From 40f227ddbb9e05bf4d04179991f809df0546c6f6 Mon Sep 17 00:00:00 2001
From: Krisztian Litkey <krisztian.litkey@intel.com>
Date: Mon, 7 Aug 2023 10:54:37 +0300
Subject: [PATCH] server: implement UnaryServerInterceptor chaining.

Add a WithChainUnaryServerInterceptor server option to allow
using more that one server side interceptor which will then
get chained and invoked in the order given.

This should allow us to implement opentelemetry instrumentation
as interceptors while allowing users to keep intercepting their
server side calls for other reasons at the same time.

Signed-off-by: Krisztian Litkey <krisztian.litkey@intel.com>
---
 config.go           |  38 ++++++++++++++-
 interceptor_test.go | 110 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 146 insertions(+), 2 deletions(-)

diff --git a/config.go b/config.go
index 0974196..f401f67 100644
--- a/config.go
+++ b/config.go
@@ -16,7 +16,10 @@
 
 package ttrpc
 
-import "errors"
+import (
+	"context"
+	"errors"
+)
 
 type serverConfig struct {
 	handshaker  Handshaker
@@ -44,9 +47,40 @@ func WithServerHandshaker(handshaker Handshaker) ServerOpt {
 func WithUnaryServerInterceptor(i UnaryServerInterceptor) ServerOpt {
 	return func(c *serverConfig) error {
 		if c.interceptor != nil {
-			return errors.New("only one interceptor allowed per server")
+			return errors.New("only one unchained interceptor allowed per server")
 		}
 		c.interceptor = i
 		return nil
 	}
 }
+
+// WithChainUnaryServerInterceptor sets the provided chain of server interceptors
+func WithChainUnaryServerInterceptor(interceptors ...UnaryServerInterceptor) ServerOpt {
+	return func(c *serverConfig) error {
+		if len(interceptors) == 0 {
+			return nil
+		}
+		if c.interceptor != nil {
+			interceptors = append([]UnaryServerInterceptor{c.interceptor}, interceptors...)
+		}
+		c.interceptor = func(
+			ctx context.Context,
+			unmarshal Unmarshaler,
+			info *UnaryServerInfo,
+			method Method) (interface{}, error) {
+			return interceptors[0](ctx, unmarshal, info,
+				chainUnaryServerInterceptors(info, method, interceptors[1:]))
+		}
+		return nil
+	}
+}
+
+func chainUnaryServerInterceptors(info *UnaryServerInfo, method Method, interceptors []UnaryServerInterceptor) Method {
+	if len(interceptors) == 0 {
+		return method
+	}
+	return func(ctx context.Context, unmarshal func(interface{}) error) (interface{}, error) {
+		return interceptors[0](ctx, unmarshal, info,
+			chainUnaryServerInterceptors(info, method, interceptors[1:]))
+	}
+}
diff --git a/interceptor_test.go b/interceptor_test.go
index 39d82f8..47fa28f 100644
--- a/interceptor_test.go
+++ b/interceptor_test.go
@@ -133,3 +133,113 @@ func TestChainUnaryClientInterceptor(t *testing.T) {
 		t.Fatalf("unexpected test service reply: %q != %q", response.Foo, reply)
 	}
 }
+
+func TestUnaryServerInterceptor(t *testing.T) {
+	var (
+		intercepted = false
+		interceptor = func(ctx context.Context, unmarshal Unmarshaler, _ *UnaryServerInfo, method Method) (interface{}, error) {
+			intercepted = true
+			return method(ctx, unmarshal)
+		}
+
+		ctx             = context.Background()
+		server          = mustServer(t)(NewServer(WithUnaryServerInterceptor(interceptor)))
+		testImpl        = &testingServer{}
+		addr, listener  = newTestListener(t)
+		client, cleanup = newTestClient(t, addr)
+		message         = strings.Repeat("a", 16)
+		reply           = strings.Repeat(message, 2)
+	)
+
+	defer listener.Close()
+	defer cleanup()
+
+	registerTestingService(server, testImpl)
+
+	go server.Serve(ctx, listener)
+	defer server.Shutdown(ctx)
+
+	request := &internal.TestPayload{
+		Foo: message,
+	}
+	response := &internal.TestPayload{}
+	if err := client.Call(ctx, serviceName, "Test", request, response); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if !intercepted {
+		t.Fatalf("ttrpc server call not intercepted")
+	}
+
+	if response.Foo != reply {
+		t.Fatalf("unexpected test service reply: %q != %q", response.Foo, reply)
+	}
+}
+
+func TestChainUnaryServerInterceptor(t *testing.T) {
+	var (
+		orderIdx  = 0
+		recorded  = []string{}
+		intercept = func(idx int, tag string) UnaryServerInterceptor {
+			return func(ctx context.Context, unmarshal Unmarshaler, _ *UnaryServerInfo, method Method) (interface{}, error) {
+				if orderIdx != idx {
+					t.Fatalf("unexpected interceptor invocation order (%d != %d)", orderIdx, idx)
+				}
+				recorded = append(recorded, tag)
+				orderIdx++
+				return method(ctx, unmarshal)
+			}
+		}
+
+		ctx    = context.Background()
+		server = mustServer(t)(NewServer(
+			WithUnaryServerInterceptor(
+				intercept(0, "seen it"),
+			),
+			WithChainUnaryServerInterceptor(
+				intercept(1, "been"),
+				intercept(2, "there"),
+				intercept(3, "done"),
+				intercept(4, "that"),
+			),
+		))
+		expected = []string{
+			"seen it",
+			"been",
+			"there",
+			"done",
+			"that",
+		}
+		testImpl        = &testingServer{}
+		addr, listener  = newTestListener(t)
+		client, cleanup = newTestClient(t, addr)
+		message         = strings.Repeat("a", 16)
+		reply           = strings.Repeat(message, 2)
+	)
+
+	defer listener.Close()
+	defer cleanup()
+
+	registerTestingService(server, testImpl)
+
+	go server.Serve(ctx, listener)
+	defer server.Shutdown(ctx)
+
+	request := &internal.TestPayload{
+		Foo: message,
+	}
+	response := &internal.TestPayload{}
+
+	if err := client.Call(ctx, serviceName, "Test", request, response); err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if !reflect.DeepEqual(recorded, expected) {
+		t.Fatalf("unexpected ttrpc chained server unary interceptor order (%s != %s)",
+			strings.Join(recorded, " "), strings.Join(expected, " "))
+	}
+
+	if response.Foo != reply {
+		t.Fatalf("unexpected test service reply: %q != %q", response.Foo, reply)
+	}
+}