os: correctly handle errno==0 in (*Process).blockUntilWaitable

CL 627478 inadvertently returns a non-nil error
containing a syscall.Errno(0).

Change-Id: I1d6a9d0575d3ed651ddc02f30505437d0d266bb3
Reviewed-on: https://go-review.googlesource.com/c/go/+/629515
Auto-Submit: Ian Lance Taylor <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Commit-Queue: Ian Lance Taylor <[email protected]>

Author: neild

src/os/wait_wait6.go

@@ -17,7 +17,10 @@ import (
 func (p *Process) blockUntilWaitable() (bool, error) {
 	err := ignoringEINTR(func() error {
 		_, errno := wait6(_P_PID, p.Pid, syscall.WEXITED|syscall.WNOWAIT)
-		return errno
+		if errno != 0 {
+			return errno
+		}
+		return nil
 	})
 	runtime.KeepAlive(p)
 	if err == syscall.ENOSYS {

src/os/wait_waitid.go

@@ -29,7 +29,10 @@ func (p *Process) blockUntilWaitable() (bool, error) {
 	psig := &siginfo[0]
 	err := ignoringEINTR(func() error {
 		_, _, errno := syscall.Syscall6(syscall.SYS_WAITID, _P_PID, uintptr(p.Pid), uintptr(unsafe.Pointer(psig)), syscall.WEXITED|syscall.WNOWAIT, 0, 0)
-		return errno
+		if errno != 0 {
+			return errno
+		}
+		return nil
 	})
 	runtime.KeepAlive(p)
 	if err != nil {