fix(channel): possible hang after connecting with TCP times out (#37813)

Problem:  Possible hang after connecting with TCP times out.
Solution: Wait for the close callback to arrive.

src/nvim/event/socket.c

@@ -79,6 +79,13 @@ int socket_watcher_init(Loop *loop, SocketWatcher *watcher, const char *endpoint
   return 0;
 }
 
+/// Callback for closing a handle initialized by socket_connect().
+static void connect_close_cb(uv_handle_t *handle)
+{
+  bool *closed = handle->data;
+  *closed = true;
+}
+
 int socket_watcher_start(SocketWatcher *watcher, int backlog, socket_cb cb)
   FUNC_ATTR_NONNULL_ALL
 {
@@ -194,7 +201,7 @@ static void connect_cb(uv_connect_t *req, int status)
   *ret_status = status;
   uv_handle_t *handle = (uv_handle_t *)req->handle;
   if (status != 0 && !uv_is_closing(handle)) {
-    uv_close(handle, NULL);
+    uv_close(handle, connect_close_cb);
   }
 }
 
@@ -202,6 +209,7 @@ bool socket_connect(Loop *loop, RStream *stream, bool is_tcp, const char *addres
                     const char **error)
 {
   bool success = false;
+  bool closed;
   int status;
   uv_connect_t req;
   req.data = &status;
@@ -243,21 +251,21 @@ tcp_retry:
     uv_pipe_connect(&req,  pipe, address, connect_cb);
     uv_stream = (uv_stream_t *)pipe;
   }
+  uv_stream->data = &closed;
+  closed = false;
   status = 1;
   LOOP_PROCESS_EVENTS_UNTIL(&main_loop, NULL, timeout, status != 1);
   if (status == 0) {
     stream_init(NULL, &stream->s, -1, uv_stream);
+    assert(uv_stream->data != &closed);  // Should have been set by stream_init().
     success = true;
   } else {
     if (!uv_is_closing((uv_handle_t *)uv_stream)) {
-      uv_close((uv_handle_t *)uv_stream, NULL);
+      uv_close((uv_handle_t *)uv_stream, connect_close_cb);
-      if (status == 1) {
-        // The uv_close() above will make libuv call connect_cb() with UV_ECANCELED.
-        // Make sure connect_cb() has been called here, as if it's called after this
-        // function ends it will cause a stack-use-after-scope.
-        LOOP_PROCESS_EVENTS_UNTIL(&main_loop, NULL, -1, status != 1);
-      }
     }
+    // Wait for the close callback to arrive before retrying or returning, otherwise
+    // it may lead to a hang or stack-use-after-return.
+    LOOP_PROCESS_EVENTS_UNTIL(&main_loop, NULL, -1, closed);
 
     if (is_tcp && addrinfo->ai_next) {
       addrinfo = addrinfo->ai_next;

test/functional/core/channels_spec.lua

@@ -475,7 +475,6 @@ describe('channels', function()
     end)
 
     it('in "tcp" mode', function()
-      skip(not is_os('linux'), 'FIXME: hangs on non-Linux')
       eq(
         'Vim:connection failed: connection refused',
         pcall_err(fn.sockconnect, 'tcp', '127.0.0.1:0')