os/exec: refactor goroutine error reporting

Use a separate channel to report the final error of the copying
goroutines, receiving a value only when all of the copying goroutines
have completed. In a followup change (CL 401835), that will allow
waiters to select on goroutine completion alongside other events (such
as Context cancellation).

Also mildly refactor the watchCtx helper method so that its structure
better matches what will be needed to implement WaitDelay.

For #50436.

Change-Id: I54b3997fb6931d204814d8382f0a388a67b520f1
Reviewed-on: https://go-review.googlesource.com/c/go/+/435995
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>

src/os/exec/exec.go

@@ -238,9 +238,16 @@ type Cmd struct {
 	// These should be closed when Wait completes.
 	userPipes []io.Closer
 
-	goroutine     []func() error
-	goroutineErrs <-chan error // one receive per goroutine
-	ctxErr        <-chan error // if non nil, receives the error from watchCtx exactly once
+	// goroutine holds a set of closures to execute to copy data
+	// to and/or from the command's I/O pipes.
+	goroutine []func() error
+
+	// If goroutineErr is non-nil, it receives the first error from a copying
+	// goroutine once all such goroutines have completed.
+	// goroutineErr is set to nil once its error has been received.
+	goroutineErr <-chan error
+
+	ctxErr <-chan error // if non nil, receives the error from watchCtx exactly once
 
 	// For a security release long ago, we created x/sys/execabs,
 	// which manipulated the unexported lookPathErr error field
@@ -567,22 +574,70 @@ func (c *Cmd) Start() error {
 	}
 	started = true
 
-	// Don't allocate the goroutineErrs channel unless there are goroutines to fire.
+	// Don't allocate the goroutineErr channel unless there are goroutines to start.
 	if len(c.goroutine) > 0 {
-		errc := make(chan error, len(c.goroutine))
-		c.goroutineErrs = errc
+		goroutineErr := make(chan error, 1)
+		c.goroutineErr = goroutineErr
+
+		type goroutineStatus struct {
+			running  int
+			firstErr error
+		}
+		statusc := make(chan goroutineStatus, 1)
+		statusc <- goroutineStatus{running: len(c.goroutine)}
 		for _, fn := range c.goroutine {
 			go func(fn func() error) {
-				errc <- fn()
+				err := fn()
+
+				status := <-statusc
+				if status.firstErr == nil {
+					status.firstErr = err
+				}
+				status.running--
+				if status.running == 0 {
+					goroutineErr <- status.firstErr
+				} else {
+					statusc <- status
+				}
 			}(fn)
 		}
+		c.goroutine = nil // Allow the goroutines' closures to be GC'd when they complete.
 	}
 
-	c.ctxErr = c.watchCtx()
+	if c.ctx != nil && c.ctx.Done() != nil {
+		errc := make(chan error)
+		c.ctxErr = errc
+		go c.watchCtx(errc)
+	}
 
 	return nil
 }
 
+// watchCtx watches c.ctx until it is able to send a result to errc.
+//
+// If c.ctx is done before a result can be sent, watchCtx terminates c.Process.
+func (c *Cmd) watchCtx(errc chan<- error) {
+	select {
+	case errc <- nil:
+		return
+	case <-c.ctx.Done():
+	}
+
+	var err error
+	if killErr := c.Process.Kill(); killErr == nil {
+		// We appear to have killed the process. c.Process.Wait should return a
+		// non-nil error to c.Wait unless the Kill signal races with a successful
+		// exit, and if that does happen we shouldn't report a spurious error,
+		// so don't set err to anything here.
+	} else if !errors.Is(killErr, os.ErrProcessDone) {
+		err = wrappedError{
+			prefix: "exec: error sending signal to Cmd",
+			err:    killErr,
+		}
+	}
+	errc <- err
+}
+
 // An ExitError reports an unsuccessful exit by a command.
 type ExitError struct {
 	*os.ProcessState
@@ -628,36 +683,33 @@ func (c *Cmd) Wait() error {
 	if c.ProcessState != nil {
 		return errors.New("exec: Wait was already called")
 	}
+
 	state, err := c.Process.Wait()
 	if err == nil && !state.Success() {
 		err = &ExitError{ProcessState: state}
 	}
 	c.ProcessState = state
 
-	// Wait for the pipe-copying goroutines to complete.
-	var copyError error
-	for range c.goroutine {
-		if err := <-c.goroutineErrs; err != nil && copyError == nil {
-			copyError = err
-		}
-	}
-	c.goroutine = nil      // Allow the goroutines' closures to be GC'd.
-	c.goroutinePipes = nil // Already closed by their respective goroutines.
-
 	if c.ctxErr != nil {
 		interruptErr := <-c.ctxErr
 		// If c.Process.Wait returned an error, prefer that.
 		// Otherwise, report any error from the interrupt goroutine.
-		if interruptErr != nil && err == nil {
+		if err == nil {
 			err = interruptErr
 		}
 	}
-	// Report errors from the copying goroutines only if the program otherwise
-	// exited normally on its own. Otherwise, the copying error may be due to the
-	// abnormal termination.
-	if err == nil {
-		err = copyError
+
+	// Wait for the pipe-copying goroutines to complete.
+	if c.goroutineErr != nil {
+		// Report an error from the copying goroutines only if the program otherwise
+		// exited normally on its own. Otherwise, the copying error may be due to the
+		// abnormal termination.
+		copyErr := <-c.goroutineErr
+		if err == nil {
+			err = copyErr
+		}
 	}
+	c.goroutinePipes = nil // Already closed by their respective goroutines.
 
 	c.closeDescriptors(c.userPipes)
 	c.userPipes = nil
@@ -665,42 +717,6 @@ func (c *Cmd) Wait() error {
 	return err
 }
 
-// watchCtx conditionally starts a goroutine that waits until either c.ctx is
-// done or c.Process.Wait has completed (called from Wait).
-// If c.ctx is done first, the goroutine terminates c.Process.
-//
-// If a goroutine was started, watchCtx returns a channel on which its result
-// must be received.
-func (c *Cmd) watchCtx() <-chan error {
-	if c.ctx == nil {
-		return nil
-	}
-
-	errc := make(chan error)
-	go func() {
-		select {
-		case errc <- nil:
-			return
-		case <-c.ctx.Done():
-		}
-
-		var err error
-		if killErr := c.Process.Kill(); killErr == nil {
-			// We appear to have successfully delivered a kill signal, so any
-			// program behavior from this point may be due to ctx.
-			err = c.ctx.Err()
-		} else if !errors.Is(killErr, os.ErrProcessDone) {
-			err = wrappedError{
-				prefix: "exec: error sending signal to Cmd",
-				err:    killErr,
-			}
-		}
-		errc <- err
-	}()
-
-	return errc
-}
-
 // Output runs the command and returns its standard output.
 // Any returned error will usually be of type *ExitError.
 // If c.Stderr was nil, Output populates ExitError.Stderr.