syscall: avoid failure in plan9 StartProcess from fd close race

Between the enumeration of fdsToClose in the parent and the
closing of fds in the child, it's possible for a file to be
closed in another thread. If that file descriptor is reused
when opening the child-parent status pipe, it will be closed
prematurely in the child and the forkExec gets out of sync.
This has been observed to cause failures in builder tests
when the link step of a build is started before the compile
step has run, with "file does not exist" messages as the
visible symptom.

The simple workaround is to check against closing the pipe.
A more comprehensive solution would be to rewrite the fd
closing code to avoid races, along the lines of the long
ago proposed https://golang.org/cl/57890043 - but meanwhile
this correction will prevent some builder failures.

Change-Id: I4ef5eaea70c21d00f4df0e0847a1c5b2966de7da
Reviewed-on: https://go-review.googlesource.com/20800
Run-TryBot: David du Colombier <0intro@gmail.com>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/syscall/exec_plan9.go

@@ -219,8 +219,10 @@ func forkAndExecInChild(argv0 *byte, argv []*byte, envv []envItem, dir *byte, at
 
 	// Close fds we don't need.
 	for i = 0; i < len(fdsToClose); i++ {
+		if fdsToClose[i] != pipe {
 			RawSyscall(SYS_CLOSE, uintptr(fdsToClose[i]), 0, 0)
 		}
+	}
 
 	if envv != nil {
 		// Write new environment variables.