net/http: add/update mp4 sniffing

Completes sniffing for mp4 signature according
to the spec at:
  https://mimesniff.spec.whatwg.org/#signature-for-mp4
Clause 6.2.1

Fixes #8773

Change-Id: Icfc4a23324ae249db52c94a21c0e8509e1833e19
Reviewed-on: https://go-review.googlesource.com/16951Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/sniff.go

@@ -102,10 +102,9 @@ var sniffSignatures = []sniffSig{
 	&exactSig{[]byte("\x50\x4B\x03\x04"), "application/zip"},
 	&exactSig{[]byte("\x1F\x8B\x08"), "application/x-gzip"},
 
-	// TODO(dsymonds): Re-enable this when the spec is sorted w.r.t. MP4.
-	//mp4Sig(0),
+	mp4Sig{},
 
-	textSig(0), // should be last
+	textSig{}, // should be last
 }
 
 type exactSig struct {
@@ -166,12 +165,14 @@ func (h htmlSig) match(data []byte, firstNonWS int) string {
 }
 
 var mp4ftype = []byte("ftyp")
+var mp4 = []byte("mp4")
 
-type mp4Sig int
+type mp4Sig struct{}
 
 func (mp4Sig) match(data []byte, firstNonWS int) string {
-	// c.f. section 6.1.
-	if len(data) < 8 {
+	// https://mimesniff.spec.whatwg.org/#signature-for-mp4
+	// c.f. section 6.2.1
+	if len(data) < 12 {
 		return ""
 	}
 	boxSize := int(binary.BigEndian.Uint32(data[:4]))
@@ -186,24 +187,14 @@ func (mp4Sig) match(data []byte, firstNonWS int) string {
 			// minor version number
 			continue
 		}
-		seg := string(data[st : st+3])
-		switch seg {
-		case "mp4", "iso", "M4V", "M4P", "M4B":
+		if bytes.Equal(data[st:st+3], mp4) {
 			return "video/mp4"
-			/* The remainder are not in the spec.
-			case "M4A":
-				return "audio/mp4"
-			case "3gp":
-				return "video/3gpp"
-			case "jp2":
-				return "image/jp2" // JPEG 2000
-			*/
 		}
 	}
 	return ""
 }
 
-type textSig int
+type textSig struct{}
 
 func (textSig) match(data []byte, firstNonWS int) string {
 	// c.f. section 5, step 4.

src/net/http/sniff_test.go

@@ -40,9 +40,7 @@ var sniffTests = []struct {
 	{"GIF 87a", []byte(`GIF87a`), "image/gif"},
 	{"GIF 89a", []byte(`GIF89a...`), "image/gif"},
 
-	// TODO(dsymonds): Re-enable this when the spec is sorted w.r.t. MP4.
-	//{"MP4 video", []byte("\x00\x00\x00\x18ftypmp42\x00\x00\x00\x00mp42isom<\x06t\xbfmdat"), "video/mp4"},
-	//{"MP4 audio", []byte("\x00\x00\x00\x20ftypM4A \x00\x00\x00\x00M4A mp42isom\x00\x00\x00\x00"), "audio/mp4"},
+	{"MP4 video", []byte("\x00\x00\x00\x18ftypmp42\x00\x00\x00\x00mp42isom<\x06t\xbfmdat"), "video/mp4"},
 }
 
 func TestDetectContentType(t *testing.T) {