dix: add per-screen window position notify hook

Right now, extension specific actions on window positioning are implemented
by wrapping the ScreenRec's PositionWindow() 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.

This commit introduces a simple approach for letting extension hook into the
window positioning path safely, w/o having to care much about side effects
with the call chain. Extensions now can simply register their hook proc
(and an opaque pointer) and get called back - w/o ever having to mess with
the ScreenRec's internal structures. These hooks are called before the original
vector (usually handled by DDX/screen driver directly) is called.

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

dix/dix_priv.h

@@ -283,6 +283,18 @@ void dixFreeScreen(ScreenPtr pScreen);
  */
 int dixScreenRaiseWindowDestroy(WindowPtr pWin);
 
+/*
+ * @brief call screen's window position notification hooks
+ * @see dixScreenHookWindowPosition
+ * @param pWin the window to notify on
+ *
+ * Call the pluggable window position hooks that extensions might have registered on
+ * the screen, and finally call ScreenRec's PositionWindow proc.
+ *
+ * Should only be called by DIX itself.
+ */
+void dixScreenRaiseWindowPosition(WindowPtr pWin, uint32_t x, uint32_t y);
+
 /*
  * @brief mark event ID as critical
  * @param event the event to add to the critical events bitmap

dix/screen.c

@@ -21,5 +21,6 @@ void dixFreeScreen(ScreenPtr pScreen)
     pScreen->CloseScreen(pScreen);
     dixFreePrivates(pScreen->devPrivates, PRIVATE_SCREEN);
     DeleteCallbackList(&pScreen->hookWindowDestroy);
+    DeleteCallbackList(&pScreen->hookWindowPosition);
     free(pScreen);
 }

dix/screen_hooks.c

@@ -24,6 +24,7 @@
     }
 
 DECLARE_HOOK_PROC(WindowDestroy, hookWindowDestroy, XorgScreenWindowDestroyProcPtr);
+DECLARE_HOOK_PROC(WindowPosition, hookWindowPosition, XorgScreenWindowPositionProcPtr);
 
 int dixScreenRaiseWindowDestroy(WindowPtr pWin)
 {
@@ -36,3 +37,19 @@ int dixScreenRaiseWindowDestroy(WindowPtr pWin)
 
     return (pScreen->DestroyWindow ? pScreen->DestroyWindow(pWin) : Success);
 }
+
+void dixScreenRaiseWindowPosition(WindowPtr pWin, uint32_t x, uint32_t y)
+{
+    ScreenPtr pScreen = pWin->drawable.pScreen;
+
+    XorgScreenWindowPositionParamRec param = {
+        .window = pWin,
+        .x = x,
+        .y = y,
+    };
+
+    CallCallbacks(&pScreen->hookWindowPosition, &param);
+
+    if (pScreen->PositionWindow)
+        pScreen->PositionWindow(pWin, x, y);
+}

dix/screen_hooks_priv.h

@@ -71,4 +71,42 @@ void dixScreenHookWindowDestroy(ScreenPtr pScreen,
 void dixScreenUnhookWindowDestroy(ScreenPtr pScreen,
                                   XorgScreenWindowDestroyProcPtr func);
 
+typedef struct {
+    WindowPtr window;
+    int32_t x;
+    int32_t y;
+} XorgScreenWindowPositionParamRec;
+
+/* prototype of a window move notification handler */
+typedef void (*XorgScreenWindowPositionProcPtr)(CallbackListPtr *pcbl,
+                                                ScreenPtr pScreen,
+                                                XorgScreenWindowPositionParamRec *param);
+
+/**
+ * @brief register a position notify hook on the given screen
+ *
+ * @param pScreen pointer to the screen to register the notify hook into
+ * @param func pointer to the window hook function
+ * @param arg opaque pointer passed to the hook
+ *
+ * When registration fails, the server aborts.
+ *
+ **/
+void dixScreenHookWindowPosition(ScreenPtr pScreen,
+                                           XorgScreenWindowPositionProcPtr func);
+
+/**
+ * @brief unregister a window position 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 dixScreenHookWindowPosition
+ *
+ * Unregister a window position notify hook registered via @ref dixScreenHookWindowPosition
+ **/
+void dixScreenUnhookWindowPosition(ScreenPtr pScreen,
+                                             XorgScreenWindowPositionProcPtr func);
+
 #endif /* DIX_SCREEN_HOOKS_H */

dix/window.c

@@ -668,7 +668,8 @@ InitRootWindow(WindowPtr pWin)
 
     if (!(*pScreen->CreateWindow) (pWin))
         return;                 /* XXX */
-    (*pScreen->PositionWindow) (pWin, 0, 0);
+
+    dixScreenRaiseWindowPosition(pWin, 0, 0);
 
     pWin->cursorIsNone = FALSE;
     pWin->optional->cursor = RefCursor(rootCursor);
@@ -926,7 +927,7 @@ CreateWindow(Window wid, WindowPtr pParent, int x, int y, unsigned w,
         return NullWindow;
     }
     /* We SHOULD check for an error value here XXX */
-    (*pScreen->PositionWindow) (pWin, pWin->drawable.x, pWin->drawable.y);
+    dixScreenRaiseWindowPosition(pWin, pWin->drawable.x, pWin->drawable.y);
 
     if (!(vmask & CWEventMask))
         RecalculateDeliverableEvents(pWin);
@@ -1846,12 +1847,9 @@ GravityTranslate(int x, int y, int oldx, int oldy,
 void
 ResizeChildrenWinSize(WindowPtr pWin, int dx, int dy, int dw, int dh)
 {
-    ScreenPtr pScreen;
     WindowPtr pSib, pChild;
     Bool resized = (dw || dh);
 
-    pScreen = pWin->drawable.pScreen;
-
     for (pSib = pWin->firstChild; pSib; pSib = pSib->nextSib) {
         if (resized && (pSib->winGravity > NorthWestGravity)) {
             int cwsx, cwsy;
@@ -1876,7 +1874,8 @@ ResizeChildrenWinSize(WindowPtr pWin, int dx, int dy, int dw, int dh)
         pSib->drawable.y = pWin->drawable.y + pSib->origin.y;
         SetWinSize(pSib);
         SetBorderSize(pSib);
-        (*pScreen->PositionWindow) (pSib, pSib->drawable.x, pSib->drawable.y);
+
+        dixScreenRaiseWindowPosition(pSib, pSib->drawable.x, pSib->drawable.y);
 
         if ((pChild = pSib->firstChild)) {
             while (1) {
@@ -1886,7 +1885,7 @@ ResizeChildrenWinSize(WindowPtr pWin, int dx, int dy, int dw, int dh)
                     pChild->origin.y;
                 SetWinSize(pChild);
                 SetBorderSize(pChild);
-                (*pScreen->PositionWindow) (pChild,
+                dixScreenRaiseWindowPosition(pChild,
                                              pChild->drawable.x,
                                              pChild->drawable.y);
                 if (pChild->firstChild) {
@@ -2582,7 +2581,8 @@ ReparentWindow(WindowPtr pWin, WindowPtr pParent,
 
     if (pScreen->ReparentWindow)
         (*pScreen->ReparentWindow) (pWin, pPriorParent);
-    (*pScreen->PositionWindow) (pWin, pWin->drawable.x, pWin->drawable.y);
+
+    dixScreenRaiseWindowPosition(pWin, pWin->drawable.x, pWin->drawable.y);
     ResizeChildrenWinSize(pWin, 0, 0, 0, 0);
 
     CheckWindowOptionalNeed(pWin);

include/scrnintstr.h

@@ -671,6 +671,10 @@ typedef struct _Screen {
     /* additional window destructors (replaces wrapping DestroyWindow).
        should NOT be touched outside of DIX core */
     CallbackListPtr hookWindowDestroy;
+
+    /* additional window position notify hooks (replaces wrapping PositionWindow)
+       should NOT be touched outside of DIX core */
+    CallbackListPtr hookWindowPosition;
 } ScreenRec;
 
 static inline RegionPtr

include/xorg-server.h.meson.in

@@ -216,5 +216,6 @@
 
 /* announce server API features */
 #define XORG_API_DIX_SCREEN_HOOK_WINDOW_DESTROY 1
+#define XORG_API_DIX_SCREEN_HOOK_WINDOW_POSITION 1
 
 #endif /* _XORG_SERVER_H_ */

mi/miwindow.c

@@ -277,7 +277,7 @@ miMoveWindow(WindowPtr pWin, int x, int y, WindowPtr pNextSib, VTKind kind)
     SetWinSize(pWin);
     SetBorderSize(pWin);
 
-    (*pScreen->PositionWindow) (pWin, x, y);
+    dixScreenRaiseWindowPosition(pWin, x, y);
 
     windowToValidate = MoveWindowInStack(pWin, pNextSib);
 
@@ -447,7 +447,7 @@ miResizeWindow(WindowPtr pWin, int x, int y, unsigned int w, unsigned int h,
     ResizeChildrenWinSize(pWin, x - oldx, y - oldy, dw, dh);
 
     /* let the hardware adjust background and border pixmaps, if any */
-    (*pScreen->PositionWindow) (pWin, x, y);
+    dixScreenRaiseWindowPosition(pWin, x, y);
 
     pFirstChange = MoveWindowInStack(pWin, pSib);