metadata: Add Get, Set, and Append methods to metadata.MD (#1940)

metadata/metadata.go

@@ -95,6 +95,30 @@ func (md MD) Copy() MD {
 	return Join(md)
 }
 
+// Get obtains the values for a given key.
+func (md MD) Get(k string) []string {
+	k = strings.ToLower(k)
+	return md[k]
+}
+
+// Set sets the value of a given key with a slice of values.
+func (md MD) Set(k string, vals ...string) {
+	if len(vals) == 0 {
+		return
+	}
+	k = strings.ToLower(k)
+	md[k] = vals
+}
+
+// Append adds the values to key k, not overwriting what was already stored at that key.
+func (md MD) Append(k string, vals ...string) {
+	if len(vals) == 0 {
+		return
+	}
+	k = strings.ToLower(k)
+	md[k] = append(md[k], vals...)
+}
+
 // Join joins any number of mds into a single MD.
 // The order of values for each key is determined by the order in which
 // the mds containing those values are presented to Join.

metadata/metadata_test.go

@@ -73,6 +73,90 @@ func TestJoin(t *testing.T) {
 	}
 }
 
+func TestGet(t *testing.T) {
+	for _, test := range []struct {
+		md       MD
+		key      string
+		wantVals []string
+	}{
+		{md: Pairs("My-Optional-Header", "42"), key: "My-Optional-Header", wantVals: []string{"42"}},
+		{md: Pairs("Header", "42", "Header", "43", "Header", "44", "other", "1"), key: "HEADER", wantVals: []string{"42", "43", "44"}},
+		{md: Pairs("HEADER", "10"), key: "HEADER", wantVals: []string{"10"}},
+	} {
+		vals := test.md.Get(test.key)
+		if !reflect.DeepEqual(vals, test.wantVals) {
+			t.Errorf("value of metadata %v is %v, want %v", test.key, vals, test.wantVals)
+		}
+	}
+}
+
+func TestSet(t *testing.T) {
+	for _, test := range []struct {
+		md      MD
+		setKey  string
+		setVals []string
+		want    MD
+	}{
+		{
+			md:      Pairs("My-Optional-Header", "42", "other-key", "999"),
+			setKey:  "Other-Key",
+			setVals: []string{"1"},
+			want:    Pairs("my-optional-header", "42", "other-key", "1"),
+		},
+		{
+			md:      Pairs("My-Optional-Header", "42"),
+			setKey:  "Other-Key",
+			setVals: []string{"1", "2", "3"},
+			want:    Pairs("my-optional-header", "42", "other-key", "1", "other-key", "2", "other-key", "3"),
+		},
+		{
+			md:      Pairs("My-Optional-Header", "42"),
+			setKey:  "Other-Key",
+			setVals: []string{},
+			want:    Pairs("my-optional-header", "42"),
+		},
+	} {
+		test.md.Set(test.setKey, test.setVals...)
+		if !reflect.DeepEqual(test.md, test.want) {
+			t.Errorf("value of metadata is %v, want %v", test.md, test.want)
+		}
+	}
+}
+
+func TestAppend(t *testing.T) {
+	for _, test := range []struct {
+		md         MD
+		appendKey  string
+		appendVals []string
+		want       MD
+	}{
+		{
+			md:         Pairs("My-Optional-Header", "42"),
+			appendKey:  "Other-Key",
+			appendVals: []string{"1"},
+			want:       Pairs("my-optional-header", "42", "other-key", "1"),
+		},
+		{
+			md:         Pairs("My-Optional-Header", "42"),
+			appendKey:  "my-OptIoNal-HeAder",
+			appendVals: []string{"1", "2", "3"},
+			want: Pairs("my-optional-header", "42", "my-optional-header", "1",
+				"my-optional-header", "2", "my-optional-header", "3"),
+		},
+		{
+			md:         Pairs("My-Optional-Header", "42"),
+			appendKey:  "my-OptIoNal-HeAder",
+			appendVals: []string{},
+			want:       Pairs("my-optional-header", "42"),
+		},
+	} {
+		test.md.Append(test.appendKey, test.appendVals...)
+		if !reflect.DeepEqual(test.md, test.want) {
+			t.Errorf("value of metadata is %v, want %v", test.md, test.want)
+		}
+	}
+}
+
 func TestAppendToOutgoingContext(t *testing.T) {
 	// Pre-existing metadata
 	ctx := NewOutgoingContext(context.Background(), Pairs("k1", "v1", "k2", "v2"))