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os/exec: document blocking Stdin/Stdout/Stderr

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WaitDelay only handles writes to Stdin and reads from Stdout/Stderr.
If Stdin is set to a blocking Reader, or Stdout/Stderr are set to
a blocking Writer, Wait can hang indefinitely. I don't see any way to
fix this with the current API, as there is no general way that the
os/exec package can interrupt the blocking Read or Write.

This CL documents the limitation and points people toward the
workaround of using StdinPipe/StdoutPipe/StderrPipe and arranging
for their own way to interrupt the blocking Read or Write.

Fixes #77227

Change-Id: I3150ae7af89dccf8d859b41eb43eaf0bbbb55fee
Reviewed-on: https://go-review.googlesource.com/c/go/+/739422
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
This commit is contained in:
Ian Lance Taylor
2026-01-26 20:03:30 -08:00
committed by Gopher Robot
parent 5ec5fdc093
commit 251f3aa6ee
1 changed files with 11 additions and 0 deletions
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11
src/os/exec/exec.go
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@@ -203,6 +203,11 @@ type Cmd struct {
// stops copying, either because it has reached the end of Stdin
// (EOF or a read error), or because writing to the pipe returned an error,
// or because a nonzero WaitDelay was set and expired.
//
// Regardless of WaitDelay, Wait can block until a Read from
// Stdin completes. If you need to use a blocking io.Reader,
// use the StdinPipe method to get a pipe, copy from the Reader
// to the pipe, and arrange to close the Reader after Wait returns.
Stdin io.Reader
// Stdout and Stderr specify the process's standard output and error.
@@ -219,6 +224,12 @@ type Cmd struct {
// goroutine reaches EOF or encounters an error or a nonzero WaitDelay
// expires.
//
// Regardless of WaitDelay, Wait can block until a Write to
// Stdout or Stderr completes. If you need to use a blocking io.Writer,
// use the StdoutPipe or StderrPipe method to get a pipe,
// copy from the pipe to the Writer, and arrange to close the
// Writer after Wait returns.
//
// If Stdout and Stderr are the same writer, and have a type that can
// be compared with ==, at most one goroutine at a time will call Write.
Stdout io.Writer