Fixed race condition at the edge of builds. Using queue channel.

The last loop collecting the remaining objectFiles may be run before the
last jobs completes.
This commit replaces the two channels used to fill objectFiles and to
signal error with direct variable access guarded by mutex, this avoids
race conditions at the end and streamlines the whole process.

Also added a 'queue' channel to feed the goroutines, this is not strictly
part of the fix, but helps to fairly distribute the workload.

legacy/builder/builder_utils/utils.go

@@ -169,57 +169,57 @@ func compileFilesWithRecipe(ctx *types.Context, sourcePath *paths.Path, sources
 	if len(sources) == 0 {
 		return objectFiles, nil
 	}
-	objectFilesChan := make(chan *paths.Path)
-	errorsChan := make(chan error)
-	doneChan := make(chan struct{})
+	var objectFilesMux sync.Mutex
+	var errors []error
+	var errorsMux sync.Mutex
 
 	ctx.Progress.Steps = ctx.Progress.Steps / float64(len(sources))
-	var wg sync.WaitGroup
 
-	// Split jobs into batches of N jobs each; wait for the completion of a batch to start the next
-	par := ctx.Jobs
-
-	go func() {
-		for total := 0; total < len(sources); total += par {
-			for i := total; i < total+par && i < len(sources); i++ {
-				wg.Add(1)
-				go func(source *paths.Path) {
-					defer wg.Done()
-					PrintProgressIfProgressEnabledAndMachineLogger(ctx)
-					objectFile, err := compileFileWithRecipe(ctx, sourcePath, source, buildPath, buildProperties, includes, recipe)
-					if err != nil {
-						errorsChan <- err
-					} else {
-						objectFilesChan <- objectFile
-					}
-				}(sources[i])
-			}
-			wg.Wait()
+	queue := make(chan *paths.Path)
+	job := func(source *paths.Path) {
+		PrintProgressIfProgressEnabledAndMachineLogger(ctx)
+		objectFile, err := compileFileWithRecipe(ctx, sourcePath, source, buildPath, buildProperties, includes, recipe)
+		if err != nil {
+			errorsMux.Lock()
+			errors = append(errors, err)
+			errorsMux.Unlock()
+		} else {
+			objectFilesMux.Lock()
+			objectFiles.Add(objectFile)
+			objectFilesMux.Unlock()
 		}
+	}
 
-		doneChan <- struct{}{}
-	}()
-
-	go func() {
-		wg.Wait()
-		doneChan <- struct{}{}
-	}()
-
-	for {
-		select {
-		case objectFile := <-objectFilesChan:
-			objectFiles.Add(objectFile)
-		case err := <-errorsChan:
-			return nil, i18n.WrapError(err)
-		case <-doneChan:
-			close(objectFilesChan)
-			for objectFile := range objectFilesChan {
-				objectFiles.Add(objectFile)
+	// Spawn jobs runners
+	var wg sync.WaitGroup
+	for i := 0; i < ctx.Jobs; i++ {
+		wg.Add(1)
+		go func() {
+			for source := range queue {
+				job(source)
 			}
-			objectFiles.Sort()
-			return objectFiles, nil
+			wg.Done()
+		}()
+	}
+
+	// Feed jobs until error or done
+	for _, source := range sources {
+		errorsMux.Lock()
+		gotError := len(errors) > 0
+		errorsMux.Unlock()
+		if gotError {
+			break
 		}
+		queue <- source
+	}
+	close(queue)
+	wg.Wait()
+	if len(errors) > 0 {
+		// output the first error
+		return nil, i18n.WrapError(errors[0])
 	}
+	objectFiles.Sort()
+	return objectFiles, nil
 }
 
 func compileFileWithRecipe(ctx *types.Context, sourcePath *paths.Path, source *paths.Path, buildPath *paths.Path, buildProperties *properties.Map, includes []string, recipe string) (*paths.Path, error) {