mime: handling invalid mime media parameters

Sometimes it's necessary to deal with emails that do not follow the specification; in particular, it's possible to download such email via gmail.
When the existing implementation handle invalid mime media parameters, it returns nils and error, although there is a valid media type, which may be returned.
If this behavior changes, it may not affect any existing programs, but it will help to parse some emails.

Fixes #19498

Change-Id: Ieb2fdbddfd93857faee941d2aa49d59e286d57fd
Reviewed-on: https://go-review.googlesource.com/38190Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/mime/mediatype.go

@@ -94,11 +94,19 @@ func checkMediaTypeDisposition(s string) error {
 	return nil
 }
 
+// ErrInvalidMediaParameter is returned by ParseMediaType if
+// the media type value was found but there was an error parsing
+// the optional parameters
+var ErrInvalidMediaParameter = errors.New("mime: invalid media parameter")
+
 // ParseMediaType parses a media type value and any optional
 // parameters, per RFC 1521.  Media types are the values in
 // Content-Type and Content-Disposition headers (RFC 2183).
 // On success, ParseMediaType returns the media type converted
 // to lowercase and trimmed of white space and a non-nil map.
+// If there is an error parsing the optional parameter,
+// the media type will be returned along with the error
+// ErrInvalidMediaParameter.
 // The returned map, params, maps from the lowercase
 // attribute to the attribute value with its case preserved.
 func ParseMediaType(v string) (mediatype string, params map[string]string, err error) {
@@ -134,7 +142,7 @@ func ParseMediaType(v string) (mediatype string, params map[string]string, err e
 				return
 			}
 			// Parse error.
-			return "", nil, errors.New("mime: invalid media parameter")
+			return mediatype, nil, ErrInvalidMediaParameter
 		}
 
 		pmap := params

src/mime/mediatype_test.go

@@ -253,13 +253,18 @@ func TestParseMediaType(t *testing.T) {
 
 type badMediaTypeTest struct {
 	in  string
+	mt  string
 	err string
 }
 
 var badMediaTypeTests = []badMediaTypeTest{
-	{"bogus ;=========", "mime: invalid media parameter"},
-	{"bogus/<script>alert</script>", "mime: expected token after slash"},
-	{"bogus/bogus<script>alert</script>", "mime: unexpected content after media subtype"},
+	{"bogus ;=========", "bogus", "mime: invalid media parameter"},
+	// The following example is from real email delivered by gmail (error: missing semicolon)
+	// and it is there to check behavior described in #19498
+	{"application/pdf; x-mac-type=\"3F3F3F3F\"; x-mac-creator=\"3F3F3F3F\" name=\"a.pdf\";",
+		"application/pdf", "mime: invalid media parameter"},
+	{"bogus/<script>alert</script>", "", "mime: expected token after slash"},
+	{"bogus/bogus<script>alert</script>", "", "mime: unexpected content after media subtype"},
 }
 
 func TestParseMediaTypeBogus(t *testing.T) {
@@ -275,8 +280,11 @@ func TestParseMediaTypeBogus(t *testing.T) {
 		if params != nil {
 			t.Errorf("ParseMediaType(%q): got non-nil params on error", tt.in)
 		}
-		if mt != "" {
-			t.Errorf("ParseMediaType(%q): got non-empty media type string on error", tt.in)
+		if err != ErrInvalidMediaParameter && mt != "" {
+			t.Errorf("ParseMediaType(%q): got unexpected non-empty media type string", tt.in)
+		}
+		if err == ErrInvalidMediaParameter && mt != tt.mt {
+			t.Errorf("ParseMediaType(%q): in case of invalid parameters: expected type %q, got %q", tt.in, tt.mt, mt)
 		}
 	}
 }