Put multipart rewrite work in functions

internal/upload/rewrite.go

@@ -37,6 +37,13 @@ var (
 	)
 )
 
+type rewriter struct {
+	writer      *multipart.Writer
+	tempPath    string
+	filter      MultipartFormProcessor
+	directories []string
+}
+
 func init() {
 	prometheus.MustRegister(multipartUploadRequests)
 	prometheus.MustRegister(multipartFileUploadBytes)
@@ -56,10 +63,14 @@ func rewriteFormFilesFromMultipart(r *http.Request, writer *multipart.Writer, te
 
 	multipartUploadRequests.Inc()
 
-	var directories []string
+	rew := &rewriter{
+		writer:   writer,
+		tempPath: tempPath,
+		filter:   filter,
+	}
 
 	cleanup = func() {
-		for _, dir := range directories {
+		for _, dir := range rew.directories {
 			os.RemoveAll(dir)
 		}
 	}
@@ -83,60 +94,77 @@ func rewriteFormFilesFromMultipart(r *http.Request, writer *multipart.Writer, te
 		}
 
 		// Copy form field
-		if filename := p.FileName(); filename != "" {
-			multipartFiles.Inc()
-
-			if strings.Contains(filename, "/") || filename == "." || filename == ".." {
-				return cleanup, fmt.Errorf("illegal filename: %q", filename)
-			}
-
-			// Create temporary directory where the uploaded file will be stored
-			if err := os.MkdirAll(tempPath, 0700); err != nil {
-				return cleanup, fmt.Errorf("mkdir for tempfile: %v", err)
-			}
-
-			tempDir, err := ioutil.TempDir(tempPath, "multipart-")
-			if err != nil {
-				return cleanup, fmt.Errorf("create tempdir: %v", err)
-			}
-			directories = append(directories, tempDir)
-
-			file, err := os.OpenFile(path.Join(tempDir, filename), os.O_WRONLY|os.O_CREATE, 0600)
-			if err != nil {
-				return cleanup, fmt.Errorf("rewriteFormFilesFromMultipart: temp file: %v", err)
-			}
-			defer file.Close()
-
-			// Add file entry
-			writer.WriteField(name+".path", file.Name())
-			writer.WriteField(name+".name", filename)
-
-			written, err := io.Copy(file, p)
-			if err != nil {
-				return cleanup, fmt.Errorf("copy from multipart to tempfile: %v", err)
-			}
-			multipartFileUploadBytes.Add(float64(written))
-
-			file.Close()
-
-			if err := filter.ProcessFile(name, file.Name(), writer); err != nil {
-				return cleanup, err
-			}
+		if p.FileName() != "" {
+			err = rew.handleFilePart(name, p)
+
 		} else {
-			np, err := writer.CreatePart(p.Header)
-			if err != nil {
-				return cleanup, fmt.Errorf("create multipart field: %v", err)
-			}
-
-			_, err = io.Copy(np, p)
-			if err != nil {
-				return cleanup, fmt.Errorf("duplicate multipart field: %v", err)
-			}
-
-			if err := filter.ProcessField(name, writer); err != nil {
-				return cleanup, fmt.Errorf("process multipart field: %v", err)
-			}
+			err = rew.copyPart(name, p)
+		}
+
+		if err != nil {
+			return cleanup, err
 		}
 	}
 	return cleanup, nil
 }
+
+func (rew *rewriter) handleFilePart(name string, p *multipart.Part) error {
+	multipartFiles.Inc()
+	filename := p.FileName()
+
+	if strings.Contains(filename, "/") || filename == "." || filename == ".." {
+		return fmt.Errorf("illegal filename: %q", filename)
+	}
+
+	// Create temporary directory where the uploaded file will be stored
+	if err := os.MkdirAll(rew.tempPath, 0700); err != nil {
+		return fmt.Errorf("mkdir for tempfile: %v", err)
+	}
+
+	tempDir, err := ioutil.TempDir(rew.tempPath, "multipart-")
+	if err != nil {
+		return fmt.Errorf("create tempdir: %v", err)
+	}
+	rew.directories = append(rew.directories, tempDir)
+
+	file, err := os.OpenFile(path.Join(tempDir, filename), os.O_WRONLY|os.O_CREATE, 0600)
+	if err != nil {
+		return fmt.Errorf("rewriteFormFilesFromMultipart: temp file: %v", err)
+	}
+	defer file.Close()
+
+	// Add file entry
+	rew.writer.WriteField(name+".path", file.Name())
+	rew.writer.WriteField(name+".name", filename)
+
+	written, err := io.Copy(file, p)
+	if err != nil {
+		return fmt.Errorf("copy from multipart to tempfile: %v", err)
+	}
+	multipartFileUploadBytes.Add(float64(written))
+
+	file.Close()
+
+	if err := rew.filter.ProcessFile(name, file.Name(), rew.writer); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (rew *rewriter) copyPart(name string, p *multipart.Part) error {
+	np, err := rew.writer.CreatePart(p.Header)
+	if err != nil {
+		return fmt.Errorf("create multipart field: %v", err)
+	}
+
+	if _, err := io.Copy(np, p); err != nil {
+		return fmt.Errorf("duplicate multipart field: %v", err)
+	}
+
+	if err := rew.filter.ProcessField(name, rew.writer); err != nil {
+		return fmt.Errorf("process multipart field: %v", err)
+	}
+
+	return nil
+}