Add pre/post interceptor hooks to grpcserver.New

New Options parameter with PreInterceptors and PostInterceptors
allows services to inject custom interceptors into the chain:

  [pre-interceptors] → logging → auth → [post-interceptors] → handler

This enables services like metacrypt to add seal-check (pre-auth)
and audit-logging (post-auth) interceptors while using the shared
auth and logging infrastructure.

Pass nil for the default chain (logging + auth only).

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

grpcserver/server.go

@@ -48,21 +48,45 @@ type Server struct {
 	listener net.Listener
 }
 
+// Options configures optional behavior for the gRPC server.
+type Options struct {
+	// PreInterceptors run before the logging and auth interceptors.
+	// Use for lifecycle gates like seal checks that should reject
+	// requests before any auth validation occurs.
+	PreInterceptors []grpc.UnaryServerInterceptor
+
+	// PostInterceptors run after auth but before the handler.
+	// Use for audit logging, rate limiting, or other cross-cutting
+	// concerns that need access to the authenticated identity.
+	PostInterceptors []grpc.UnaryServerInterceptor
+}
+
 // New creates a gRPC server with TLS (if certFile and keyFile are
-// non-empty) and an interceptor chain: logging → auth → handler.
+// non-empty) and an interceptor chain:
+//
+//	[pre-interceptors] → logging → auth → [post-interceptors] → handler
 //
 // The auth interceptor uses methods to determine the access level for
 // each RPC. Methods not in any map are denied by default.
 //
 // If certFile and keyFile are empty, TLS is skipped (for testing).
-func New(certFile, keyFile string, authenticator *auth.Authenticator, methods MethodMap, logger *slog.Logger) (*Server, error) {
-	chain := grpc.ChainUnaryInterceptor(
+// opts is optional; pass nil for the default chain (logging + auth only).
+func New(certFile, keyFile string, authenticator *auth.Authenticator, methods MethodMap, logger *slog.Logger, opts *Options) (*Server, error) {
+	var interceptors []grpc.UnaryServerInterceptor
+	if opts != nil {
+		interceptors = append(interceptors, opts.PreInterceptors...)
+	}
+	interceptors = append(interceptors,
 		loggingInterceptor(logger),
 		authInterceptor(authenticator, methods),
 	)
+	if opts != nil {
+		interceptors = append(interceptors, opts.PostInterceptors...)
+	}
+	chain := grpc.ChainUnaryInterceptor(interceptors...)
 
-	var opts []grpc.ServerOption
-	opts = append(opts, chain)
+	var serverOpts []grpc.ServerOption
+	serverOpts = append(serverOpts, chain)
 
 	if certFile != "" && keyFile != "" {
 		cert, err := tls.LoadX509KeyPair(certFile, keyFile)
@@ -73,11 +97,11 @@ func New(certFile, keyFile string, authenticator *auth.Authenticator, methods Me
 			Certificates: []tls.Certificate{cert},
 			MinVersion:   tls.VersionTLS13,
 		}
-		opts = append(opts, grpc.Creds(credentials.NewTLS(tlsCfg)))
+		serverOpts = append(serverOpts, grpc.Creds(credentials.NewTLS(tlsCfg)))
 	}
 
 	return &Server{
-		GRPCServer: grpc.NewServer(opts...),
+		GRPCServer: grpc.NewServer(serverOpts...),
 		Logger:     logger,
 	}, nil
 }

grpcserver/server_test.go

@@ -216,7 +216,7 @@ func TestNewWithoutTLS(t *testing.T) {
 	defer srv.Close()
 	a := testAuth(t, srv.URL)
 
-	s, err := New("", "", a, testMethods, slog.Default())
+	s, err := New("", "", a, testMethods, slog.Default(), nil)
 	if err != nil {
 		t.Fatalf("New: %v", err)
 	}