Add xcb_xlib_lock and xcb_xlib_unlock, a special-purpose two-level recursive lock just for libX11.

src/xcb_conn.c

@@ -59,6 +59,18 @@ static int set_fd_flags(const int fd)
     return 1;
 }
 
+static int _xcb_xlib_init(_xcb_xlib *xlib)
+{
+    xlib->lock = 0;
+    pthread_cond_init(&xlib->cond, 0);
+    return 1;
+}
+
+static void _xcb_xlib_destroy(_xcb_xlib *xlib)
+{
+    pthread_cond_destroy(&xlib->cond);
+}
+
 static int write_setup(xcb_connection_t *c, xcb_auth_info_t *auth_info)
 {
     static const char pad[3];
@@ -215,6 +227,7 @@ xcb_connection_t *xcb_connect_to_fd(int fd, xcb_auth_info_t *auth_info)
     if(!(
         set_fd_flags(fd) &&
         pthread_mutex_init(&c->iolock, 0) == 0 &&
+        _xcb_xlib_init(&c->xlib) &&
         _xcb_in_init(&c->in) &&
         _xcb_out_init(&c->out) &&
         write_setup(c, auth_info) &&
@@ -239,6 +252,7 @@ void xcb_disconnect(xcb_connection_t *c)
     close(c->fd);
 
     pthread_mutex_destroy(&c->iolock);
+    _xcb_xlib_destroy(&c->xlib);
     _xcb_in_destroy(&c->in);
     _xcb_out_destroy(&c->out);
 
@@ -258,6 +272,12 @@ void _xcb_conn_shutdown(xcb_connection_t *c)
 void _xcb_lock_io(xcb_connection_t *c)
 {
     pthread_mutex_lock(&c->iolock);
+    while(c->xlib.lock)
+    {
+        if(pthread_equal(c->xlib.thread, pthread_self()))
+            break;
+        pthread_cond_wait(&c->xlib.cond, &c->iolock);
+    }
 }
 
 void _xcb_unlock_io(xcb_connection_t *c)

src/xcb_xlib.c

@@ -26,6 +26,8 @@
 #include "xcbxlib.h"
 #include "xcbint.h"
 
+#include <assert.h>
+
 unsigned int xcb_get_request_sent(xcb_connection_t *c)
 {
     if(c->has_error)
@@ -39,3 +41,22 @@ pthread_mutex_t *xcb_get_io_lock(xcb_connection_t *c)
         return 0;
     return &c->iolock;
 }
+
+void xcb_xlib_lock(xcb_connection_t *c)
+{
+    _xcb_lock_io(c);
+    assert(!c->xlib.lock);
+    c->xlib.lock = 1;
+    c->xlib.thread = pthread_self();
+    _xcb_unlock_io(c);
+}
+
+void xcb_xlib_unlock(xcb_connection_t *c)
+{
+    _xcb_lock_io(c);
+    assert(c->xlib.lock);
+    assert(pthread_equal(c->xlib.thread, pthread_self()));
+    c->xlib.lock = 0;
+    pthread_cond_broadcast(&c->xlib.cond);
+    _xcb_unlock_io(c);
+}

src/xcbint.h

@@ -113,6 +113,15 @@ int _xcb_in_read(xcb_connection_t *c);
 int _xcb_in_read_block(xcb_connection_t *c, void *buf, int nread);
 
 
+/* xcb_xlib.c */
+
+typedef struct _xcb_xlib {
+    int lock;
+    pthread_t thread;
+    pthread_cond_t cond;
+} _xcb_xlib;
+
+
 /* xcb_xid.c */
 
 typedef struct _xcb_xid {
@@ -150,6 +159,7 @@ struct xcb_connection_t {
 
     /* I/O data */
     pthread_mutex_t iolock;
+    _xcb_xlib xlib;
     _xcb_in in;
     _xcb_out out;
 
@@ -159,8 +169,6 @@ struct xcb_connection_t {
 };
 
 void _xcb_conn_shutdown(xcb_connection_t *c);
-void _xcb_lock_io(xcb_connection_t *c);
-void _xcb_unlock_io(xcb_connection_t *c);
 int _xcb_conn_wait(xcb_connection_t *c, pthread_cond_t *cond, struct iovec **vector, int *count);
 
 
@@ -172,4 +180,10 @@ int _xcb_get_auth_info(int fd, xcb_auth_info_t *info);
 #pragma GCC visibility pop
 #endif
 
+
+/* xcb_conn.c symbols visible to xcb-xlib */
+
+void _xcb_lock_io(xcb_connection_t *c);
+void _xcb_unlock_io(xcb_connection_t *c);
+
 #endif

src/xcbxlib.h

@@ -36,4 +36,7 @@ unsigned int xcb_get_request_sent(xcb_connection_t *c);
 
 pthread_mutex_t *xcb_get_io_lock(xcb_connection_t *c);
 
+void xcb_xlib_lock(xcb_connection_t *c);
+void xcb_xlib_unlock(xcb_connection_t *c);
+
 #endif