context: document that Err is unspecified before Done

It could have been defined the other way, but since the behavior has
been unspecified, this is the conservative approach for people writing
different implementations of the Context interface.

Change-Id: I7334a4c674bc2330cca6874f7cac1eb0eaea3cff
Reviewed-on: https://go-review.googlesource.com/37375
Reviewed-by: Matt Layher <mdlayher@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Sameer Ajmani <sameer@golang.org>
Run-TryBot: Sameer Ajmani <sameer@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

diff --git a/src/context/context.go b/src/context/context.go
index c60d3788184633776c0eedd0b25c26e11c48ae9b..cfd1c09fcc1844a52c4e64c99ac7c42b751646e2 100644 (file)
--- a/src/context/context.go
+++ b/src/context/context.go
@@ -100,6 +100,7 @@ type Context interface {
        // Canceled if the context was canceled or DeadlineExceeded if the
        // context's deadline passed. No other values for Err are defined.
        // After Done is closed, successive calls to Err return the same value.
+       // Err's return value is unspecified before Done is closed.
        Err() error
 
        // Value returns the value associated with this context for key, or nil