Support handing off socket write permission to external code.

Libraries like Xlib, some XCB language bindings, and potentially others
have a common problem: they want to share the X connection with XCB. This
requires coordination of request sequence numbers.  Previously, XCB had an
Xlib-specific lock, and allowed Xlib to block XCB from making requests.
Now we've replaced that lock with a handoff mechanism, xcb_take_socket,
allowing external code to ask XCB for permission to take over the write
side of the socket and send raw data with xcb_writev.  The caller of
xcb_take_socket must supply a callback which XCB can call when it wants
the write side of the socket back to make a request.  This callback
synchronizes with the external socket owner, flushes any output queues if
appropriate, and then returns the sequence number of the last request sent
over the socket.

Commit by Josh Triplett and Jamey Sharp.
Handoff mechanism inspired by Keith Packard.

1 files changed, 59 insertions, 0 deletions

diff --git a/src/xcb_out.c b/src/xcb_out.c
index 1094ceb..4c6ab13 100644
--- a/src/xcb_out.c
+++ b/src/xcb_out.c
@@ -55,6 +55,25 @@ static int write_block(xcb_connection_t *c, struct iovec *vector, int count)
     return _xcb_out_send(c, &vector, &count);
 }
 
+static void get_socket_back(xcb_connection_t *c)
+{
+    while(c->out.return_socket && c->out.socket_moving)
+        pthread_cond_wait(&c->out.socket_cond, &c->iolock);
+    if(!c->out.return_socket)
+        return;
+
+    c->out.socket_moving = 1;
+    pthread_mutex_unlock(&c->iolock);
+    c->out.return_socket(c->out.socket_closure);
+    pthread_mutex_lock(&c->iolock);
+    c->out.socket_moving = 0;
+
+    pthread_cond_broadcast(&c->out.socket_cond);
+    c->out.return_socket = 0;
+    c->out.socket_closure = 0;
+    _xcb_in_replies_done(c);
+}
+
 /* Public interface */
 
 void xcb_prefetch_maximum_request_length(xcb_connection_t *c)
@@ -191,6 +210,7 @@ unsigned int xcb_send_request(xcb_connection_t *c, int flags, struct iovec *vect
     /* wait for other writing threads to get out of my way. */
     while(c->out.writing)
         pthread_cond_wait(&c->out.cond, &c->iolock);
+    get_socket_back(c);
 
     request = ++c->out.request;
     /* send GetInputFocus (sync_req) when 64k-2 requests have been sent without
@@ -235,6 +255,39 @@ unsigned int xcb_send_request(xcb_connection_t *c, int flags, struct iovec *vect
     return request;
 }
 
+int xcb_take_socket(xcb_connection_t *c, void (*return_socket)(void *closure), void *closure, int flags, uint64_t *sent)
+{
+    int ret;
+    if(c->has_error)
+        return 0;
+    pthread_mutex_lock(&c->iolock);
+    get_socket_back(c);
+    ret = _xcb_out_flush_to(c, c->out.request);
+    if(ret)
+    {
+        c->out.return_socket = return_socket;
+        c->out.socket_closure = closure;
+        if(flags)
+            _xcb_in_expect_reply(c, c->out.request, WORKAROUND_EXTERNAL_SOCKET_OWNER, flags);
+        assert(c->out.request == c->out.request_written);
+        *sent = c->out.request;
+    }
+    pthread_mutex_unlock(&c->iolock);
+    return ret;
+}
+
+int xcb_writev(xcb_connection_t *c, struct iovec *vector, int count, uint64_t requests)
+{
+    int ret;
+    if(c->has_error)
+        return 0;
+    pthread_mutex_lock(&c->iolock);
+    c->out.request += requests;
+    ret = _xcb_out_send(c, &vector, &count);
+    pthread_mutex_unlock(&c->iolock);
+    return ret;
+}
+
 int xcb_flush(xcb_connection_t *c)
 {
     int ret;
@@ -250,6 +303,12 @@ int xcb_flush(xcb_connection_t *c)
 
 int _xcb_out_init(_xcb_out *out)
 {
+    if(pthread_cond_init(&out->socket_cond, 0))
+        return 0;
+    out->return_socket = 0;
+    out->socket_closure = 0;
+    out->socket_moving = 0;
+
     if(pthread_cond_init(&out->cond, 0))
         return 0;
     out->writing = 0;