http: implement MP4 sniffing.

R=rsc
CC=golang-dev
https://golang.org/cl/4809044

src/pkg/http/sniff.go

@@ -6,6 +6,7 @@ package http
 
 import (
 	"bytes"
+	"encoding/binary"
 )
 
 // Content-type sniffing algorithm.
@@ -97,7 +98,7 @@ var sniffSignatures = []sniffSig{
 	&exactSig{[]byte("\x50\x4B\x03\x04"), "application/zip"},
 	&exactSig{[]byte("\x1F\x8B\x08"), "application/x-gzip"},
 
-	// TODO(dsymonds): MP4.
+	mp4Sig(0),
 
 	textSig(0), // should be last
 }
@@ -159,6 +160,32 @@ func (h htmlSig) match(data []byte, firstNonWS int) string {
 	return "text/html; charset=utf-8"
 }
 
+type mp4Sig int
+
+func (mp4Sig) match(data []byte, firstNonWS int) string {
+	// c.f. section 6.1.
+	if len(data) < 8 {
+		return ""
+	}
+	boxSize := int(binary.BigEndian.Uint32(data[:4]))
+	if boxSize%4 != 0 || len(data) < boxSize {
+		return ""
+	}
+	if !bytes.Equal(data[4:8], []byte("ftyp")) {
+		return ""
+	}
+	for st := 8; st < boxSize; st += 4 {
+		if st == 12 {
+			// minor version number
+			continue
+		}
+		if bytes.Equal(data[st:st+3], []byte("mp4")) {
+			return "video/mp4"
+		}
+	}
+	return ""
+}
+
 type textSig int
 
 func (textSig) match(data []byte, firstNonWS int) string {

src/pkg/http/sniff_test.go

@@ -34,6 +34,8 @@ var sniffTests = []struct {
 	// Image types.
 	{"GIF 87a", []byte(`GIF87a`), "image/gif"},
 	{"GIF 89a", []byte(`GIF89a...`), "image/gif"},
+
+	{"MP4", []byte("\x00\x00\x00\x18ftypmp42\x00\x00\x00\x00mp42isom<\x06t\xbfmdat"), "video/mp4"},
 }
 
 func TestDetectContentType(t *testing.T) {