[v4] breaking change: modify API for working with response metadata (#255)

* implement functions to append/get metadata

* сhanged behavior to return nil instead of empty metadata for getResponseMetadata()

* removed metadata copy when passing to gRPC headers

---------

Co-authored-by: Vasiliy Tolstov <v.tolstov@unistack.org>

go.mod

@@ -5,6 +5,7 @@ go 1.23.0
 toolchain go1.24.2
 
 require (
+	github.com/stretchr/testify v1.10.0
 	go.unistack.org/micro/v4 v4.1.8
 	golang.org/x/net v0.39.0
 	google.golang.org/grpc v1.72.0
@@ -13,8 +14,10 @@ require (
 
 require (
 	github.com/ash3in/uuidv8 v1.2.0 // indirect
+	github.com/davecgh/go-spew v1.1.2-0.20180830191138-d8f796af33cc // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/matoous/go-nanoid v1.5.1 // indirect
+	github.com/pmezard/go-difflib v1.0.1-0.20181226105442-5d4384ee4fb2 // indirect
 	github.com/spf13/cast v1.7.1 // indirect
 	go.unistack.org/micro-proto/v4 v4.1.0 // indirect
 	golang.org/x/sys v0.32.0 // indirect

grpc.go

@@ -263,6 +263,7 @@ func (g *Server) handler(srv interface{}, stream grpc.ServerStream) error {
 	// create new context
 	ctx = metadata.NewIncomingContext(ctx, md)
 	ctx = metadata.NewOutgoingContext(ctx, metadata.New(0))
+	ctx = context.WithValue(ctx, rspMetadataKey{}, &rspMetadataVal{})
 
 	stream = &streamWrapper{ctx, stream}
 
@@ -397,8 +398,8 @@ func (g *Server) processRequest(ctx context.Context, stream grpc.ServerStream, s
 	statusDesc := ""
 	// execute the handler
 	appErr := fn(ctx, r, replyv.Interface())
-	if outmd, ok := metadata.FromOutgoingContext(ctx); ok {
+	if md := getResponseMetadata(ctx); len(md) > 0 {
-		if err = stream.SendHeader(gmetadata.MD(outmd.Copy())); err != nil {
+		if err = stream.SendHeader(gmetadata.MD(md)); err != nil {
 			return err
 		}
 	}
@@ -481,8 +482,8 @@ func (g *Server) processStream(ctx context.Context, stream grpc.ServerStream, se
 	statusDesc := ""
 
 	appErr := fn(ctx, r, ss)
-	if outmd, ok := metadata.FromOutgoingContext(ctx); ok {
+	if md := getResponseMetadata(ctx); len(md) > 0 {
-		if err := stream.SendHeader(gmetadata.MD(outmd.Copy())); err != nil {
+		if err := stream.SendHeader(gmetadata.MD(md)); err != nil {
 			return err
 		}
 	}

metadata.go

@@ -0,0 +1,47 @@
+package grpc
+
+import (
+	"context"
+
+	"go.unistack.org/micro/v4/metadata"
+)
+
+type (
+	rspMetadataKey struct{}
+	rspMetadataVal struct {
+		m metadata.Metadata
+	}
+)
+
+// AppendResponseMetadata adds metadata entries to metadata.Metadata stored in the context.
+// It expects the context to contain a *rspMetadataVal value under the rspMetadataKey{} key.
+// If the value is missing or invalid, the function does nothing.
+//
+// Note: this function is not thread-safe. Synchronization is required if used from multiple goroutines.
+func AppendResponseMetadata(ctx context.Context, md metadata.Metadata) {
+	if md == nil {
+		return
+	}
+
+	val, ok := ctx.Value(rspMetadataKey{}).(*rspMetadataVal)
+	if !ok || val == nil || val.m == nil {
+		return
+	}
+
+	for key, values := range md {
+		val.m.Append(key, values...)
+	}
+}
+
+// getResponseMetadata retrieves the metadata.Metadata stored in the context.
+//
+// Note: this function is not thread-safe. Synchronization is required if used from multiple goroutines.
+// If you plan to modify the returned metadata, make a full copy to avoid affecting shared state.
+func getResponseMetadata(ctx context.Context) metadata.Metadata {
+	val, ok := ctx.Value(rspMetadataKey{}).(*rspMetadataVal)
+	if !ok || val == nil || val.m == nil {
+		return nil
+	}
+
+	return val.m
+}

metadata_test.go

@@ -0,0 +1,136 @@
+package grpc
+
+import (
+	"context"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"go.unistack.org/micro/v4/metadata"
+)
+
+func TestAppendResponseMetadata(t *testing.T) {
+	tests := []struct {
+		name     string
+		ctx      context.Context
+		md       metadata.Metadata
+		expected context.Context
+	}{
+		{
+			name:     "nil metadata",
+			ctx:      context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{m: metadata.Metadata{}}),
+			md:       nil,
+			expected: context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{m: metadata.Metadata{}}),
+		},
+		{
+			name:     "empty metadata",
+			ctx:      context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{m: metadata.Metadata{}}),
+			md:       metadata.Metadata{},
+			expected: context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{m: metadata.Metadata{}}),
+		},
+		{
+			name:     "context without response metadata key",
+			ctx:      context.Background(),
+			md:       metadata.Pairs("key1", "val1"),
+			expected: context.Background(),
+		},
+		{
+			name:     "context with nil response metadata value",
+			ctx:      context.WithValue(context.Background(), rspMetadataKey{}, nil),
+			md:       metadata.Pairs("key1", "val1"),
+			expected: context.WithValue(context.Background(), rspMetadataKey{}, nil),
+		},
+		{
+			name:     "context with incorrect type in response metadata value",
+			ctx:      context.WithValue(context.Background(), rspMetadataKey{}, struct{}{}),
+			md:       metadata.Pairs("key1", "val1"),
+			expected: context.WithValue(context.Background(), rspMetadataKey{}, struct{}{}),
+		},
+		{
+			name:     "context with response metadata value, but nil metadata",
+			ctx:      context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{m: nil}),
+			md:       metadata.Pairs("key1", "val1"),
+			expected: context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{m: nil}),
+		},
+		{
+			name: "basic metadata append",
+			ctx:  context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{m: metadata.Metadata{}}),
+			md:   metadata.Pairs("key1", "val1"),
+			expected: context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{
+				m: metadata.Metadata{
+					"key1": []string{"val1"},
+				},
+			}),
+		},
+		{
+			name: "multiple values for same key",
+			ctx:  context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{m: metadata.Metadata{}}),
+			md:   metadata.Pairs("key1", "val1", "key1", "val2"),
+			expected: context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{
+				m: metadata.Metadata{
+					"key1": []string{"val1", "val2"},
+				},
+			}),
+		},
+		{
+			name: "multiple values for different keys",
+			ctx:  context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{m: metadata.Metadata{}}),
+			md:   metadata.Pairs("key1", "val1", "key1", "val2", "key2", "val3", "key2", "val4", "key3", "val5"),
+			expected: context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{
+				m: metadata.Metadata{
+					"key1": []string{"val1", "val2"},
+					"key2": []string{"val3", "val4"},
+					"key3": []string{"val5"},
+				},
+			}),
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			AppendResponseMetadata(tt.ctx, tt.md)
+			require.Equal(t, tt.expected, tt.ctx)
+		})
+	}
+}
+
+func TestGetResponseMetadata(t *testing.T) {
+	tests := []struct {
+		name     string
+		ctx      context.Context
+		expected metadata.Metadata
+	}{
+		{
+			name:     "context without response metadata key",
+			ctx:      context.Background(),
+			expected: nil,
+		},
+		{
+			name:     "context with nil response metadata value",
+			ctx:      context.WithValue(context.Background(), rspMetadataKey{}, nil),
+			expected: nil,
+		},
+		{
+			name:     "context with incorrect type in response metadata value",
+			ctx:      context.WithValue(context.Background(), rspMetadataKey{}, &struct{}{}),
+			expected: nil,
+		},
+		{
+			name:     "context with response metadata value, but nil metadata",
+			ctx:      context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{m: nil}),
+			expected: nil,
+		},
+		{
+			name: "valid metadata",
+			ctx: context.WithValue(context.Background(), rspMetadataKey{}, &rspMetadataVal{
+				m: metadata.Pairs("key1", "value1"),
+			}),
+			expected: metadata.Metadata{"key1": {"value1"}},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.expected, getResponseMetadata(tt.ctx))
+		})
+	}
+}