Merge bitcoin#30675: http: set TCP_NODELAY when creating HTTP server

03d49d0 http: set TCP_NODELAY when creating HTTP server (Roman Zeyde)

Pull request description:

  Otherwise, the default HTTP server config may result in high latency, due to Nagle's algorithm (on the server) and delayed ACK (on the client):

  [1] https://www.extrahop.com/blog/tcp-nodelay-nagle-quickack-best-practices
  [2] https://eklitzke.org/the-caveats-of-tcp-nodelay

  Without the fix, fetching a small block takes ~40ms (when connection keep-alive is enabled):
  ```
  $ ab -k -c 1 -n 100 http://localhost:8332/rest/block/00000000000002b5898f7cdc80d9c84e9747bc6b9388cc989971d443f05713ee.bin

  Server Software:
  Server Hostname:        localhost
  Server Port:            8332

  Document Path:          /rest/block/00000000000002b5898f7cdc80d9c84e9747bc6b9388cc989971d443f05713ee.bin
  Document Length:        25086 bytes

  Concurrency Level:      1
  Time taken for tests:   4.075 seconds
  Complete requests:      100
  Failed requests:        0
  Keep-Alive requests:    100
  Total transferred:      2519200 bytes
  HTML transferred:       2508600 bytes
  Requests per second:    24.54 [#/sec] (mean)
  Time per request:       40.747 [ms] (mean)
  Time per request:       40.747 [ms] (mean, across all concurrent requests)
  Transfer rate:          603.76 [Kbytes/sec] received

  Connection Times (ms)
                min  mean[+/-sd] median   max
  Connect:        0    0   0.0      0       0
  Processing:     0   41   4.1     41      42
  Waiting:        0    0   0.1      0       1
  Total:          0   41   4.1     41      42

  Percentage of the requests served within a certain time (ms)
    50%     41
    66%     41
    75%     41
    80%     41
    90%     42
    95%     42
    98%     42
    99%     42
   100%     42 (longest request)
  ```

  With the fix, it takes ~0.2ms:
  ```
  $ ab -k -c 1 -n 1000 http://localhost:8332/rest/block/00000000000002b5898f7cdc80d9c84e9747bc6b9388cc989971d443f05713ee.bin

  Benchmarking localhost (be patient)
  Completed 100 requests
  Completed 200 requests
  Completed 300 requests
  Completed 400 requests
  Completed 500 requests
  Completed 600 requests
  Completed 700 requests
  Completed 800 requests
  Completed 900 requests
  Completed 1000 requests
  Finished 1000 requests

  Server Software:
  Server Hostname:        localhost
  Server Port:            8332

  Document Path:          /rest/block/00000000000002b5898f7cdc80d9c84e9747bc6b9388cc989971d443f05713ee.bin
  Document Length:        25086 bytes

  Concurrency Level:      1
  Time taken for tests:   0.194 seconds
  Complete requests:      1000
  Failed requests:        0
  Keep-Alive requests:    1000
  Total transferred:      25192000 bytes
  HTML transferred:       25086000 bytes
  Requests per second:    5147.05 [#/sec] (mean)
  Time per request:       0.194 [ms] (mean)
  Time per request:       0.194 [ms] (mean, across all concurrent requests)
  Transfer rate:          126625.50 [Kbytes/sec] received

  Connection Times (ms)
                min  mean[+/-sd] median   max
  Connect:        0    0   0.0      0       0
  Processing:     0    0   0.0      0       0
  Waiting:        0    0   0.0      0       0
  Total:          0    0   0.0      0       0

  Percentage of the requests served within a certain time (ms)
    50%      0
    66%      0
    75%      0
    80%      0
    90%      0
    95%      0
    98%      0
    99%      0
   100%      0 (longest request)
  ```

ACKs for top commit:
  achow101:
    ACK 03d49d0
  theStack:
    re-ACK 03d49d0
  tdb3:
    ACK 03d49d0

Tree-SHA512: bbf3d78b8521f569430850ec4315a75711303547df1a3de213a4ad34c9700105e374e0a649352fd05f8e4badb5b59debd3720e1c5d392c5113d7816648f7fcaa

src/httpserver.cpp

@@ -388,6 +388,12 @@ static bool HTTPBindAddresses(struct evhttp* http)
             if (i->first.empty() || (addr.has_value() && addr->IsBindAny())) {
                 LogPrintf("WARNING: the RPC server is not safe to expose to untrusted networks such as the public internet\n");
             }
+            // Set the no-delay option (disable Nagle's algorithm) on the TCP socket.
+            evutil_socket_t fd = evhttp_bound_socket_get_fd(bind_handle);
+            int one = 1;
+            if (setsockopt(fd, IPPROTO_TCP, TCP_NODELAY, (sockopt_arg_type)&one, sizeof(one)) == SOCKET_ERROR) {
+                LogInfo("WARNING: Unable to set TCP_NODELAY on RPC server socket, continuing anyway\n");
+            }
             boundSockets.push_back(bind_handle);
         } else {
             LogPrintf("Binding RPC on address %s port %i failed.\n", i->first, i->second);