Check in unrelated fence code

* fence_ring.c: New file.

fence_ring.c

@@ -0,0 +1,145 @@
+/* Wayland compositor running on top of an X server.
+
+Copyright (C) 2022 to various contributors.
+
+This file is part of 12to11.
+
+12to11 is free software: you can redistribute it and/or modify it
+under the terms of the GNU General Public License as published by the
+Free Software Foundation, either version 3 of the License, or (at your
+option) any later version.
+
+12to11 is distributed in the hope that it will be useful, but WITHOUT
+ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with 12to11.  If not, see <https://www.gnu.org/licenses/>.  */
+
+#include <sys/fcntl.h>
+
+#include <stdio.h>
+
+#include "compositor.h"
+
+#include <X11/xshmfence.h>
+#include <X11/extensions/sync.h>
+
+#include <xcb/dri3.h>
+
+#define FenceFree	(1U << 31)
+
+struct _Fence
+{
+  /* The next and last fences on this list.  */
+  Fence *next;
+
+  /* The xshmfence.  */
+  struct xshmfence *fence;
+
+  /* The sync fence.  High bit is a flag meaning that the fence is on
+     the free list.  */
+  XSyncFence fence_id;
+
+  /* The number of references to this fence.  Incremented by
+     FenceRetain, decremented by FenceAwait.  */
+  int refcount;
+};
+
+/* Chain of all free fences.  */
+static Fence *all_fences;
+
+Fence *
+GetFence (void)
+{
+  Fence *fence;
+  int fd;
+  Window drawable;
+
+  drawable = DefaultRootWindow (compositor.display);
+
+  /* Get one free fence.  */
+
+  for (fence = all_fences; fence; fence = fence->next)
+    {
+      /* Unlink this fence.  */
+      all_fences = fence->next;
+      fence->next = NULL;
+
+      /* Mark the fence as used.  */
+      fence->fence_id &= ~FenceFree;
+
+      /* Return it.  */
+      return fence;
+    }
+
+  /* Otherwise, allocate a new fence.  */
+  fence = XLCalloc (1, sizeof *fence);
+  fd = xshmfence_alloc_shm ();
+
+  if (fd < 0)
+    {
+      perror ("xshmfence_alloc_shm");
+      abort ();
+    }
+
+  /* Map it.  */
+  fence->fence = xshmfence_map_shm (fd);
+
+  if (!fence->fence)
+    {
+      perror ("xshmfence_map_shm");
+      abort ();
+    }
+
+  /* Upload the fence to the X server.  XCB will close the file
+     descriptor.  */
+  fence->fence_id = xcb_generate_id (compositor.conn);
+
+  /* Make the file descriptor CLOEXEC, since it isn't closed
+     immediately.  */
+  XLAddFdFlag (fd, FD_CLOEXEC, False);
+  xcb_dri3_fence_from_fd (compositor.conn, drawable,
+			  fence->fence_id, 0, fd);
+
+  /* Return the fence.  */
+  return fence;
+}
+
+void
+FenceAwait (Fence *fence)
+{
+  XLAssert (fence->refcount);
+  fence->refcount -= 1;
+
+  if (!(fence->fence_id & FenceFree))
+    {
+      /* Wait for the fence to be triggered.  */
+      xshmfence_await (fence->fence);
+
+      /* Reset the fence.  */
+      xshmfence_reset (fence->fence);
+      fence->fence_id |= FenceFree;
+    }
+
+  if (fence->refcount)
+    return;
+
+  /* Now that the fence is no longer referenced, it can be put back on
+     the list of free fences.  */
+  fence->next = all_fences;
+  all_fences = fence;
+}
+
+void
+FenceRetain (Fence *fence)
+{
+  fence->refcount++;
+}
+
+XSyncFence
+FenceToXFence (Fence *fence)
+{
+  return fence->fence_id & ~FenceFree;
+}