The Go bytes.Buffer isn't thread-safe. Yet, when I read the source code I notice that os/exec.CombinedOutput() uses the same buffer for both c.Stdout and c.Stderr. Further reading the implementation of the package it looks like there is no synchronisation when writing to the c.Stderr/c.Stdout here.
Am I missing something or have I found a possible synchronisation issue? AFAIK stderr and stdout can be written to concurrently by the child process.
Stderr and Stdout are not written concurrently if they are the same writer as stated in the Cmd documentation:
If Stdout and Stderr are the same writer, at most one goroutine at a time will call Write.
This feature is implemented in the Cmd.stderr function. If Stdout and Stderr are the same, then the same fd is passed the child process stdout and stderr. In the case where the fd is a pipe with a goroutine to pump it, there's only one goroutine writing to Stdout/Stderr.