Fix hang in xcb_request_check()

This fixes https://gitlab.freedesktop.org/xorg/lib/libxcb/-/issues/53

The issue was that libxcb expected to get a reply based on the request_expected
variable, but a reply would never arrive because the request was never actually
written.  To resolve this, a separate request_expected_written variable is
added.

src/xcb_in.c

@@ -736,12 +736,17 @@ xcb_generic_error_t *xcb_request_check(xcb_connection_t *c, xcb_void_cookie_t co
         return 0;
     pthread_mutex_lock(&c->iolock);
     request = widen(c, cookie.sequence);
-    if(XCB_SEQUENCE_COMPARE(request, >=, c->in.request_expected)
-       && XCB_SEQUENCE_COMPARE(request, >, c->in.request_completed))
+    if (XCB_SEQUENCE_COMPARE(request, >, c->in.request_completed))
+    {
+        if(XCB_SEQUENCE_COMPARE(request, >=, c->in.request_expected))
         {
             _xcb_out_send_sync(c);
+        }
+        if (XCB_SEQUENCE_COMPARE(request, >=, c->out.request_expected_written))
+        {
             _xcb_out_flush_to(c, c->out.request);
         }
+    }
     reply = wait_for_reply(c, request, &ret);
     assert(!reply);
     pthread_mutex_unlock(&c->iolock);

src/xcb_out.c

@@ -447,6 +447,7 @@ int _xcb_out_init(_xcb_out *out)
 
     out->request = 0;
     out->request_written = 0;
+    out->request_expected_written = 0;
 
     if(pthread_mutex_init(&out->reqlenlock, 0))
         return 0;
@@ -467,6 +468,7 @@ int _xcb_out_send(xcb_connection_t *c, struct iovec *vector, int count)
     while(ret && count)
         ret = _xcb_conn_wait(c, &c->out.cond, &vector, &count);
     c->out.request_written = c->out.request;
+    c->out.request_expected_written = c->in.request_expected;
     pthread_cond_broadcast(&c->out.cond);
     _xcb_in_wake_up_next_reader(c);
     return ret;

src/xcbint.h

@@ -113,6 +113,7 @@ typedef struct _xcb_out {
 
     uint64_t request;
     uint64_t request_written;
+    uint64_t request_expected_written;
     uint64_t total_written;
 
     pthread_mutex_t reqlenlock;