fix(channel): crash on failed sockconnect() (#37811)

Problem:  Crash on failed sockconnect() if a new connection is accepted
          while polling for uv events.
Solution: Don't use channel_destroy_early().

Also test "tcp" mode failure properly.

src/nvim/channel.c

@@ -474,7 +474,9 @@ uint64_t channel_connect(bool tcp, const char *address, bool rpc, CallbackReader
   channel = channel_alloc(kChannelStreamSocket);
   if (!socket_connect(&main_loop, &channel->stream.socket,
                       tcp, address, timeout, error)) {
-    channel_destroy_early(channel);
+    // Don't use channel_destroy_early() as new channels may have been allocated
+    // by channel_from_connection() while polling for uv events.
+    channel_decref(channel);
     return 0;
   }
 

test/functional/core/channels_spec.lua

@@ -478,9 +478,31 @@ 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, 'pipe', '127.0.0.1:0')
+        pcall_err(fn.sockconnect, 'tcp', '127.0.0.1:0')
+      )
+    end)
+
+    it('with another connection accepted while polling #37807', function()
+      local server = api.nvim_get_vvar('servername')
+      local invalid_pipe = n.new_pipename()
+      exec_lua(function()
+        vim.defer_fn(function()
+          vim.uv.sleep(50) -- Block the uv event loop.
+          vim.fn.sockconnect('pipe', invalid_pipe)
+        end, 10)
+      end)
+      vim.uv.sleep(20)
+      -- The server uv event loop is currently blocked, so the connection will
+      -- be accepted when sockconnect() polls.
+      local other_session = n.connect(server)
+      eq({ true, { 1000 } }, { other_session:request('nvim_list_wins') })
+      other_session:close()
+      matches(
+        '^vim.schedule callback: Vim:connection failed: connection refused\n',
+        api.nvim_get_vvar('errmsg')
       )
     end)
   end)