library: fix xcursor-related crash

X11 usually doesn't expect to be running user code from inside the
XFlush function, so, when libX11 ran an xcb function, hit my hook, and
my hook called another X11 function, this resulted in libX11 locking a
mutex that it had already locked for XFlush.

src/library/so/main.c

@@ -68,12 +68,16 @@ static xcb_get_geometry_reply_t* (*real_xcb_get_geometry_reply)(xcb_connection_t
 
 static int (*real_XDefineCursor)(void*, XWindow, XCursor) = NULL;
 static int (*real_XUndefineCursor)(void*, XWindow) = NULL;
+static int (*real_XFreeCursor)(void*, XCursor) = NULL;
+static int (*real_XFlush)(void*) = NULL;
 
 static struct GLLibFunctions libgl = {0};
 
 static void* xcursor_display = NULL;
 static XCursor xcursor_cursor;
 static uint8_t xcursor_defined = false;
+static uint8_t xcursor_change_pending = 0;
+static uint8_t xcursor_is_in_xflush = false;
 
 static ElfW(Word) _bolt_hash_elf(const char* name) {
 	ElfW(Word) tmp, hash = 0;
@@ -234,6 +238,10 @@ static void _bolt_init_libx11(unsigned long addr, const Elf32_Word* gnu_hash_tab
     if (sym) real_XDefineCursor = sym->st_value + libx11_addr;
     sym = _bolt_lookup_symbol("XUndefineCursor", gnu_hash_table, hash_table, string_table, symbol_table);
     if (sym) real_XUndefineCursor = sym->st_value + libx11_addr;
+    sym = _bolt_lookup_symbol("XFreeCursor", gnu_hash_table, hash_table, string_table, symbol_table);
+    if (sym) real_XFreeCursor = sym->st_value + libx11_addr;
+    sym = _bolt_lookup_symbol("XFlush", gnu_hash_table, hash_table, string_table, symbol_table);
+    if (sym) real_XFlush = sym->st_value + libx11_addr;
 }
 
 static int _bolt_dl_iterate_callback(struct dl_phdr_info* info, size_t size, void* args) {
@@ -445,9 +453,11 @@ static uint8_t handle_mouse_event(int16_t x, int16_t y, uint32_t detail, ptrdiff
     struct MouseEvent event;
     _bolt_mouseevent_from_xcb(x, y, detail, &event);
     const uint8_t mousein_fake_old = mousein_fake;
-    uint8_t ret = _bolt_plugin_handle_mouse_event(&event, bool_offset, event_offset, grab_type, &mousein_fake, &mousein_real);
+    const uint8_t ret = _bolt_plugin_handle_mouse_event(&event, bool_offset, event_offset, grab_type, &mousein_fake, &mousein_real);
     if (mousein_fake_old != mousein_fake && xcursor_display) {
-        if (ret) {
+        if (xcursor_is_in_xflush) {
+            xcursor_change_pending = ret ? 1 : 2;
+        } else if (ret) {
             if (xcursor_defined) real_XDefineCursor(xcursor_display, main_window_xcb, xcursor_cursor);
         } else {
             real_XUndefineCursor(xcursor_display, main_window_xcb);
@@ -648,14 +658,35 @@ xcb_get_geometry_reply_t* xcb_get_geometry_reply(xcb_connection_t* c, xcb_get_ge
     return ret;
 }
 
+int XFlush(void* display) {
+    xcursor_is_in_xflush = true;
+    const int ret = real_XFlush(display);
+    xcursor_is_in_xflush = false;
+    switch (xcursor_change_pending) {
+        case 1:
+            if (xcursor_defined) {
+                real_XDefineCursor(xcursor_display, main_window_xcb, xcursor_cursor);
+                xcursor_change_pending = 0;
+            }
+            break;
+        case 2:
+            real_XUndefineCursor(xcursor_display, main_window_xcb);
+            xcursor_change_pending = 0;
+            break;
+    }
+    return ret;
+}
+
 int XDefineCursor(void* display, XWindow w, XCursor cursor) {
-    int ret = real_XDefineCursor(display, w, cursor);
-    if (!ret) return ret;
-    if (w != main_window_xcb) return ret;
+    if (w != main_window_xcb) return real_XDefineCursor(display, w, cursor);
+    if (mousein_fake) {
+        const int ret = real_XDefineCursor(display, w, cursor);
+        if (!ret) return ret;
+    }
     if (!xcursor_display) xcursor_display = display;
     xcursor_cursor = cursor;
     xcursor_defined = true;
-    return ret;
+    return 1;
 }
 
 int XUndefineCursor(void* display, XWindow w) {
@@ -667,6 +698,15 @@ int XUndefineCursor(void* display, XWindow w) {
     return ret;
 }
 
+int XFreeCursor(void* display, XCursor cursor) {
+    int ret = real_XFreeCursor(display, cursor);
+    if (!ret) return ret;
+    if (cursor == xcursor_cursor) {
+        xcursor_defined = false;
+    }
+    return ret;
+}
+
 void* dlopen(const char*, int);
 void* dlsym(void*, const char*);
 void* dlvsym(void*, const char*, const char*);
@@ -703,6 +743,8 @@ static void* _bolt_dl_lookup(void* handle, const char* symbol) {
     } else if (handle == libx11_addr) {
         if (strcmp(symbol, "XDefineCursor") == 0) return XDefineCursor;
         if (strcmp(symbol, "XUndefineCursor") == 0) return XUndefineCursor;
+        if (strcmp(symbol, "XFreeCursor") == 0) return XFreeCursor;
+        if (strcmp(symbol, "XFlush") == 0) return XFlush;
     }
     return NULL;
 }