documentation: on server, use FromIncomingContext for retrieving cont…

…ext and `SetHeader`, `SetTrailer` to send metadata to client (#7238)

Documentation/grpc-metadata.md

@@ -12,11 +12,11 @@ Four kinds of service method:
 - [Client streaming RPC](https://grpc.io/docs/guides/concepts.html#client-streaming-rpc)
 - [Bidirectional streaming RPC](https://grpc.io/docs/guides/concepts.html#bidirectional-streaming-rpc)
 
-And concept of [metadata](https://grpc.io/docs/guides/concepts.html#metadata).
+And concept of [metadata].
 
 ## Constructing metadata
 
-A metadata can be created using package [metadata](https://godoc.org/google.golang.org/grpc/metadata).
+A metadata can be created using package [metadata].
 The type MD is actually a map from string to a list of strings:
 
 ```go
@@ -64,20 +64,10 @@ md := metadata.Pairs(
 )
 ```
 
-## Retrieving metadata from context
-
-Metadata can be retrieved from context using `FromIncomingContext`:
-
-```go
-func (s *server) SomeRPC(ctx context.Context, in *pb.SomeRequest) (*pb.SomeResponse, err) {
-    md, ok := metadata.FromIncomingContext(ctx)
-    // do something with metadata
-}
-```
-
 ## Sending and receiving metadata - client side
 
-Client side metadata sending and receiving examples are available [here](../examples/features/metadata/client/main.go).
+Client side metadata sending and receiving examples are available
+[here](../examples/features/metadata/client/main.go).
 
 ### Sending metadata
 
@@ -127,7 +117,8 @@ Metadata that a client can receive includes header and trailer.
 
 #### Unary call
 
-Header and trailer sent along with a unary call can be retrieved using function [Header](https://godoc.org/google.golang.org/grpc#Header) and [Trailer](https://godoc.org/google.golang.org/grpc#Trailer) in [CallOption](https://godoc.org/google.golang.org/grpc#CallOption):
+Header and trailer sent along with a unary call can be retrieved using function
+[Header] and [Trailer] in [CallOption]:
 
 ```go
 var header, trailer metadata.MD // variable to store header and trailer
@@ -149,7 +140,8 @@ For streaming calls including:
 - Client streaming RPC
 - Bidirectional streaming RPC
 
-Header and trailer can be retrieved from the returned stream using function `Header` and `Trailer` in interface [ClientStream](https://godoc.org/google.golang.org/grpc#ClientStream):
+Header and trailer can be retrieved from the returned stream using function
+`Header` and `Trailer` in interface [ClientStream]:
 
 ```go
 stream, err := client.SomeStreamingRPC(ctx)
@@ -164,11 +156,13 @@ trailer := stream.Trailer()
 
 ## Sending and receiving metadata - server side
 
-Server side metadata sending and receiving examples are available [here](../examples/features/metadata/server/main.go).
+Server side metadata sending and receiving examples are available
+[here](../examples/features/metadata/server/main.go).
 
 ### Receiving metadata
 
-To read metadata sent by the client, the server needs to retrieve it from RPC context.
+To read metadata sent by the client, the server needs to retrieve it from RPC
+context using [FromIncomingContext].
 If it is a unary call, the RPC handler's context can be used.
 For streaming calls, the server needs to get context from the stream.
 
@@ -194,15 +188,16 @@ func (s *server) SomeStreamingRPC(stream pb.Service_SomeStreamingRPCServer) erro
 
 #### Unary call
 
-To send header and trailer to client in unary call, the server can call [SendHeader](https://godoc.org/google.golang.org/grpc#SendHeader) and [SetTrailer](https://godoc.org/google.golang.org/grpc#SetTrailer) functions in module [grpc](https://godoc.org/google.golang.org/grpc).
+To send header and trailer to client in unary call, the server can call
+[SetHeader] and [SetTrailer] functions in module [grpc].
 These two functions take a context as the first parameter.
 It should be the RPC handler's context or one derived from it:
 
 ```go
 func (s *server) SomeRPC(ctx context.Context, in *pb.someRequest) (*pb.someResponse, error) {
-    // create and send header
+    // create and set header
     header := metadata.Pairs("header-key", "val")
-    grpc.SendHeader(ctx, header)
+    grpc.SetHeader(ctx, header)
     // create and set trailer
     trailer := metadata.Pairs("trailer-key", "val")
     grpc.SetTrailer(ctx, trailer)
@@ -211,20 +206,39 @@ func (s *server) SomeRPC(ctx context.Context, in *pb.someRequest) (*pb.someRespo
 
 #### Streaming call
 
-For streaming calls, header and trailer can be sent using function `SendHeader` and `SetTrailer` in interface [ServerStream](https://godoc.org/google.golang.org/grpc#ServerStream):
+For streaming calls, header and trailer can be sent using function
+[SetHeader] and [SetTrailer] in interface [ServerStream]:
 
 ```go
 func (s *server) SomeStreamingRPC(stream pb.Service_SomeStreamingRPCServer) error {
-    // create and send header
+    // create and set header
     header := metadata.Pairs("header-key", "val")
-    stream.SendHeader(header)
+    stream.SetHeader(header)
     // create and set trailer
     trailer := metadata.Pairs("trailer-key", "val")
     stream.SetTrailer(trailer)
 }
 ```
 
+**Important**
+
+Do not use
+[FromOutgoingContext] on the server to write metadata to be sent to the client.
+[FromOutgoingContext] is for client-side use only.
+
 ## Updating metadata from a server interceptor
 
 An example for updating metadata from a server interceptor is
 available [here](../examples/features/metadata_interceptor/server/main.go).
+
+[FromIncomingContext]: <https://pkg.go.dev/google.golang.org/grpc/metadata#FromIncomingContext>
+[SetHeader]: <https://godoc.org/google.golang.org/grpc#SetHeader>
+[SetTrailer]: https://godoc.org/google.golang.org/grpc#SetTrailer
+[FromOutgoingContext]: https://pkg.go.dev/google.golang.org/grpc/metadata#FromOutgoingContext
+[ServerStream]: https://godoc.org/google.golang.org/grpc#ServerStream
+[grpc]: https://godoc.org/google.golang.org/grpc
+[ClientStream]: https://godoc.org/google.golang.org/grpc#ClientStream
+[Header]: https://godoc.org/google.golang.org/grpc#Header
+[Trailer]: https://godoc.org/google.golang.org/grpc#Trailer
+[CallOption]: https://godoc.org/google.golang.org/grpc#CallOption
+[metadata]: https://godoc.org/google.golang.org/grpc/metadata

examples/features/metadata_interceptor/client/main.go

@@ -29,18 +29,30 @@ import (
 
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/metadata"
 
 	pb "google.golang.org/grpc/examples/features/proto/echo"
 )
 
 var addr = flag.String("addr", "localhost:50051", "the address to connect to")
 
 func callUnaryEcho(ctx context.Context, client pb.EchoClient) {
-	resp, err := client.UnaryEcho(ctx, &pb.EchoRequest{Message: "hello world"})
+	var header, trailer metadata.MD
+	resp, err := client.UnaryEcho(ctx, &pb.EchoRequest{Message: "hello world"}, grpc.Header(&header), grpc.Trailer(&trailer))
 	if err != nil {
 		log.Fatalf("UnaryEcho: %v", err)
 	}
 	fmt.Println("UnaryEcho: ", resp.Message)
+
+	fmt.Println("Received headers:")
+	for k, v := range header {
+		fmt.Printf("%s: %v\n", k, v)
+	}
+
+	fmt.Println("Received trailers:")
+	for k, v := range trailer {
+		fmt.Printf("%s: %v\n", k, v)
+	}
 }
 
 func callBidiStreamingEcho(ctx context.Context, client pb.EchoClient) {
@@ -64,6 +76,21 @@ func callBidiStreamingEcho(ctx context.Context, client pb.EchoClient) {
 		}
 		fmt.Println("BidiStreaming Echo: ", resp.Message)
 	}
+
+	header, err := c.Header()
+	if err != nil {
+		log.Fatalf("Receiving headers: %v", err)
+	}
+	fmt.Println("Received headers:")
+	for k, v := range header {
+		fmt.Printf("%s: %v\n", k, v)
+	}
+
+	trailer := c.Trailer()
+	fmt.Println("Received tailers:")
+	for k, v := range trailer {
+		fmt.Printf("%s: %v\n", k, v)
+	}
 }
 
 func main() {

examples/features/metadata_interceptor/server/main.go

@@ -49,10 +49,22 @@ func unaryInterceptor(ctx context.Context, req any, info *grpc.UnaryServerInfo,
 		return nil, errMissingMetadata
 	}
 
+	// Create and set metadata from interceptor to server.
 	md.Append("key1", "value1")
 	ctx = metadata.NewIncomingContext(ctx, md)
 
-	return handler(ctx, req)
+	// Call the handler to complete the normal execution of the RPC.
+	resp, err := handler(ctx, req)
+
+	// Create and set header metadata from interceptor to client.
+	header := metadata.Pairs("header-key", "val")
+	grpc.SetHeader(ctx, header)
+
+	// Create and set trailer metadata from interceptor to client.
+	trailer := metadata.Pairs("trailer-key", "val")
+	grpc.SetTrailer(ctx, trailer)
+
+	return resp, err
 }
 
 func (s *server) UnaryEcho(ctx context.Context, in *pb.EchoRequest) (*pb.EchoResponse, error) {
@@ -89,10 +101,22 @@ func streamInterceptor(srv any, ss grpc.ServerStream, info *grpc.StreamServerInf
 		return errMissingMetadata
 	}
 
+	// Create and set metadata from interceptor to server.
 	md.Append("key1", "value1")
 	ctx := metadata.NewIncomingContext(ss.Context(), md)
 
-	return handler(srv, &wrappedStream{ss, ctx})
+	// Call the handler to complete the normal execution of the RPC.
+	err := handler(srv, &wrappedStream{ss, ctx})
+
+	// Create and set header metadata from interceptor to client.
+	header := metadata.Pairs("header-key", "val")
+	ss.SetHeader(header)
+
+	// Create and set trailer metadata from interceptor to client.
+	trailer := metadata.Pairs("trailer-key", "val")
+	ss.SetTrailer(trailer)
+
+	return err
 }
 
 func (s *server) BidirectionalStreamingEcho(stream pb.Echo_BidirectionalStreamingEchoServer) error {