api/server/router/grpc: fix linting for deprecated http2 code

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit c47560390b)

The code lives in api/server/router/grpc on this branch. The comment
drops upstream's pointer to native gRPC over the Docker socket and the
TODO to remove the /grpc endpoint: this branch has no native gRPC
support, so /grpc is the only way to reach the daemon's gRPC services.

Signed-off-by: Cory Snider <csnider@mirantis.com>

api/server/router/grpc/grpc.go

@@ -20,7 +20,7 @@ import (
 type grpcRouter struct {
 	routes     []router.Route
 	grpcServer *grpc.Server
-	h2Server   *http2.Server
+	h2Server   *http2.Server //nolint:staticcheck // http2.Server is deprecated; kept for backward compatibility.
 }
 
 // NewRouter initializes a new grpc http router
@@ -32,7 +32,7 @@ func NewRouter(backends ...Backend) router.Router {
 	}
 
 	r := &grpcRouter{
-		h2Server:   &http2.Server{},
+		h2Server:   &http2.Server{}, //nolint:staticcheck // http2.Server is deprecated; kept for backward compatibility.
 		grpcServer: grpc.NewServer(opts...),
 	}
 	for _, b := range backends {

api/server/router/grpc/grpc_routes.go

@@ -38,8 +38,12 @@ func (gr *grpcRouter) serveGRPC(ctx context.Context, w http.ResponseWriter, r *h
 	conn.Write([]byte{})
 	resp.Write(conn)
 
-	// https://godoc.org/golang.org/x/net/http2#Server.ServeConn
-	// TODO: is it a problem that conn has already been written to?
-	gr.h2Server.ServeConn(conn, &http2.ServeConnOpts{Handler: gr.grpcServer})
+	// Keep using the deprecated http2.Server for compatibility with clients
+	// using the /grpc endpoint, which performs an HTTP/1.1 Upgrade: h2c
+	// handshake. net/http's unencrypted HTTP/2 support uses HTTP/2 with prior
+	// knowledge and does not support Upgrade: h2c.
+	gr.h2Server.ServeConn(conn, &http2.ServeConnOpts{ //nolint:staticcheck // http2.ServeConn and http2.ServeConnOpts are deprecated.
+		Handler: gr.grpcServer,
+	})
 	return nil
 }