metadata: Use strings.EqualFold for ValueFromIncomingContext (#6743)

metadata/metadata.go

@@ -153,14 +153,16 @@ func Join(mds ...MD) MD {
 type mdIncomingKey struct{}
 type mdOutgoingKey struct{}
 
-// NewIncomingContext creates a new context with incoming md attached.
+// NewIncomingContext creates a new context with incoming md attached. md must
+// not be modified after calling this function.
 func NewIncomingContext(ctx context.Context, md MD) context.Context {
 	return context.WithValue(ctx, mdIncomingKey{}, md)
 }
 
 // NewOutgoingContext creates a new context with outgoing md attached. If used
 // in conjunction with AppendToOutgoingContext, NewOutgoingContext will
-// overwrite any previously-appended metadata.
+// overwrite any previously-appended metadata. md must not be modified after
+// calling this function.
 func NewOutgoingContext(ctx context.Context, md MD) context.Context {
 	return context.WithValue(ctx, mdOutgoingKey{}, rawMD{md: md})
 }
@@ -203,7 +205,8 @@ func FromIncomingContext(ctx context.Context) (MD, bool) {
 }
 
 // ValueFromIncomingContext returns the metadata value corresponding to the metadata
-// key from the incoming metadata if it exists. Key must be lower-case.
+// key from the incoming metadata if it exists. Keys are matched in a case insensitive
+// manner.
 //
 // # Experimental
 //
@@ -219,17 +222,16 @@ func ValueFromIncomingContext(ctx context.Context, key string) []string {
 		return copyOf(v)
 	}
 	for k, v := range md {
-		// We need to manually convert all keys to lower case, because MD is a
-		// map, and there's no guarantee that the MD attached to the context is
-		// created using our helper functions.
-		if strings.ToLower(k) == key {
+		// Case insenitive comparison: MD is a map, and there's no guarantee
+		// that the MD attached to the context is created using our helper
+		// functions.
+		if strings.EqualFold(k, key) {
 			return copyOf(v)
 		}
 	}
 	return nil
 }
 
-// the returned slice must not be modified in place
 func copyOf(v []string) []string {
 	vals := make([]string, len(v))
 	copy(vals, v)

metadata/metadata_test.go

@@ -198,13 +198,48 @@ func (s) TestDelete(t *testing.T) {
 	}
 }
 
+func (s) TestFromIncomingContext(t *testing.T) {
+	md := Pairs(
+		"X-My-Header-1", "42",
+	)
+	// Verify that we lowercase if callers directly modify md
+	md["X-INCORRECT-UPPERCASE"] = []string{"foo"}
+	ctx := NewIncomingContext(context.Background(), md)
+
+	result, found := FromIncomingContext(ctx)
+	if !found {
+		t.Fatal("FromIncomingContext must return metadata")
+	}
+	expected := MD{
+		"x-my-header-1":         []string{"42"},
+		"x-incorrect-uppercase": []string{"foo"},
+	}
+	if !reflect.DeepEqual(result, expected) {
+		t.Errorf("FromIncomingContext returned %#v, expected %#v", result, expected)
+	}
+
+	// ensure modifying result does not modify the value in the context
+	result["new_key"] = []string{"foo"}
+	result["x-my-header-1"][0] = "mutated"
+
+	result2, found := FromIncomingContext(ctx)
+	if !found {
+		t.Fatal("FromIncomingContext must return metadata")
+	}
+	if !reflect.DeepEqual(result2, expected) {
+		t.Errorf("FromIncomingContext after modifications returned %#v, expected %#v", result2, expected)
+	}
+}
+
 func (s) TestValueFromIncomingContext(t *testing.T) {
 	md := Pairs(
 		"X-My-Header-1", "42",
 		"X-My-Header-2", "43-1",
 		"X-My-Header-2", "43-2",
 		"x-my-header-3", "44",
 	)
+	// Verify that we lowercase if callers directly modify md
+	md["X-INCORRECT-UPPERCASE"] = []string{"foo"}
 	ctx := NewIncomingContext(context.Background(), md)
 
 	for _, test := range []struct {
@@ -227,6 +262,10 @@ func (s) TestValueFromIncomingContext(t *testing.T) {
 			key:  "x-unknown",
 			want: nil,
 		},
+		{
+			key:  "x-incorrect-uppercase",
+			want: []string{"foo"},
+		},
 	} {
 		v := ValueFromIncomingContext(ctx, test.key)
 		if !reflect.DeepEqual(v, test.want) {
@@ -348,8 +387,22 @@ func BenchmarkFromIncomingContext(b *testing.B) {
 func BenchmarkValueFromIncomingContext(b *testing.B) {
 	md := Pairs("X-My-Header-1", "42")
 	ctx := NewIncomingContext(context.Background(), md)
-	b.ResetTimer()
-	for n := 0; n < b.N; n++ {
-		ValueFromIncomingContext(ctx, "x-my-header-1")
-	}
+
+	b.Run("key-found", func(b *testing.B) {
+		for n := 0; n < b.N; n++ {
+			result := ValueFromIncomingContext(ctx, "x-my-header-1")
+			if len(result) != 1 {
+				b.Fatal("ensures not optimized away")
+			}
+		}
+	})
+
+	b.Run("key-not-found", func(b *testing.B) {
+		for n := 0; n < b.N; n++ {
+			result := ValueFromIncomingContext(ctx, "key-not-found")
+			if len(result) != 0 {
+				b.Fatal("ensures not optimized away")
+			}
+		}
+	})
 }