Re-raise i3lock when obscured in a separate process

This is cleaner than commit a305e62 (running authentication in a
separate process) because we don’t lose the feature that X11 will buffer
KeyPresses for us while the process blocks on PAM. See
http://cr.i3wm.org/patch/305 for the amount of code (and new known bugs)
that would have to be introduced to make a305e62 work.

i3lock.c

@@ -364,7 +364,8 @@ static void handle_key_press(xcb_key_press_event_t *event) {
  * hiding us) gets hidden.
  *
  */
-static void handle_visibility_notify(xcb_visibility_notify_event_t *event) {
+static void handle_visibility_notify(xcb_connection_t *conn,
+    xcb_visibility_notify_event_t *event) {
     if (event->state != XCB_VISIBILITY_UNOBSCURED) {
         uint32_t values[] = { XCB_STACK_MODE_ABOVE };
         xcb_configure_window(conn, event->window, XCB_CONFIG_WINDOW_STACK_MODE, values);
@@ -501,7 +502,7 @@ static void xcb_check_cb(EV_P_ ev_check *w, int revents) {
                 break;
 
             case XCB_VISIBILITY_NOTIFY:
-                handle_visibility_notify((xcb_visibility_notify_event_t*)event);
+                handle_visibility_notify(conn, (xcb_visibility_notify_event_t*)event);
                 break;
 
             case XCB_MAP_NOTIFY:
@@ -531,6 +532,63 @@ static void xcb_check_cb(EV_P_ ev_check *w, int revents) {
     }
 }
 
+/*
+ * This function is called from a fork()ed child and will raise the i3lock
+ * window when the window is obscured, even when the main i3lock process is
+ * blocked due to PAM.
+ *
+ */
+static void raise_loop(xcb_window_t window) {
+    xcb_connection_t *conn;
+    xcb_generic_event_t *event;
+    int screens;
+
+    if ((conn = xcb_connect(NULL, &screens)) == NULL ||
+        xcb_connection_has_error(conn))
+        errx(EXIT_FAILURE, "Cannot open display\n");
+
+    /* We need to know about the window being obscured or getting destroyed. */
+    xcb_change_window_attributes(conn, window, XCB_CW_EVENT_MASK,
+        (uint32_t[]){
+            XCB_EVENT_MASK_VISIBILITY_CHANGE |
+            XCB_EVENT_MASK_STRUCTURE_NOTIFY
+        });
+    xcb_flush(conn);
+
+    DEBUG("Watching window 0x%08x\n", window);
+    while ((event = xcb_wait_for_event(conn)) != NULL) {
+        if (event->response_type == 0) {
+            xcb_generic_error_t *error = (xcb_generic_error_t*)event;
+            DEBUG("X11 Error received! sequence 0x%x, error_code = %d\n",
+                 error->sequence, error->error_code);
+            free(event);
+            continue;
+        }
+        /* Strip off the highest bit (set if the event is generated) */
+        int type = (event->response_type & 0x7F);
+        DEBUG("Read event of type %d\n", type);
+        switch (type) {
+            case XCB_VISIBILITY_NOTIFY:
+                handle_visibility_notify(conn, (xcb_visibility_notify_event_t*)event);
+                break;
+            case XCB_UNMAP_NOTIFY:
+                DEBUG("UnmapNotify for 0x%08x\n", (((xcb_unmap_notify_event_t*)event)->window));
+                if (((xcb_unmap_notify_event_t*)event)->window == window)
+                    exit(EXIT_SUCCESS);
+                break;
+            case XCB_DESTROY_NOTIFY:
+                DEBUG("DestroyNotify for 0x%08x\n", (((xcb_destroy_notify_event_t*)event)->window));
+                if (((xcb_destroy_notify_event_t*)event)->window == window)
+                    exit(EXIT_SUCCESS);
+                break;
+            default:
+                DEBUG("Unhandled event type %d\n", type);
+                break;
+        }
+        free(event);
+    }
+}
+
 int main(int argc, char *argv[]) {
     char *username;
     char *image_path = NULL;
@@ -692,6 +750,17 @@ int main(int argc, char *argv[]) {
     win = open_fullscreen_window(conn, screen, color, bg_pixmap);
     xcb_free_pixmap(conn, bg_pixmap);
 
+    pid_t pid = fork();
+    /* The pid == -1 case is intentionally ignored here:
+     * While the child process is useful for preventing other windows from
+     * popping up while i3lock blocks, it is not critical. */
+    if (pid == 0) {
+        /* Child */
+        close(xcb_get_file_descriptor(conn));
+        raise_loop(win);
+        exit(EXIT_SUCCESS);
+    }
+
     cursor = create_cursor(conn, screen, win, curs_choice);
 
     grab_pointer_and_keyboard(conn, screen, cursor);