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wmsetbg: fix "X connection broken" crash when setting background

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setPixmapProperty() called XKillClient(dpy, old_pixmap) to free the
previous background pixmap stored in _XROOTPMAP_ID.  The pixmap was
created by duplicatePixmap() via a short-lived second X connection
that used XSetCloseDownMode(RetainPermanent).  After that connection
closed, the X server eventually recycled its client ID for a new
connection.  If wmsetbg's own 'dpy' connection received that recycled
ID, XKillClient() would kill wmsetbg itself, causing the fatal error:

  X connection to :0 broken (explicit kill or server shutdown).

Fix by switching from RetainPermanent to RetainTemporary in
duplicatePixmap().  With RetainTemporary, background pixmaps are kept
alive for the lifetime of the X session and freed automatically at
logout — no explicit XKillClient() is needed.

Remove the now-dead XKillClient() block from setPixmapProperty():
- dummyErrorHandler() was only used to suppress its X errors; remove.
- The mode variable toggled PropModeReplace/PropModeAppend based on
  whether an old pixmap was present; since we always want to replace,
  use PropModeReplace unconditionally.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
This commit is contained in:
Haroldo Santos
2026-06-15 08:20:24 -04:00
committed by Carlos R. Mafra
parent 7c80e917d7
commit 6c107e0ce5
1 changed files with 16 additions and 23 deletions
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util/wmsetbg.c
+16 -23
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@@ -781,8 +781,12 @@ static Pixmap duplicatePixmap(Pixmap pixmap, int width, int height)
Display *tmpDpy;
Pixmap copyP;
/* must open a new display or the RetainPermanent will
* leave stuff allocated in RContext unallocated after exit */
/* Open a separate connection so the pixmap survives our exit.
* RetainTemporary (not RetainPermanent) is used intentionally:
* the pixmap lives until the X session ends, avoiding the need
* for XKillClient() on old pixmaps (which can crash when the
* X server reuses a client ID that now belongs to our own
* connection, causing "X connection broken"). */
tmpDpy = XOpenDisplay(display);
if (!tmpDpy) {
wwarning("could not open display to update background image information");
@@ -795,21 +799,13 @@ static Pixmap duplicatePixmap(Pixmap pixmap, int width, int height)
XCopyArea(tmpDpy, pixmap, copyP, DefaultGC(tmpDpy, scr), 0, 0, width, height, 0, 0);
XSync(tmpDpy, False);
XSetCloseDownMode(tmpDpy, RetainPermanent);
XSetCloseDownMode(tmpDpy, RetainTemporary);
XCloseDisplay(tmpDpy);
}
return copyP;
}
static int dummyErrorHandler(Display * dpy, XErrorEvent * err)
{
/* Parameter not used, but tell the compiler that it is ok */
(void) dpy;
(void) err;
return 0;
}
static void setPixmapProperty(Pixmap pixmap)
{
@@ -818,7 +814,6 @@ static void setPixmapProperty(Pixmap pixmap)
int format;
unsigned long length, after;
unsigned char *data;
int mode;
if (!prop) {
prop = XInternAtom(dpy, "_XROOTPMAP_ID", False);
@@ -826,23 +821,21 @@ static void setPixmapProperty(Pixmap pixmap)
XGrabServer(dpy);
/* Clear out the old pixmap */
/* Read and discard the old property data. We no longer call
* XKillClient() on the old pixmap: since duplicatePixmap() uses
* RetainTemporary, old pixmaps are freed automatically when the
* X session ends. Calling XKillClient() here was unsafe because
* the X server reuses client IDs; if the stored resource ID was
* reassigned to our own connection the server would kill us,
* producing "X connection to :0 broken". */
XGetWindowProperty(dpy, root, prop, 0L, 1L, False, AnyPropertyType,
&type, &format, &length, &after, &data);
if (data)
XFree(data);
if ((type == XA_PIXMAP) && (format == 32) && (length == 1)) {
XSetErrorHandler(dummyErrorHandler);
XKillClient(dpy, *((Pixmap *) data));
XSync(dpy, False);
XSetErrorHandler(NULL);
mode = PropModeReplace;
} else {
mode = PropModeAppend;
}
if (pixmap)
XChangeProperty(dpy, root, prop, XA_PIXMAP, 32, mode, (unsigned char *)&pixmap, 1);
XChangeProperty(dpy, root, prop, XA_PIXMAP, 32, PropModeReplace,
(unsigned char *)&pixmap, 1);
else
XDeleteProperty(dpy, root, prop);