add internal/grpc package (#46461)

Just a place to start putting some shared gRPC code. The only thing in
it right now is the handler multiplexing code that will be shared across
all initial implementations.

go.mod

@@ -439,7 +439,7 @@ require (
 	golang.org/x/text v0.5.0
 	golang.org/x/xerrors v0.0.0-20220907171357-04be3eba64a2 // indirect
 	google.golang.org/appengine v1.6.7 // indirect
-	google.golang.org/grpc v1.51.0 // indirect
+	google.golang.org/grpc v1.51.0
 	gopkg.in/alexcesaro/statsd.v2 v2.0.0 // indirect
 	gopkg.in/inf.v0 v0.9.1 // indirect
 	gopkg.in/square/go-jose.v2 v2.6.0 // indirect

internal/grpc/grpc.go

@@ -0,0 +1,29 @@
+// Package grpc is a set of shared code for implementing gRPC.
+package grpc
+
+import (
+	"net/http"
+	"strings"
+
+	"golang.org/x/net/http2"
+	"golang.org/x/net/http2/h2c"
+	"google.golang.org/grpc"
+)
+
+// MultiplexHandlers takes a gRPC server and a plain HTTP handler and multiplexes the
+// request handling. Any requests that declare themselves as gRPC requests are routed
+// to the gRPC server, all others are routed to the httpHandler.
+func MultiplexHandlers(grpcServer *grpc.Server, httpHandler http.Handler) http.Handler {
+	newHandler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.ProtoMajor == 2 && strings.Contains(r.Header.Get("Content-Type"), "application/grpc") {
+			grpcServer.ServeHTTP(w, r)
+		} else {
+			httpHandler.ServeHTTP(w, r)
+		}
+	})
+
+	// Until we enable TLS, we need to fall back to the h2c protocol, which is
+	// basically HTTP2 without TLS. The standard library does not implement the
+	// h2s protocol, so this hijacks h2s requests and handles them correctly.
+	return h2c.NewHandler(newHandler, &http2.Server{})
+}

internal/grpc/grpc_test.go

@@ -0,0 +1,39 @@
+package grpc
+
+import (
+	"bytes"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+	"google.golang.org/grpc"
+)
+
+func TestMultiplexHandlers(t *testing.T) {
+	grpcServer := grpc.NewServer()
+	called := false
+	httpHandler := http.HandlerFunc(func(http.ResponseWriter, *http.Request) {
+		called = true
+	})
+	multiplexedHandler := MultiplexHandlers(grpcServer, httpHandler)
+
+	{ // Basic HTTP request is routed to HTTP handler
+		req, err := http.NewRequest("GET", "", bytes.NewReader(nil))
+		require.NoError(t, err)
+		called = false
+		multiplexedHandler.ServeHTTP(httptest.NewRecorder(), req)
+		require.True(t, called)
+	}
+
+	{ // Request with HTTP2 and application/grpc header is not routed to HTTP handler
+		req, err := http.NewRequest("GET", "", bytes.NewReader(nil))
+		require.NoError(t, err)
+		req.Header.Add("content-type", "application/grpc")
+		req.ProtoMajor = 2
+
+		called = false
+		multiplexedHandler.ServeHTTP(httptest.NewRecorder(), req)
+		require.False(t, called)
+	}
+}