net/http: in ServeContent, don't seek on content until necessary

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/11080043

src/pkg/net/http/fs.go

@@ -105,23 +105,31 @@ func dirList(w ResponseWriter, f File) {
 //
 // Note that *os.File implements the io.ReadSeeker interface.
 func ServeContent(w ResponseWriter, req *Request, name string, modtime time.Time, content io.ReadSeeker) {
-	size, err := content.Seek(0, os.SEEK_END)
-	if err != nil {
-		Error(w, "seeker can't seek", StatusInternalServerError)
-		return
-	}
-	_, err = content.Seek(0, os.SEEK_SET)
-	if err != nil {
-		Error(w, "seeker can't seek", StatusInternalServerError)
-		return
+	sizeFunc := func() (int64, error) {
+		size, err := content.Seek(0, os.SEEK_END)
+		if err != nil {
+			return 0, errSeeker
+		}
+		_, err = content.Seek(0, os.SEEK_SET)
+		if err != nil {
+			return 0, errSeeker
+		}
+		return size, nil
 	}
-	serveContent(w, req, name, modtime, size, content)
+	serveContent(w, req, name, modtime, sizeFunc, content)
 }
 
+// errSeeker is returned by ServeContent's sizeFunc when the content
+// doesn't seek properly. The underlying Seeker's error text isn't
+// included in the sizeFunc reply so it's not sent over HTTP to end
+// users.
+var errSeeker = errors.New("seeker can't seek")
+
 // if name is empty, filename is unknown. (used for mime type, before sniffing)
 // if modtime.IsZero(), modtime is unknown.
 // content must be seeked to the beginning of the file.
-func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time, size int64, content io.ReadSeeker) {
+// The sizeFunc is called at most once. Its error, if any, is sent in the HTTP response.
+func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time, sizeFunc func() (int64, error), content io.ReadSeeker) {
 	if checkLastModified(w, r, modtime) {
 		return
 	}
@@ -151,6 +159,12 @@ func serveContent(w ResponseWriter, r *Request, name string, modtime time.Time,
 		w.Header().Set("Content-Type", ctype)
 	}
 
+	size, err := sizeFunc()
+	if err != nil {
+		Error(w, err.Error(), StatusInternalServerError)
+		return
+	}
+
 	// handle Content-Range header.
 	sendSize := size
 	var sendContent io.Reader = content
@@ -378,7 +392,8 @@ func serveFile(w ResponseWriter, r *Request, fs FileSystem, name string, redirec
 	}
 
 	// serverContent will check modification time
-	serveContent(w, r, d.Name(), d.ModTime(), d.Size(), f)
+	sizeFunc := func() (int64, error) { return d.Size(), nil }
+	serveContent(w, r, d.Name(), d.ModTime(), sizeFunc, f)
 }
 
 // localRedirect gives a Moved Permanently response.

src/pkg/net/http/fs_test.go

@@ -567,7 +567,10 @@ func TestServeContent(t *testing.T) {
 	defer ts.Close()
 
 	type testCase struct {
-		file             string
+		// One of file or content must be set:
+		file    string
+		content io.ReadSeeker
+
 		modtime          time.Time
 		serveETag        string // optional
 		serveContentType string // optional
@@ -615,6 +618,14 @@ func TestServeContent(t *testing.T) {
 			},
 			wantStatus: 304,
 		},
+		"not_modified_etag_no_seek": {
+			content:   panicOnSeek{nil}, // should never be called
+			serveETag: `"foo"`,
+			reqHeader: map[string]string{
+				"If-None-Match": `"foo"`,
+			},
+			wantStatus: 304,
+		},
 		"range_good": {
 			file:      "testdata/style.css",
 			serveETag: `"A"`,
@@ -638,15 +649,21 @@ func TestServeContent(t *testing.T) {
 		},
 	}
 	for testName, tt := range tests {
-		f, err := os.Open(tt.file)
-		if err != nil {
-			t.Fatalf("test %q: %v", testName, err)
+		var content io.ReadSeeker
+		if tt.file != "" {
+			f, err := os.Open(tt.file)
+			if err != nil {
+				t.Fatalf("test %q: %v", testName, err)
+			}
+			defer f.Close()
+			content = f
+		} else {
+			content = tt.content
 		}
-		defer f.Close()
 
 		servec <- serveParam{
 			name:        filepath.Base(tt.file),
-			content:     f,
+			content:     content,
 			modtime:     tt.modtime,
 			etag:        tt.serveETag,
 			contentType: tt.serveContentType,
@@ -763,3 +780,5 @@ func TestLinuxSendfileChild(*testing.T) {
 		panic(err)
 	}
 }
+
+type panicOnSeek struct{ io.ReadSeeker }