http: Add EncodeQuery, a handy helper function for constructing URL query strings.

R=stephenm, rsc
CC=golang-dev
https://golang.org/cl/2985042

src/pkg/http/client.go

@@ -199,20 +199,13 @@ func PostForm(url string, data map[string]string) (r *Response, err os.Error) {
 	return send(&req)
 }
 
+// TODO: remove this function when PostForm takes a multimap.
 func urlencode(data map[string]string) (b *bytes.Buffer) {
-	b = new(bytes.Buffer)
-	first := true
+	m := make(map[string][]string, len(data))
 	for k, v := range data {
-		if first {
-			first = false
-		} else {
-			b.WriteByte('&')
-		}
-		b.WriteString(URLEscape(k))
-		b.WriteByte('=')
-		b.WriteString(URLEscape(v))
+		m[k] = []string{v}
 	}
-	return
+	return bytes.NewBuffer([]byte(EncodeQuery(m)))
 }
 
 // Head issues a HEAD to the specified URL.

src/pkg/http/url.go

@@ -515,3 +515,15 @@ func (url *URL) String() string {
 	}
 	return result
 }
+
+// EncodeQuery encodes the query represented as a multimap.
+func EncodeQuery(m map[string][]string) string {
+	parts := make([]string, 0, len(m)) // will be large enough for most uses
+	for k, vs := range m {
+		prefix := URLEscape(k) + "="
+		for _, v := range vs {
+			parts = append(parts, prefix+URLEscape(v))
+		}
+	}
+	return strings.Join(parts, "&")
+}

src/pkg/http/url_test.go

@@ -507,3 +507,24 @@ func TestUnescapeUserinfo(t *testing.T) {
 		}
 	}
 }
+
+type qMap map[string][]string
+
+type EncodeQueryTest struct {
+	m        qMap
+	expected string
+}
+
+var encodeQueryTests = []EncodeQueryTest{
+	{nil, ""},
+	{qMap{"q": {"puppies"}, "oe": {"utf8"}}, "q=puppies&oe=utf8"},
+	{qMap{"q": {"dogs", "&", "7"}}, "q=dogs&q=%26&q=7"},
+}
+
+func TestEncodeQuery(t *testing.T) {
+	for _, tt := range encodeQueryTests {
+		if q := EncodeQuery(tt.m); q != tt.expected {
+			t.Errorf(`EncodeQuery(%+v) = %q, want %q`, tt.m, q, tt.expected)
+		}
+	}
+}