feat(pushqueue): add example gateway server and client

Wires up noop VCS and passthrough queue implementations so the
pushqueue gateway can start and serve Ping/Land/CheckMergeability
RPCs for local development.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: kevinlnew

example/pushqueue/gateway/client/BUILD.bazel

@@ -0,0 +1,19 @@
+load("@rules_go//go:def.bzl", "go_binary", "go_library")
+
+go_library(
+    name = "client_lib",
+    srcs = ["main.go"],
+    importpath = "github.com/uber/submitqueue/example/pushqueue/gateway/client",
+    visibility = ["//visibility:private"],
+    deps = [
+        "//pushqueue/gateway/protopb",
+        "@org_golang_google_grpc//:grpc",
+        "@org_golang_google_grpc//credentials/insecure",
+    ],
+)
+
+go_binary(
+    name = "client",
+    embed = [":client_lib"],
+    visibility = ["//visibility:public"],
+)

example/pushqueue/gateway/client/main.go

@@ -0,0 +1,73 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// 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 (
+	"context"
+	"flag"
+	"fmt"
+	"os"
+	"time"
+
+	pb "github.com/uber/submitqueue/pushqueue/gateway/protopb"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/credentials/insecure"
+)
+
+func main() {
+	addr := flag.String("addr", "localhost:8084", "pushqueue gateway server address")
+	message := flag.String("message", "", "message to send in ping request")
+	timeout := flag.Duration("timeout", 5*time.Second, "request timeout")
+	flag.Parse()
+
+	if err := run(*addr, *message, *timeout); err != nil {
+		fmt.Fprintf(os.Stderr, "Error: %v\n", err)
+		os.Exit(1)
+	}
+}
+
+func run(addr, message string, timeout time.Duration) error {
+	conn, err := grpc.NewClient(
+		addr,
+		grpc.WithTransportCredentials(insecure.NewCredentials()),
+	)
+	if err != nil {
+		return fmt.Errorf("failed to connect: %w", err)
+	}
+	defer conn.Close()
+
+	client := pb.NewPushQueueGatewayClient(conn)
+
+	ctx, cancel := context.WithTimeout(context.Background(), timeout)
+	defer cancel()
+
+	req := &pb.PingRequest{
+		Message: message,
+	}
+
+	fmt.Printf("Sending ping to pushqueue gateway at %s...\n", addr)
+	resp, err := client.Ping(ctx, req)
+	if err != nil {
+		return fmt.Errorf("ping failed: %w", err)
+	}
+
+	fmt.Printf("\nResponse:\n")
+	fmt.Printf("  Message:      %s\n", resp.Message)
+	fmt.Printf("  Service Name: %s\n", resp.ServiceName)
+	fmt.Printf("  Timestamp:    %d (%s)\n", resp.Timestamp, time.Unix(resp.Timestamp, 0))
+	fmt.Printf("  Hostname:     %s\n", resp.Hostname)
+
+	return nil
+}

example/pushqueue/gateway/server/BUILD.bazel

@@ -0,0 +1,25 @@
+load("@rules_go//go:def.bzl", "go_binary", "go_library")
+
+go_library(
+    name = "server_lib",
+    srcs = ["main.go"],
+    importpath = "github.com/uber/submitqueue/example/pushqueue/gateway/server",
+    visibility = ["//visibility:private"],
+    deps = [
+        "//pushqueue/entity",
+        "//pushqueue/extension/landqueue",
+        "//pushqueue/extension/vcs",
+        "//pushqueue/gateway/controller",
+        "//pushqueue/gateway/protopb",
+        "@com_github_uber_go_tally_v4//:tally",
+        "@org_golang_google_grpc//:grpc",
+        "@org_golang_google_grpc//reflection",
+        "@org_uber_go_zap//:zap",
+    ],
+)
+
+go_binary(
+    name = "server",
+    embed = [":server_lib"],
+    visibility = ["//visibility:public"],
+)

example/pushqueue/gateway/server/main.go

@@ -0,0 +1,200 @@
+// Copyright (c) 2025 Uber Technologies, Inc.
+//
+// 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 (
+	"context"
+	"errors"
+	"fmt"
+	"net"
+	"os"
+	"os/signal"
+	"sync"
+	"syscall"
+	"time"
+
+	"github.com/uber-go/tally/v4"
+	"github.com/uber/submitqueue/pushqueue/entity"
+	"github.com/uber/submitqueue/pushqueue/extension/landqueue"
+	"github.com/uber/submitqueue/pushqueue/extension/vcs"
+	"github.com/uber/submitqueue/pushqueue/gateway/controller"
+	pb "github.com/uber/submitqueue/pushqueue/gateway/protopb"
+	"go.uber.org/zap"
+	"google.golang.org/grpc"
+	"google.golang.org/grpc/reflection"
+)
+
+// GatewayServer wraps the controllers and implements the gRPC service interface.
+type GatewayServer struct {
+	pb.UnimplementedPushQueueGatewayServer
+	pingController *controller.PingController
+	landController *controller.LandController
+}
+
+// Ping delegates to the controller.
+func (s *GatewayServer) Ping(ctx context.Context, req *pb.PingRequest) (*pb.PingResponse, error) {
+	return s.pingController.Ping(ctx, req)
+}
+
+// Land delegates to the controller.
+func (s *GatewayServer) Land(ctx context.Context, req *pb.LandRequest) (*pb.LandResponse, error) {
+	return s.landController.Land(ctx, req)
+}
+
+// CheckMergeability delegates to the controller.
+func (s *GatewayServer) CheckMergeability(ctx context.Context, req *pb.CheckMergeabilityRequest) (*pb.CheckMergeabilityResponse, error) {
+	return s.landController.CheckMergeability(ctx, req)
+}
+
+func main() {
+	code := 0
+	if err := run(); err != nil {
+		if errors.Is(err, context.Canceled) {
+			fmt.Println("PushQueue gateway server stopped by signal")
+			code = 128 + int(syscall.SIGTERM)
+		} else {
+			fmt.Fprintf(os.Stderr, "PushQueue gateway server failure: %v\n", err)
+			code = 1
+		}
+	}
+	os.Exit(code)
+}
+
+func run() error {
+	ctx, cancel := signal.NotifyContext(context.Background(), syscall.SIGINT, syscall.SIGTERM)
+	defer cancel()
+
+	logger, err := zap.NewDevelopment()
+	if err != nil {
+		return fmt.Errorf("failed to create logger: %w", err)
+	}
+	defer logger.Sync()
+
+	scope := tally.NewTestScope("pushqueue_gateway", nil)
+	metricsStopCh := make(chan any, 1)
+	metricsWgDone := sync.WaitGroup{}
+	metricsWgDone.Add(1)
+	go func() {
+		defer metricsWgDone.Done()
+
+		ticker := time.NewTicker(10 * time.Second)
+		defer ticker.Stop()
+
+		for {
+			select {
+			case <-metricsStopCh:
+				return
+			case <-ticker.C:
+				snapshot := scope.Snapshot()
+				logger.Info("metrics snapshot",
+					zap.Any("counters", snapshot.Counters()),
+					zap.Any("gauges", snapshot.Gauges()),
+					zap.Any("timers", snapshot.Timers()),
+				)
+			}
+		}
+	}()
+
+	defer func() {
+		close(metricsStopCh)
+		metricsWgDone.Wait()
+	}()
+
+	grpcServer := grpc.NewServer()
+
+	pingController := controller.NewPingController(logger.Sugar(), scope)
+	landController := controller.NewLandController(logger.Sugar(), scope, noopVCS{}, noopQueue{})
+	srv := &GatewayServer{
+		pingController: pingController,
+		landController: landController,
+	}
+	pb.RegisterPushQueueGatewayServer(grpcServer, srv)
+
+	reflection.Register(grpcServer)
+
+	port := os.Getenv("PORT")
+	if port == "" {
+		port = ":8084"
+	}
+	listener, err := net.Listen("tcp", port)
+	if err != nil {
+		return fmt.Errorf("failed to listen on port %s: %w", port, err)
+	}
+
+	fmt.Printf("PushQueue gateway gRPC server is running on %s\n", port)
+	fmt.Println("Press Ctrl+C to stop, or send a SIGTERM.")
+
+	serverErrCh := make(chan error, 1)
+	go func() {
+		serverErrCh <- grpcServer.Serve(listener)
+	}()
+
+	var serverErr error
+	select {
+	case <-ctx.Done():
+		fmt.Println("Shutting down pushqueue gateway server due to interruption signal...")
+		err = ctx.Err()
+		grpcServer.GracefulStop()
+		serverErr = <-serverErrCh
+	case serverErr = <-serverErrCh:
+		fmt.Println("Shutting down pushqueue gateway server due to critical GRPC server error...")
+	}
+
+	if serverErr != nil {
+		err = fmt.Errorf("GRPC server exited with error: %w", serverErr)
+	}
+
+	return err
+}
+
+// noopVCS is a placeholder VCS that errors on every operation.
+type noopVCS struct{}
+
+func (noopVCS) CheckMergeability(_ context.Context, _ entity.QueueTarget, items []entity.LandItem) ([]vcs.MergeabilityResult, error) {
+	results := make([]vcs.MergeabilityResult, len(items))
+	for i := range results {
+		results[i] = vcs.MergeabilityResult{Mergeable: false, Reason: "noop VCS: not configured"}
+	}
+	return results, nil
+}
+
+func (noopVCS) Prepare(_ context.Context, _ entity.QueueTarget, _ []entity.LandItem) error {
+	return fmt.Errorf("noop VCS: not configured")
+}
+
+func (noopVCS) Push(_ context.Context, _ entity.QueueTarget, _ []entity.LandItem) (vcs.PushResult, error) {
+	return vcs.PushResult{}, fmt.Errorf("noop VCS: not configured")
+}
+
+func (noopVCS) Finalize(_ context.Context, _ entity.QueueTarget, _ []entity.LandItem) error {
+	return fmt.Errorf("noop VCS: not configured")
+}
+
+// noopQueue is a placeholder Queue that passes through immediately.
+type noopQueue struct{}
+
+var _ landqueue.Queue = noopQueue{}
+
+func (noopQueue) Enqueue(_ context.Context, _ entity.QueueTarget, _ []entity.LandItem) error {
+	return nil
+}
+
+func (noopQueue) Wait(_ context.Context, _ entity.QueueTarget) error {
+	return nil
+}
+
+func (noopQueue) Dequeue(_ context.Context, _ entity.QueueTarget) error {
+	return nil
+}