Transfer completed Go output without copies

internal/gotool/gotool.go

@@ -105,7 +105,7 @@ func PrepareBuildDependencies(ctx context.Context, directory string) error {
 		Stderr: captured,
 	})
 	if err != nil {
-		if output := strings.TrimSpace(string(captured.Bytes())); output != "" {
+		if output := strings.TrimSpace(string(captured.take())); output != "" {
 			return fmt.Errorf("go mod download: %w: %s", err, output)
 		}
 		return fmt.Errorf("go mod download: %w", err)
@@ -121,8 +121,19 @@ func (tool *Tool) HostTarget() Target {
 // Run executes a Go command with ReleaseKit's shared offline baseline. Native
 // commands use the host target and effective cgo configuration. Static commands
 // use target and disable cgo. Output, when supplied, receives stdout and stderr
-// as the command runs; the returned bytes retain both streams for diagnostics.
-func (tool *Tool) Run(ctx context.Context, directory string, mode Mode, target Target, output io.Writer, args ...string) ([]byte, error) {
+// as the command runs.
+func (tool *Tool) Run(ctx context.Context, directory string, mode Mode, target Target, output io.Writer, args ...string) error {
+	_, err := tool.execute(ctx, directory, mode, target, output, false, args...)
+	return err
+}
+
+// Output executes a Go command and returns its combined stdout and stderr.
+// Output, when supplied, also receives the bytes as the command runs.
+func (tool *Tool) Output(ctx context.Context, directory string, mode Mode, target Target, output io.Writer, args ...string) ([]byte, error) {
+	return tool.execute(ctx, directory, mode, target, output, true, args...)
+}
+
+func (tool *Tool) execute(ctx context.Context, directory string, mode Mode, target Target, output io.Writer, keepOutput bool, args ...string) ([]byte, error) {
 	if len(args) == 0 {
 		return nil, errors.New("Go command requires arguments")
 	}
@@ -136,14 +147,24 @@ func (tool *Tool) Run(ctx context.Context, directory string, mode Mode, target T
 		Stdout: captured,
 		Stderr: captured,
 	})
-	contents := captured.Bytes()
+	if forwardErr := captured.forwardingError(); forwardErr != nil {
+		err = errors.Join(err, fmt.Errorf("forward Go output: %w", forwardErr))
+	}
+	contents := captured.take()
 	if err != nil {
 		if len(contents) != 0 {
-			return contents, fmt.Errorf("go %s: %w: %s", strings.Join(args, " "), err, strings.TrimSpace(string(contents)))
+			return outputBytes(contents, keepOutput), fmt.Errorf("go %s: %w: %s", strings.Join(args, " "), err, strings.TrimSpace(string(contents)))
 		}
-		return contents, fmt.Errorf("go %s: %w", strings.Join(args, " "), err)
+		return outputBytes(contents, keepOutput), fmt.Errorf("go %s: %w", strings.Join(args, " "), err)
 	}
-	return contents, nil
+	return outputBytes(contents, keepOutput), nil
+}
+
+func outputBytes(contents []byte, keep bool) []byte {
+	if keep {
+		return contents
+	}
+	return nil
 }
 
 func (tool *Tool) environment(mode Mode, target Target) map[string]string {
@@ -206,9 +227,10 @@ func discover(ctx context.Context, directory string, environment map[string]stri
 }
 
 type combinedWriter struct {
-	mu     sync.Mutex
-	output io.Writer
-	buffer bytes.Buffer
+	mu         sync.Mutex
+	output     io.Writer
+	buffer     bytes.Buffer
+	forwardErr error
 }
 
 func (writer *combinedWriter) Write(contents []byte) (int, error) {
@@ -217,11 +239,29 @@ func (writer *combinedWriter) Write(contents []byte) (int, error) {
 	if writer.output == nil {
 		return writer.buffer.Write(contents)
 	}
-	return io.MultiWriter(&writer.buffer, writer.output).Write(contents)
+	if _, err := writer.buffer.Write(contents); err != nil {
+		return 0, err
+	}
+	written, err := writer.output.Write(contents)
+	if err == nil && written != len(contents) {
+		err = io.ErrShortWrite
+	}
+	if err != nil && writer.forwardErr == nil {
+		writer.forwardErr = err
+	}
+	return written, err
 }
 
-func (writer *combinedWriter) Bytes() []byte {
+func (writer *combinedWriter) forwardingError() error {
 	writer.mu.Lock()
 	defer writer.mu.Unlock()
-	return append([]byte(nil), writer.buffer.Bytes()...)
+	return writer.forwardErr
+}
+
+// take transfers the completed capture to the caller. It must only be used
+// after the child process has stopped writing.
+func (writer *combinedWriter) take() []byte {
+	writer.mu.Lock()
+	defer writer.mu.Unlock()
+	return writer.buffer.Bytes()
 }