fix data race on p.stderr (#209)

Co-authored-by: John Arundel <john@bitfieldconsulting.com>

.github/workflows/ci.yml

@@ -13,7 +13,7 @@ jobs:
         with:
           go-version: ${{ matrix.go-version }}
       - uses: actions/checkout@v3
-      - run: go test ./...
+      - run: go test -race ./...
 
   gocritic:
     runs-on: ubuntu-latest

script.go

@@ -28,13 +28,14 @@ import (
 // Pipe represents a pipe object with an associated [ReadAutoCloser].
 type Pipe struct {
 	// Reader is the underlying reader.
-	Reader         ReadAutoCloser
-	stdout, stderr io.Writer
-	httpClient     *http.Client
+	Reader     ReadAutoCloser
+	stdout     io.Writer
+	httpClient *http.Client
 
-	// because pipe stages are concurrent, protect 'err'
-	mu  *sync.Mutex
-	err error
+	// because pipe stages are concurrent, protect 'err' and 'stderr'
+	mu     *sync.Mutex
+	err    error
+	stderr io.Writer
 }
 
 // Args creates a pipe containing the program's command-line arguments from
@@ -414,8 +415,9 @@ func (p *Pipe) Exec(cmdLine string) *Pipe {
 		cmd.Stdin = r
 		cmd.Stdout = w
 		cmd.Stderr = w
-		if p.stderr != nil {
-			cmd.Stderr = p.stderr
+		pipeStderr := p.stdErr()
+		if pipeStderr != nil {
+			cmd.Stderr = pipeStderr
 		}
 		err = cmd.Start()
 		if err != nil {
@@ -454,8 +456,9 @@ func (p *Pipe) ExecForEach(cmdLine string) *Pipe {
 			cmd := exec.Command(args[0], args[1:]...)
 			cmd.Stdout = w
 			cmd.Stderr = w
-			if p.stderr != nil {
-				cmd.Stderr = p.stderr
+			pipeStderr := p.stdErr()
+			if pipeStderr != nil {
+				cmd.Stderr = pipeStderr
 			}
 			err = cmd.Start()
 			if err != nil {
@@ -839,6 +842,18 @@ func (p *Pipe) Slice() ([]string, error) {
 	return result, p.Error()
 }
 
+// stdErr returns the pipe's configured standard error writer for commands run
+// via [Pipe.Exec] and [Pipe.ExecForEach]. The default is nil, which means that
+// error output will go to the pipe.
+func (p *Pipe) stdErr() io.Writer {
+	if p.mu == nil { // uninitialised pipe
+		return nil
+	}
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	return p.stderr
+}
+
 // Stdout copies the pipe's contents to its configured standard output (using
 // [Pipe.WithStdout]), or to [os.Stdout] otherwise, and returns the number of
 // bytes successfully written, together with any error.
@@ -913,10 +928,11 @@ func (p *Pipe) WithReader(r io.Reader) *Pipe {
 	return p
 }
 
-// WithStderr redirects the standard error output for commands run via
-// [Pipe.Exec] or [Pipe.ExecForEach] to the writer w, instead of going to the
-// pipe as it normally would.
+// WithStderr sets the standard error output for [Pipe.Exec] or
+// [Pipe.ExecForEach] commands to w, instead of the pipe.
 func (p *Pipe) WithStderr(w io.Writer) *Pipe {
+	p.mu.Lock()
+	defer p.mu.Unlock()
 	p.stderr = w
 	return p
 }

script_test.go

@@ -1971,6 +1971,14 @@ func TestEncodeBase64_CorrectlyEncodesInputBytes(t *testing.T) {
 	}
 }
 
+func TestWithStdErr_IsConcurrencySafeAfterExec(t *testing.T) {
+	t.Parallel()
+	err := script.Exec("echo").WithStderr(nil).Wait()
+	if err != nil {
+		t.Fatal(err)
+	}
+}
+
 func ExampleArgs() {
 	script.Args().Stdout()
 	// prints command-line arguments