dix: add per-screen pixmap destructor mechanism

Right now, extension specific pixmap destruction procedures are implemented
by wrapping the ScreenRec's DestroyPixmap() proc pointer: the extensions are
storing the original pointer in their private data and putting in their own one.
On each call, their proc restores the original one, calls it, and switches back
again. When multiple extensions doing so, they're forming a kind of daisy chain.
(the same is done for lots of other procs)

While that approach is looking nice and elegant on the drawing board, it's
complicated, dangerous like a chainsaw and makes debugging hard, leading to
pretty blurred API borders.

It's even getting worse: the proc also has to do ref counting, and only destroy
the pixmap if refconter reaching zero - that's all done in the individual screen
drivers. Therefore, all extensions must check for refcnt == 1, in order to know
when to really act.

This commit introduces a simple approach for letting extension hook into the
pixmap destruction safely, w/o having to care much about side effects with
the call chain. Extensions now can simply register their destructor proc
(and an opaque pointer) and get called back - w/o ever having to mess with
the ScreenRec's internal structures.

Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>

dix/dispatch.c

@@ -1447,6 +1447,8 @@ int
 dixDestroyPixmap(void *value, XID pid)
 {
     PixmapPtr pPixmap = (PixmapPtr) value;
+    if (pPixmap && pPixmap->refcnt == 1)
+        dixScreenRaisePixmapDestroy(pPixmap);
     if (pPixmap && pPixmap->drawable.pScreen && pPixmap->drawable.pScreen->DestroyPixmap)
         return pPixmap->drawable.pScreen->DestroyPixmap(pPixmap);
     return TRUE;

dix/dix_priv.h

@@ -307,6 +307,22 @@ void dixScreenRaiseWindowPosition(WindowPtr pWin, uint32_t x, uint32_t y);
  */
 void dixScreenRaiseClose(ScreenPtr pScreen);
 
+/*
+ * @brief call screen's PixmapDestroy hook
+ * @see dixScreenHookPixmapDestroy
+ * @param pPixmap the pixmap being destroyed
+ *
+ * Call the pluggable pixmap destroy pixmap hooks that extensions might have
+ * registered on the screen.
+ * Note that it's *only* called, when the pixmap is really being destroyed
+ * (instead of just unref'ed)
+ *
+ * Should only be called by DIX itself, by dixDestroyPixmap()
+ * It must be called *before* the ScreenRec->DestroyPixmap() is called, any
+ * only if the reference counter reaches 1.
+ */
+void dixScreenRaisePixmapDestroy(PixmapPtr pPixmap);
+
 /*
  * @brief mark event ID as critical
  * @param event the event to add to the critical events bitmap

dix/screen.c

@@ -23,5 +23,6 @@ void dixFreeScreen(ScreenPtr pScreen)
     DeleteCallbackList(&pScreen->hookWindowDestroy);
     DeleteCallbackList(&pScreen->hookWindowPosition);
     DeleteCallbackList(&pScreen->hookClose);
+    DeleteCallbackList(&pScreen->hookPixmapDestroy);
     free(pScreen);
 }

dix/screen_hooks.c

@@ -26,6 +26,7 @@
 DECLARE_HOOK_PROC(WindowDestroy, hookWindowDestroy, XorgScreenWindowDestroyProcPtr);
 DECLARE_HOOK_PROC(WindowPosition, hookWindowPosition, XorgScreenWindowPositionProcPtr);
 DECLARE_HOOK_PROC(Close, hookClose, XorgScreenCloseProcPtr);
+DECLARE_HOOK_PROC(PixmapDestroy, hookPixmapDestroy, XorgScreenPixmapDestroyProcPtr);
 
 int dixScreenRaiseWindowDestroy(WindowPtr pWin)
 {
@@ -67,3 +68,13 @@ void dixScreenRaiseClose(ScreenPtr pScreen) {
     if (pScreen->CloseScreen)
         pScreen->CloseScreen(pScreen);
 }
+
+void dixScreenRaisePixmapDestroy(PixmapPtr pPixmap)
+{
+    if (!pPixmap)
+        return;
+
+    ScreenPtr pScreen = pPixmap->drawable.pScreen;
+    CallCallbacks(&pScreen->hookPixmapDestroy, pPixmap);
+    /* we must not call the original ScreenRec->DestroyPixmap() here */
+}

dix/screen_hooks_priv.h

@@ -23,6 +23,7 @@
 #include <X11/Xfuncproto.h>
 
 #include "include/callback.h" /* CallbackListPtr */
+#include "include/pixmap.h" /* PixmapPtr */
 #include "include/screenint.h" /* ScreenPtr */
 #include "include/window.h" /* WindowPtr */
 
@@ -145,4 +146,43 @@ _X_EXPORT
 void dixScreenUnhookClose(ScreenPtr pScreen,
                           XorgScreenCloseProcPtr func);
 
+/* prototype of pixmap destroy notification handler */
+typedef void (*XorgScreenPixmapDestroyProcPtr)(CallbackListPtr *pcbl,
+                                               ScreenPtr pScreen,
+                                               PixmapPtr pPixmap);
+
+/**
+ * @brief register a pixmap destroy hook on the given screen
+ *
+ * @param pScreen pointer to the screen to register the notify hook into
+ * @param func pointer to the hook function
+ * @param arg opaque pointer passed to the hook
+ *
+ * When registration fails, the server aborts.
+ * This hook is called only when the pixmap is really to be destroyed,
+ * (unlike ScreenRec->DestroyPixmap())
+ *
+ * NOTE: only exported for libglamoregl, not supposed to be used by drivers.
+ **/
+_X_EXPORT
+void dixScreenHookPixmapDestroy(ScreenPtr pScreen,
+                                XorgScreenPixmapDestroyProcPtr func);
+
+/**
+ * @brief unregister a pixmap destroy notify hook on the given screen
+ *
+ * @param pScreen pointer to the screen to unregister the hook from
+ * @param func pointer to the hook function
+ * @param arg opaque pointer passed to the destructor
+ *
+ * @see dixScreenHookClose
+ *
+ * Unregister a screen close notify hook registered via @ref dixScreenHookPixmapDestroy
+ *
+ * NOTE: only exported for libglamoregl, not supposed to be used by drivers.
+ **/
+_X_EXPORT
+void dixScreenUnhookPixmapDestroy(ScreenPtr pScreen,
+                                  XorgScreenPixmapDestroyProcPtr func);
+
 #endif /* DIX_SCREEN_HOOKS_H */

include/scrnintstr.h

@@ -679,6 +679,10 @@ typedef struct _Screen {
     /* additional screen close notify hooks (replaces wrapping CloseScreen)
        should NOT be touched outside of DIX core */
     CallbackListPtr hookClose;
+
+    /* additional pixmap destroy notify hooks (replaces wrapping DestroyPixmap)
+       should NOT be touched outside of DIX core */
+    CallbackListPtr hookPixmapDestroy;
 } ScreenRec;
 
 static inline RegionPtr

include/xorg-server.h.meson.in

@@ -218,5 +218,6 @@
 #define XORG_API_DIX_SCREEN_HOOK_WINDOW_DESTROY 1
 #define XORG_API_DIX_SCREEN_HOOK_WINDOW_POSITION 1
 #define XORG_API_DIX_SCREEN_HOOK_CLOSE 1
+#define XORG_API_DIX_SCREEN_HOOK_PIXMAP_DESTROY 1
 
 #endif /* _XORG_SERVER_H_ */