Modified error message for clarity.

Previously, error message only showed "<nil>" when child and pid were the
same (since no error is returned by the Wait4 syscall in this case) which
occurs when the process has incorrectly terminated. A new error message
was added to improve clarity for such a case. Tests for this function were
modified to reflect the improved distinction between process termination
and error.

PiperOrigin-RevId: 204018107
Change-Id: Ib38481c9590405e5bafcb6efe27fd49b3948910c

runsc/specutils/specutils.go

@@ -324,9 +324,14 @@ func WaitForReady(pid int, timeout time.Duration, ready func() (bool, error)) er
 		// Check if the process is still running.
 		var ws syscall.WaitStatus
 		var ru syscall.Rusage
+
+		// If the process is alive, child is 0 because of the NOHANG option.
+		// If the process has terminated, child equals the process id.
 		child, err := syscall.Wait4(pid, &ws, syscall.WNOHANG, &ru)
-		if err != nil || child == pid {
-			return fmt.Errorf("process (%d) is not running, err: %v", pid, err)
+		if err != nil {
+			return fmt.Errorf("error waiting for process: %v", err)
+		} else if child == pid {
+			return fmt.Errorf("process %d has terminated", pid)
 		}
 
 		// Process continues to run, backoff and retry.

runsc/specutils/specutils_test.go

@@ -76,8 +76,11 @@ func TestWaitForReadyNotRunning(t *testing.T) {
 	err := WaitForReady(cmd.Process.Pid, 5*time.Second, func() (bool, error) {
 		return false, nil
 	})
-	if !strings.Contains(err.Error(), "not running") {
-		t.Errorf("ProcessWaitReady got: %v, expected: not running", err)
+	if err != nil && !strings.Contains(err.Error(), "terminated") {
+		t.Errorf("ProcessWaitReady got: %v, expected: process terminated", err)
+	}
+	if err == nil {
+		t.Errorf("ProcessWaitReady incorrectly succeeded")
 	}
 }