syscall: fix fork-exec/wait inconsistencies for Plan 9

Fixes the fork-exec/wait race condition for ForkExec
as well, by making it use startProcess. This makes the
comment for StartProcess consistent as well.

Further, the passing of Waitmsg data in startProcess
and WaitProcess is protected against possible forks
from outside of ForkExec and StartProcess, which might
cause interference with the Await call.

R=rsc, rminnich, npe, ality
CC=golang-dev
https://golang.org/cl/7128059

src/pkg/syscall/exec_plan9.go

@@ -495,11 +495,6 @@ func forkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error)
 	return pid, nil
 }
 
-// Combination of fork and exec, careful to be thread safe.
-func ForkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error) {
-	return forkExec(argv0, argv, attr)
-}
-
 type waitErr struct {
 	Waitmsg
 	err error
@@ -551,7 +546,9 @@ func startProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, err err
 		forkc <- ret
 
 		var w waitErr
-		w.err = Await(&w.Waitmsg)
+		for w.err == nil && w.Pid != ret.pid {
+			w.err = Await(&w.Waitmsg)
+		}
 		waitc <- &w
 		close(waitc)
 	}()
@@ -559,6 +556,11 @@ func startProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, err err
 	return ret.pid, ret.err
 }
 
+// Combination of fork and exec, careful to be thread safe.
+func ForkExec(argv0 string, argv []string, attr *ProcAttr) (pid int, err error) {
+	return startProcess(argv0, argv, attr)
+}
+
 // StartProcess wraps ForkExec for package os.
 func StartProcess(argv0 string, argv []string, attr *ProcAttr) (pid int, handle uintptr, err error) {
 	pid, err = startProcess(argv0, argv, attr)
@@ -612,8 +614,8 @@ func Exec(argv0 string, argv []string, envv []string) (err error) {
 // WaitProcess waits until the pid of a
 // running process is found in the queue of
 // wait messages. It is used in conjunction
-// with StartProcess to wait for a running
-// process to exit.
+// with ForkExec/StartProcess to wait for a
+// running process to exit.
 func WaitProcess(pid int, w *Waitmsg) (err error) {
 	procs.Lock()
 	ch := procs.waits[pid]