStatus.Load()&fingRunningFinalizer == 0
        }
        return hasPrefix(funcname(f), "runtime.")
}

// SetCgoTraceback records three C functions to use to gather
// traceback information from C code and to convert that traceback
// information into symbolic information. These are used when printing
// stack traces for a program that uses cgo.
//
// The traceback and context functions may be called from a signal
// handler, and must therefore use only async-signal safe functions.
// The symbolizer function may be called while the program is
// crashing, and so must be cautious about using memory.  None of the
// functions may call back into Go.
//
// The context function will be called with a single argument, a
// pointer to a struct:
//
//      struct {
//              Context uintptr
//      }
//
// In C syntax, this struct will be
//
//      struct {
//              uintptr_t Context;
//      };
//
// If the Context field is 0, the context function is being called to
// record the current traceback context. It should record in the
// Context field whatever information is needed about the current
// point of execution to later produce a stack trace, probably the
// stack pointer and PC. In this case the context function will be
// called from C code.
//
// If the Context field is not 0, then it is a value returned by a
// previous call to the context function. This case is called when the
// context is no longer needed; that is, when the Go code is returning
// to its C code caller. This permits the context function to release
// any associated resources.
//
// While it would be correct for the context function to record a
// complete a stack trace whenever it is called, and simply copy that
// out in the traceback function, in a typical program the context
// function will be called many times without ever recording a
// traceback for that context. Recording a complete stack trace in a
// call to the context function is likely to be inefficient.
//
// The traceback function will be called with a single argument, a
// pointer to a struct:
//
//      struct {
//              Context    uintptr
//              SigContext uintptr
//              Buf        *uintptr
//              Max        uintptr
//      }
//
// In C syntax, this struct will be
//
//      struct {
//              uintptr_t  Context;
//              uintptr_t  SigContext;
//              uintptr_t* Buf;
//              uintptr_t  Max;
//      };
//
// The Context field will be zero to gather a traceback from the
// current program execution point. In this case, the traceback
// function will be called from C code.
//
// Otherwise Context will be a value previously returned by a call to
// the context function. The traceback function should gather a stack
// trace from that saved point in the program execution. The traceback
// function may be called from an execution thread other than the one
// that recorded the context, but only when the context is known to be
// valid and unchanging. The traceback function may also be called
// deeper in the call stack on the same thread that recorded the
// context. The traceback function may be called multiple times with
// the same Context value; it will usually be appropriate to cache the
// result, if possible, the first time this is called for a specific
// context value.
//
// If the traceback function is called from a signal handler on a Unix
// system, SigContext will be the signal context argument passed to
// the signal handler (a C ucontext_t* cast to uintptr_t). This may be
// used to start tracing at the point where the signal occurred. If
// the traceback function is not called from a signal handler,
// SigContext will be zero.
//
// Buf is where the traceback information should be stored. It should
// be PC values, such that Buf[0] is the PC of the caller, Buf[1] is
// the PC of that function's caller, and so on.  Max is the maximum
// number of entries to store.  The function should store a zero to
// indicate the top of the stack, or that the caller is on a different
// stack, presumably a Go stack.
//
// Unlike runtime.Callers, the PC values returned should, when passed
// to the symbolizer function, return the file/line of the call
// instruction.  No additional subtraction is required or appropriate.
//
// On all platforms, the traceback function is invoked when a call from
// Go to C to Go requests a stack trace. On linux/amd64, linux/ppc64le,
// linux/arm64, and freebsd/amd64, the traceback function is also invoked
// when a signal is received by a thread that is executing a cgo call.
// The traceback function should not make assumptions about when it is
// called, as future versions of Go may make additional calls.
//
// The symbolizer function will be called with a single argument, a
// pointer to a struct:
//
//      struct {
//              PC      uintptr // program counter to fetch information for
//              File    *byte   // file name (NUL terminated)
//              Lineno  uintptr // line number
//              Func    *byte   // function name (NUL terminated)
//              Entry   uintptr // function entry point
//              More    uintptr // set non-zero if more info for this PC
//              Data    uintptr // unused by runtime, available for function
//      }
//
// In C syntax, this struct will be
//
//      struct {
//              uintptr_t PC;
//              char*     File;
//              uintptr_t Lineno;
//              char*     Func;
//              uintptr_t Entry;
//              uintptr_t More;
//              uintptr_t Data;
//      };
//
// The PC field will be a value returned by a call to the traceback
// function.
//
// The first time the function is called for a particular traceback,
// all the fields except PC will be 0. The function should fill in the
// other fields if possible, setting them to 0/nil if the information
// is not available. The Data field may be used to store any useful
// information across calls. The More field should be set to non-zero
// if there is more information for this PC, zero otherwise. If More
// is set non-zero, the function will be called again with the same
// PC, and may return different information (this is intended for use
// with inlined functions). If More is zero, the function will be
// called with the next PC value in the traceback. When the traceback
// is complete, the function will be called once more with PC set to
// zero; this may be used to free any information. Each call will
// leave the fields of the struct set to the same values they had upon
// return, except for the PC field when the More field is zero. The
// function must not keep a copy of the struct pointer between calls.
//
// When calling SetCgoTraceback, the version argument is the version
// number of the structs that the functions expect to receive.
// Currently this must be zero.
//
// The symbolizer function may be nil, in which case the results of
// the traceback function will be displayed as numbers. If the
// traceback function is nil, the symbolizer function will never be
// called. The context function may be nil, in which case the
// traceback function will only be called with the context field set
// to zero.  If the context function is nil, then calls from Go to C
// to Go will not show a traceback for the C portion of the call stack.
//
// SetCgoTraceback should be called only once, ideally from an init function.
func SetCgoTraceback(version int, traceback, context, symbolizer unsafe.Pointer) {
        if version != 0 {
                panic("unsupported version")
        }

        if cgoTraceback != nil && cgoTraceback != traceback ||
                cgoContext != nil && cgoContext != context ||
                cgoSymbolizer != nil && cgoSymbolizer != symbolizer {
                panic("call SetCgoTraceback only once")
        }

        cgoTraceback = traceback
        cgoContext = context
        cgoSymbolizer = symbolizer

        // The context function is called when a C function calls a Go
        // function. As such it is only called by C code in runtime/cgo.
        if _cgo_set_context_function != nil {
                cgocall(_cgo_set_context_function, context)
        }
}

var cgoTraceback unsafe.Pointer
var cgoContext unsafe.Pointer
var cgoSymbolizer unsafe.Pointer

// cgoTracebackArg is the type passed to cgoTraceback.
type cgoTracebackArg struct {
        context    uintptr
        sigContext uintptr
        buf        *uintptr
        max        uintptr
}

// cgoContextArg is the type passed to the context function.
type cgoContextArg struct {
        context uintptr
}

// cgoSymbolizerArg is the type passed to cgoSymbolizer.
type cgoSymbolizerArg struct {
        pc       uintptr
        file     *byte
        lineno   uintptr
        funcName *byte
        entry    uintptr
        more     uintptr
        data     uintptr
}

// printCgoTraceback prints a traceback of callers.
func printCgoTraceback(callers *cgoCallers) {
        if cgoSymbolizer == nil {
                for _, c := range callers {
                        if c == 0 {
                                break
                        }
                        print("non-Go function at pc=", hex(c), "\n")
                }
                return
        }

        commitFrame := func() (pr, stop bool) { return true, false }
        var arg cgoSymbolizerArg
        for _, c := range callers {
                if c == 0 {
                        break
                }
                printOneCgoTraceback(c, commitFrame, &arg)
        }
        arg.pc = 0
        callCgoSymbolizer(&arg)
}

// printOneCgoTraceback prints the traceback of a single cgo caller.
// This can print more than one line because of inlining.
// It returns the "stop" result of commitFrame.
func printOneCgoTraceback(pc uintptr, commitFrame func() (pr, stop bool), arg *cgoSymbolizerArg) bool {
        arg.pc = pc
        for {
                if pr, stop := commitFrame(); stop {
                        return true
                } else if !pr {
                        continue
                }

                callCgoSymbolizer(arg)
                if arg.funcName != nil {
                        // Note that we don't print any argument
                        // information here, not even parentheses.
                        // The symbolizer must add that if appropriate.
                        println(gostringnocopy(arg.funcName))
                } else {
                        println("non-Go function")
                }
                print("\t")
                if arg.file != nil {
                        print(gostringnocopy(arg.file), ":", arg.lineno, " ")
                }
                print("pc=", hex(pc), "\n")
                if arg.more == 0 {
                        return false
                }
        }
}

// callCgoSymbolizer calls the cgoSymbolizer function.
func callCgoSymbolizer(arg *cgoSymbolizerArg) {
        call := cgocall
        if panicking.Load() > 0 || getg().m.curg != getg() {
                // We do not want to call into the scheduler when panicking
                // or when on the system stack.
                call = asmcgocall
        }
        if msanenabled {
                msanwrite(unsafe.Pointer(arg), unsafe.Sizeof(cgoSymbolizerArg{}))
        }
        if asanenabled {
                asanwrite(unsafe.Pointer(arg), unsafe.Sizeof(cgoSymbolizerArg{}))
        }
        call(cgoSymbolizer, noescape(unsafe.Pointer(arg)))
}

// cgoContextPCs gets the PC values from a cgo traceback.
func cgoContextPCs(ctxt uintptr, buf []uintptr) {
        if cgoTraceback == nil {
                return
        }
        call := cgocall
        if panicking.Load() > 0 || getg().m.curg != getg() {
                // We do not want to call into the scheduler when panicking
                // or when on the system stack.
                call = asmcgocall
        }
        arg := cgoTracebackArg{
                context: ctxt,
                buf:     (*uintptr)(noescape(unsafe.Pointer(&buf[0]))),
                max:     uintptr(len(buf)),
        }
        if msanenabled {
                msanwrite(unsafe.Pointer(&arg), unsafe.Sizeof(arg))
        }
        if asanenabled {
                asanwrite(unsafe.Pointer(&arg), unsafe.Sizeof(arg))
        }
        call(cgoTraceback, noescape(unsafe.Pointer(&arg)))
}