Add documentation and example of adding details to errors (#1915)

Documentation/rpc-errors.md

@@ -0,0 +1,67 @@
+# RPC Errors
+
+All service method handlers should return `nil` or errors from the
+`status.Status` type. Clients have direct access to the errors.
+
+Upon encountering an error, a gRPC server method handler should create a
+`status.Status`. In typical usage, one would use [status.New][new-status]
+passing in an appropriate [codes.Code][code] as well as a description of the
+error to produce a `status.Status`. Calling [status.Err][status-err] converts
+the `status.Status` type into an `error`. As a convenience method, there is also
+[status.Error][status-error] which obviates the conversion step. Compare:
+
+```
+st := status.New(codes.NotFound, "some description")
+err := st.Err()
+
+// vs.
+
+err := status.Error(codes.NotFound, "some description")
+```
+
+## Adding additional details to errors
+
+In some cases, it may be necessary to add details for a particular error on the
+server side. The [status.WithDetails][with-details] method exists for this
+purpose. Clients may then read those details by first converting the plain
+`error` type back to a [status.Status][status] and then using
+[status.Details][details].
+
+## Example
+
+The [example][example] demonstrates the API discussed above and shows how to add
+information about rate limits to the error message using `status.Status`.
+
+To run the example, first start the server:
+
+```
+$ go run examples/rpc_errors/server/main.go
+```
+
+In a separate sesssion, run the client:
+
+```
+$ go run examples/rpc_errors/client/main.go
+```
+
+On the first run of the client, all is well:
+
+```
+2018/03/12 19:39:33 Greeting: Hello world
+```
+
+Upon running the client a second time, the client exceeds the rate limit and
+receives an error with details:
+
+```
+2018/03/19 16:42:01 Quota failure: violations:<subject:"name:world" description:"Limit one greeting per person" >
+exit status 1
+```
+
+[status]:       https://godoc.org/google.golang.org/grpc/status#Status
+[new-status]:   https://godoc.org/google.golang.org/grpc/status#New
+[code]:         https://godoc.org/google.golang.org/grpc/codes#Code
+[with-details]: https://godoc.org/google.golang.org/grpc/status#Status.WithDetails
+[details]:      https://godoc.org/google.golang.org/grpc/status#Status.Details
+[status-err]:   https://godoc.org/google.golang.org/grpc/status#Status.Err
+[example]:      https://github.com/grpc/grpc-go/blob/master/examples/rpc_errors

examples/rpc_errors/client/main.go

@@ -0,0 +1,62 @@
+/*
+ *
+ * Copyright 2018 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package main
+
+import (
+	"log"
+	"os"
+	"time"
+
+	"golang.org/x/net/context"
+	epb "google.golang.org/genproto/googleapis/rpc/errdetails"
+	"google.golang.org/grpc"
+	pb "google.golang.org/grpc/examples/helloworld/helloworld"
+	"google.golang.org/grpc/status"
+)
+
+func main() {
+	// Set up a connection to the server.
+	conn, err := grpc.Dial("localhost:50051", grpc.WithInsecure())
+	if err != nil {
+		log.Fatalf("did not connect: %v", err)
+	}
+	defer func() {
+		if e := conn.Close(); e != nil {
+			log.Printf("failed to close connection: %s", e)
+		}
+	}()
+	c := pb.NewGreeterClient(conn)
+
+	ctx, cancel := context.WithTimeout(context.Background(), time.Second)
+	defer cancel()
+	r, err := c.SayHello(ctx, &pb.HelloRequest{Name: "world"})
+	if err != nil {
+		s := status.Convert(err)
+		for _, d := range s.Details() {
+			switch info := d.(type) {
+			case *epb.QuotaFailure:
+				log.Printf("Quota failure: %s", info)
+			default:
+				log.Printf("Unexpected type: %s", info)
+			}
+		}
+		os.Exit(1)
+	}
+	log.Printf("Greeting: %s", r.Message)
+}

examples/rpc_errors/server/main.go

@@ -0,0 +1,80 @@
+/*
+ *
+ * Copyright 2018 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package main
+
+import (
+	"fmt"
+	"log"
+	"net"
+	"sync"
+
+	"golang.org/x/net/context"
+	epb "google.golang.org/genproto/googleapis/rpc/errdetails"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/codes"
+	pb "google.golang.org/grpc/examples/helloworld/helloworld"
+	"google.golang.org/grpc/status"
+)
+
+const (
+	port = ":50051"
+)
+
+// server is used to implement helloworld.GreeterServer.
+type server struct {
+	mu    sync.Mutex
+	count map[string]int
+}
+
+// SayHello implements helloworld.GreeterServer
+func (s *server) SayHello(ctx context.Context, in *pb.HelloRequest) (*pb.HelloReply, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	// Track the number of times the user has been greeted.
+	s.count[in.Name]++
+	if s.count[in.Name] > 1 {
+		st := status.New(codes.ResourceExhausted, "Request limit exceeded.")
+		ds, err := st.WithDetails(
+			&epb.QuotaFailure{
+				Violations: []*epb.QuotaFailure_Violation{{
+					Subject:     fmt.Sprintf("name:%s", in.Name),
+					Description: "Limit one greeting per person",
+				}},
+			},
+		)
+		if err != nil {
+			return nil, st.Err()
+		}
+		return nil, ds.Err()
+	}
+	return &pb.HelloReply{Message: "Hello " + in.Name}, nil
+}
+
+func main() {
+	log.Printf("server starting on port %s...", port)
+	lis, err := net.Listen("tcp", port)
+	if err != nil {
+		log.Fatalf("failed to listen: %v", err)
+	}
+	s := grpc.NewServer()
+	pb.RegisterGreeterServer(s, &server{count: make(map[string]int)})
+	if err := s.Serve(lis); err != nil {
+		log.Fatalf("failed to serve: %v", err)
+	}
+}