strconv: avoid panic on invalid call to FormatFloat

Calling FormatFloat with an invalid value of fmt is expected
to return a string containing '%' and the input fmt character.
Since even before Go 1.0, the code has been panicking in the
case where prec=0.

Fixes #52187

Change-Id: I74fec601eedb7fe28efc5132c4253674661452aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/402817
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Auto-Submit: Emmanuel Odeke <emmanuel@orijtech.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>

diff --git a/src/strconv/ftoa.go b/src/strconv/ftoa.go
index eca04b851c3ad0e21c295077d8f20f40e833a0b0..f602d0ffe6cc9b6a6060815f7de1b9406c7e4ca2 100644 (file)
--- a/src/strconv/ftoa.go
+++ b/src/strconv/ftoa.go
@@ -138,6 +138,9 @@ func genericFtoa(dst []byte, val float64, fmt byte, prec, bitSize int) []byte {
                                prec = 1
                        }
                        digits = prec
+               default:
+                       // Invalid mode.
+                       digits = 1
                }
                var buf [24]byte
                if bitSize == 32 && digits <= 9 {

diff --git a/src/strconv/ftoa_test.go b/src/strconv/ftoa_test.go
index 73008b1c6219132b231e220b8d12a0bc8ebe3668..3512ccf58070ec42ade315e26cd20d6094dda91f 100644 (file)
--- a/src/strconv/ftoa_test.go
+++ b/src/strconv/ftoa_test.go
@@ -151,6 +151,11 @@ var ftoatests = []ftoaTest{
        {498484681984085570, 'f', -1, "498484681984085570"},
        {-5.8339553793802237e+23, 'g', -1, "-5.8339553793802237e+23"},
 
+       // Issue 52187
+       {123.45, '?', 0, "%?"},
+       {123.45, '?', 1, "%?"},
+       {123.45, '?', -1, "%?"},
+
        // rounding
        {2.275555555555555, 'x', -1, "0x1.23456789abcdep+01"},
        {2.275555555555555, 'x', 0, "0x1p+01"},