mime: make FormatMediaType take full type for consistency

Fixes #2405

R=rsc
CC=golang-dev
https://golang.org/cl/5539048

src/pkg/mime/mediatype.go

@@ -12,17 +12,22 @@ import (
 	"unicode"
 )
 
-// FormatMediaType serializes type t, subtype sub and the paramaters
-// param as a media type conform RFC 2045 and RFC 2616.
-// The type, subtype, and parameter names are written in lower-case.
+// FormatMediaType serializes mediatype t and the parameters
+// param as a media type conforming to RFC 2045 and RFC 2616.
+// The type and parameter names are written in lower-case.
 // When any of the arguments result in a standard violation then
 // FormatMediaType returns the empty string.
-func FormatMediaType(t, sub string, param map[string]string) string {
-	if !(IsToken(t) && IsToken(sub)) {
+func FormatMediaType(t string, param map[string]string) string {
+	slash := strings.Index(t, "/")
+	if slash == -1 {
+		return ""
+	}
+	major, sub := t[:slash], t[slash+1:]
+	if !IsToken(major) || !IsToken(sub) {
 		return ""
 	}
 	var b bytes.Buffer
-	b.WriteString(strings.ToLower(t))
+	b.WriteString(strings.ToLower(major))
 	b.WriteByte('/')
 	b.WriteString(strings.ToLower(sub))
 

src/pkg/mime/mediatype_test.go

@@ -253,3 +253,24 @@ func TestParseMediaTypeBogus(t *testing.T) {
 		t.Errorf("expected invalid media parameter; got error %q", err)
 	}
 }
+
+type formatTest struct {
+	typ    string
+	params map[string]string
+	want   string
+}
+
+var formatTests = []formatTest{
+	{"noslash", nil, ""},
+	{"foo/BAR", nil, "foo/bar"},
+	{"foo/BAR", map[string]string{"X": "Y"}, "foo/bar; x=Y"},
+}
+
+func TestFormatMediaType(t *testing.T) {
+	for i, tt := range formatTests {
+		got := FormatMediaType(tt.typ, tt.params)
+		if got != tt.want {
+			t.Errorf("%d. FormatMediaType(%q, %v) = %q; want %q", i, tt.typ, tt.params, got, tt.want)
+		}
+	}
+}

src/pkg/mime/type.go

@@ -62,21 +62,14 @@ func AddExtensionType(ext, typ string) error {
 }
 
 func setExtensionType(extension, mimeType string) error {
-	full, param, err := ParseMediaType(mimeType)
+	_, param, err := ParseMediaType(mimeType)
 	if err != nil {
 		return err
 	}
-	if split := strings.Index(full, "/"); split < 0 {
-		return fmt.Errorf(`mime: malformed MIME type "%s"`, mimeType)
-	} else {
-		main := full[:split]
-		sub := full[split+1:]
-		if main == "text" && param["charset"] == "" {
-			param["charset"] = "utf-8"
-		}
-		mimeType = FormatMediaType(main, sub, param)
+	if strings.HasPrefix(mimeType, "text/") && param["charset"] == "" {
+		param["charset"] = "utf-8"
+		mimeType = FormatMediaType(mimeType, param)
 	}
-
 	mimeLock.Lock()
 	mimeTypes[extension] = mimeType
 	mimeLock.Unlock()