context: add APIs for writing and reading cancelation cause

Extend the context package to allow users to specify why a context was
canceled in the form of an error, the "cause". Users write the cause
by calling WithCancelCause to construct a derived context, then
calling cancel(cause) to cancel the context with the provided cause.
Users retrieve the cause by calling context.Cause(ctx), which returns
the cause of the first cancelation for ctx or any of its parents.

The cause is implemented as a field of cancelCtx, since only cancelCtx
can be canceled. Calling cancel copies the cause to all derived (child)
cancelCtxs. Calling Cause(ctx) finds the nearest parent cancelCtx by
looking up the context value keyed by cancelCtxKey.

API changes:
+pkg context, func Cause(Context) error
+pkg context, func WithCancelCause(Context) (Context, CancelCauseFunc)
+pkg context, type CancelCauseFunc func(error)

Fixes #26356
Fixes #51365

Change-Id: I15b62bd454c014db3f4f1498b35204451509e641
Reviewed-on: https://go-review.googlesource.com/c/go/+/375977
Reviewed-by: Damien Neil <dneil@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Sameer Ajmani <sameer@golang.org>
Auto-Submit: Sameer Ajmani <sameer@golang.org>

1 files changed, 3 insertions, 0 deletions

diff --git a/api/next/51365.txt b/api/next/51365.txt
new file mode 100644
index 0000000000..df629f1852
--- /dev/null
+++ b/api/next/51365.txt
@@ -0,0 +1,3 @@
+pkg context, func Cause(Context) error #51365
+pkg context, func WithCancelCause(Context) (Context, CancelCauseFunc) #51365
+pkg context, type CancelCauseFunc func(error) #51365