Fix #1041

httplib.h

@@ -2337,6 +2337,12 @@ inline bool wait_until_socket_is_ready(socket_t sock, time_t sec, time_t usec) {
 #endif
 }
 
+inline bool is_socket_alive(socket_t sock) {
+  char buf[1];
+  return detail::select_read(sock, 0, 0) &&
+         recv(sock, &buf[0], sizeof(buf), MSG_PEEK) != 0;
+}
+
 class SocketStream : public Stream {
 public:
   SocketStream(socket_t sock, time_t read_timeout_sec, time_t read_timeout_usec,
@@ -5723,13 +5729,14 @@ inline bool ClientImpl::send(Request &req, Response &res, Error &error) {
 
   {
     std::lock_guard<std::mutex> guard(socket_mutex_);
+
     // Set this to false immediately - if it ever gets set to true by the end of
     // the request, we know another thread instructed us to close the socket.
     socket_should_be_closed_when_request_is_done_ = false;
 
     auto is_alive = false;
     if (socket_.is_open()) {
-      is_alive = detail::select_write(socket_.sock, 0, 0) > 0;
+      is_alive = detail::is_socket_alive(socket_.sock);
       if (!is_alive) {
         // Attempt to avoid sigpipe by shutting down nongracefully if it seems
         // like the other side has already closed the connection Also, there

test/test.cc

@@ -3756,6 +3756,36 @@ TEST(KeepAliveTest, ReadTimeout) {
   ASSERT_FALSE(svr.is_running());
 }
 
+TEST(KeepAliveTest, Issue1041) {
+  const auto resourcePath = "/hi";
+
+  Server svr;
+  svr.set_keep_alive_timeout(3);
+
+  svr.Get(resourcePath, [](const httplib::Request &, httplib::Response &res) {
+    res.set_content("Hello World!", "text/plain");
+  });
+
+  auto a2 = std::async(std::launch::async, [&svr]{ svr.listen(HOST, PORT); });
+  std::this_thread::sleep_for(std::chrono::milliseconds(200));
+
+  Client cli(HOST, PORT);
+  cli.set_keep_alive(true);
+
+  auto result = cli.Get(resourcePath);
+  ASSERT_TRUE(result);
+  EXPECT_EQ(200, result->status);
+
+  std::this_thread::sleep_for(std::chrono::seconds(5));
+
+  result = cli.Get(resourcePath);
+  ASSERT_TRUE(result);
+  EXPECT_EQ(200, result->status);
+
+  svr.stop();
+  a2.wait();
+}
+
 TEST(ClientProblemDetectionTest, ContentProvider) {
   Server svr;