Implement common file server to return consistent Content-Type (#191)

* Implement common file server to return consistent Content-Type

When `/etc/mime.types` has a unusual mime type, Content-Type of response becomes the type and you may get unexpected result.
This server returns consistent Content-Type header for js and css files

Signed-off-by: mrasu <m.rasu.hitsuji@gmail.com>

README.md

@@ -9,4 +9,5 @@ components and libraries.
 * **log**: A logging wrapper around [logrus](https://github.com/sirupsen/logrus)
 * **model**: Shared data structures
 * **route**: A routing wrapper around [httprouter](https://github.com/julienschmidt/httprouter) using `context.Context`
+* **server**: Common servers
 * **version**: Version information and metrics

server/static_file_server.go

@@ -0,0 +1,41 @@
+// Copyright 2019 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package server
+
+import (
+	"net/http"
+	"path/filepath"
+)
+
+var mimeTypes = map[string]string{
+	".js":  "application/javascript",
+	".css": "text/css",
+	".png": "image/png",
+	".jpg": "image/jpeg",
+	".gif": "image/gif",
+}
+
+func StaticFileServer(root http.FileSystem) http.Handler {
+	return http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			fileExt := filepath.Ext(r.URL.Path)
+
+			if t, ok := mimeTypes[fileExt]; ok {
+				w.Header().Set("Content-Type", t)
+			}
+
+			http.FileServer(root).ServeHTTP(w, r)
+		},
+	)
+}

server/static_file_server_test.go

@@ -0,0 +1,83 @@
+// Copyright 2019 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package server
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+type dummyFileSystem struct{}
+
+func (fs dummyFileSystem) Open(path string) (http.File, error) {
+	return http.Dir(".").Open(".")
+}
+
+func TestServeHttp(t *testing.T) {
+	cases := []struct {
+		name        string
+		path        string
+		contentType string
+	}{
+		{
+			name:        "normal file",
+			path:        "index.html",
+			contentType: "",
+		},
+		{
+			name:        "javascript",
+			path:        "test.js",
+			contentType: "application/javascript",
+		},
+		{
+			name:        "css",
+			path:        "test.css",
+			contentType: "text/css",
+		},
+		{
+			name:        "png",
+			path:        "test.png",
+			contentType: "image/png",
+		},
+		{
+			name:        "jpg",
+			path:        "test.jpg",
+			contentType: "image/jpeg",
+		},
+		{
+			name:        "gif",
+			path:        "test.gif",
+			contentType: "image/gif",
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			rr := httptest.NewRecorder()
+			req, err := http.NewRequest("GET", "http://localhost/"+c.path, nil)
+
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			s := StaticFileServer(dummyFileSystem{})
+			s.ServeHTTP(rr, req)
+
+			if rr.Header().Get("Content-Type") != c.contentType {
+				t.Fatalf("Unexpected Content-Type: %s", rr.Header().Get("Content-Type"))
+			}
+		})
+	}
+}