Is there a race in exec.CombinedOutput?

[王富民awaw]

exec.CombinedOutput uses the same io.Writer for both stdout and stderr in  exec.go - Go (opensource.google) .

This io.Writer is written in  exec.go - Go (opensource.google) .

These two writes happens concurrently in two goroutines in exec.go - Go (opensource.google) .

As far as I know, bytes.Buffer is not safe to concurrent writes.

Is this not a race?

[Jason Phillips]

From the Stdout/Stderr field documentation on the os/exec.Cmd type:

"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."

[Brian Candler]

Code reference:

https://cs.opensource.google/go/go/+/refs/tags/go1.21.1:src/os/exec/exec.go;l=523-565

* c.writerDescriptor creates the pipe, and a copying function is appended to the slice c.goroutine. This slice contains the functions which will be started in their own goroutines.

* c.childStdout calls c.writerDescriptor

* c.childStderr reuses the exiting childStdout if Stdout and Stderr are the same (and therefore there is still only one function in c.goroutine)

* interfaceEqual implements the check that Jason quoted:

https://cs.opensource.google/go/go/+/refs/tags/go1.21.1:src/os/exec/exec.go;l=473-480

[王富民awaw]

That's interesting!

Thanks Jason and Brian for educating me! Thanks!