os/exec: clarify requirements for stdin/stdout

[dvyukov]

Here is code from golang-nuts:
        cmd := exec.Command("cat")
        in, _ := cmd.StdinPipe()
        out, _ := cmd.StdoutPipe()
        cmd.Start()
        go func() { defer os.Stdout.Close(); io.Copy(os.Stdout, out) }()
        go func() { defer in.Close(); io.Copy(in, os.Stdin) }()
        cmd.Wait()
https://groups.google.com/forum/#!topic/golang-nuts/gjkbC6M4cAU

This has 2 data races:
1. Both cmd and user close 'in'.
2. Cmd closes 'out', while user reads from it.

I've seen similar races in several projects.

os/exec documentation must clearly explain:
1. When it is allowed to close stdin/stdout (only after Wait?)
2. How to stream stdout (it's easy to race with stdout.Close in Wait)
3. How to close stdin to notify the process about end of input data (e.g. cat)
4. How/when to use thread-safe Reader/Writer

It all is basically impossible to get right first time.

[robpike]

Comment 1:

Labels changed: added priority-later, go1.2, removed priority-triage, go1.2maybe.

Status changed to Accepted.

[bradfitz]

Comment 2:

See the somewhat unfortunate issue #4290 for history.
Don't use goroutines.

[rsc]

Comment 3:

Labels changed: added documentation.

[rsc]

Comment 4:

This issue was closed by revision 22e8f82.

Status changed to Fixed.