[skip-changelog] Fixed 'compile --preprocess' output (#2339)

arduino/builder/builder.go

@@ -243,10 +243,17 @@ func (b *Builder) ImportedLibraries() libraries.List {
 }
 
 // Preprocess fixdoc
-func (b *Builder) Preprocess() error {
+func (b *Builder) Preprocess() ([]byte, error) {
 	b.Progress.AddSubSteps(6)
 	defer b.Progress.RemoveSubSteps()
-	return b.preprocess()
+
+	if err := b.preprocess(); err != nil {
+		return nil, err
+	}
+
+	// Return arduino-preprocessed source
+	preprocessedSketch, err := b.sketchBuildPath.Join(b.sketch.MainFile.Base() + ".cpp").ReadFile()
+	return preprocessedSketch, err
 }
 
 func (b *Builder) preprocess() error {
@@ -303,13 +310,6 @@ func (b *Builder) preprocess() error {
 	b.Progress.CompleteStep()
 	b.Progress.PushProgress()
 
-	// Output arduino-preprocessed source
-	preprocessedSketch, err := b.sketchBuildPath.Join(b.sketch.MainFile.Base() + ".cpp").ReadFile()
-	if err != nil {
-		return err
-	}
-	b.logger.WriteStdout(preprocessedSketch)
-
 	return nil
 }
 

commands/compile/compile.go

@@ -240,11 +240,13 @@ func Compile(ctx context.Context, req *rpc.CompileRequest, outStream, errStream
 
 	if req.GetPreprocess() {
 		// Just output preprocessed source code and exit
-		compileErr := sketchBuilder.Preprocess()
-		if compileErr != nil {
-			compileErr = &arduino.CompileFailedError{Message: compileErr.Error()}
+		preprocessedSketch, err := sketchBuilder.Preprocess()
+		if err != nil {
+			err = &arduino.CompileFailedError{Message: err.Error()}
+			return r, err
 		}
-		return r, compileErr
+		_, err = outStream.Write(preprocessedSketch)
+		return r, err
 	}
 
 	defer func() {