notify: fix crash in notify_done() when terminal has been closed before notification

In server mode, if a terminal instance triggered a tracked
notification (e.g. a bell notification, or a kitty notification), then
the foot server crashed if the terminal instance was closed before the
notification.

This happened because the notify_done() callback's user supplied
argument was the terminal instance, and this pointer was not verified
before it was de-referenced.

Fix by searching the wayland global's list of terminal instances, and
ignore the callback if the wanted terminal instance is gone.

Closes #2397

CHANGELOG.md

@@ -114,6 +114,9 @@
   ([#2364][2364]).
 * Escape quotes in file names being DnD:ed on the command line
   ([#2363][2363]).
+* Crash in `--server` mode, when a tracked notification is closed
+  after the associated terminal instance has been closed
+  ([#2397][2397]).
 
 [2353]: https://codeberg.org/dnkl/foot/issues/2353
 [2352]: https://codeberg.org/dnkl/foot/issues/2352
@@ -122,6 +125,7 @@
 [2360]: https://codeberg.org/dnkl/foot/issues/2360
 [2370]: https://codeberg.org/dnkl/foot/issues/2370
 [2364]: https://codeberg.org/dnkl/foot/issues/2364
+[2397]: https://codeberg.org/dnkl/foot/issues/2397
 
 
 ### Security

notify.c

@@ -233,10 +233,31 @@ fdm_notify_stdout(struct fdm *fdm, int fd, int events, void *data)
     return true;
 }
 
+struct done_context {
+    struct wayland *wayl;
+    struct terminal *term;
+};
+
 static void
 notif_done(struct reaper *reaper, pid_t pid, int status, void *data)
 {
-    struct terminal *term = data;
+    struct done_context *ctx = data;
+    const struct wayland *wayl = ctx->wayl;
+    const struct terminal *wanted_term = ctx->term;
+    free(ctx);
+
+    struct terminal *term = NULL;
+    tll_foreach(wayl->terms, it) {
+        if (it->item == wanted_term) {
+            term = it->item;
+            break;
+        }
+    }
+
+    if (term == NULL) {
+        LOG_WARN("notification closed, but the associated terminal instance is already gone");
+        return;
+    }
 
     tll_foreach(term->active_notifications, it) {
         struct notification *notif = &it->item;
@@ -567,11 +588,18 @@ notify_notify(struct terminal *term, struct notification *notif)
                 &fdm_notify_stdout, (void *)term);
     }
 
+    struct done_context *ctx = NULL;
+    if (track_notification) {
+        ctx = xmalloc(sizeof(*ctx));
+        ctx->wayl = term->wl;
+        ctx->term = term;
+    }
+
     /* Redirect stdin to /dev/null, but ignore failure to open */
     int devnull = open("/dev/null", O_RDONLY);
     pid_t pid = spawn(
         term->reaper, NULL, argv, devnull, stdout_fds[1], -1,
-        track_notification ? &notif_done : NULL, (void *)term, NULL);
+        track_notification ? &notif_done : NULL, ctx, NULL);
 
     if (stdout_fds[1] >= 0) {
         /* Close write-end of stdout pipe */
@@ -649,9 +677,7 @@ notify_close(struct terminal *term, const char *id)
             }
 
             int devnull = open("/dev/null", O_RDONLY);
-            spawn(
+            spawn(term->reaper, NULL, argv, devnull, -1, -1, NULL, NULL, NULL);
-                term->reaper, NULL, argv, devnull, -1, -1,
-                NULL, (void *)term, NULL);
 
             if (devnull >= 0)
                 close(devnull);