From 631da8cc2fe01f74271a9230d101ab56a66234ca Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Camilla=20L=C3=B6wy?= <elmindreda@elmindreda.org>
Date: Wed, 25 May 2022 19:43:55 +0200
Subject: [PATCH] X11: Fix previous error handler not being restored

GLFW did not restore the previous Xlib error handler when removing its
own, instead resetting to the default handler.

This commit saves and restores the previous error handler.

None of this is thread-safe or could ever be.

Fixes #2108

(cherry picked from commit 26920a1a38d7c4c5b7908cf2e04b750d941f1faa)
---
 README.md          | 1 +
 src/x11_init.c     | 7 +++++--
 src/x11_platform.h | 2 ++
 3 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/README.md b/README.md
index 26a6092a..6005f81b 100644
--- a/README.md
+++ b/README.md
@@ -136,6 +136,7 @@ information on what to include when reporting a bug.
  - [Cocoa] Bugfix: The EGL and OSMesa libraries were not unloaded on termination
  - [X11] Bugfix: The OSMesa libray was not unloaded on termination
  - [X11] Bugfix: A malformed response during selection transfer could cause a segfault
+ - [X11] Bugfix: Some calls would reset Xlib to the default error handler (#2108)
  - [Wayland] Added support for file path drop events (#2040)
  - [Wayland] Bugfix: `glfwSetClipboardString` would fail if set to result of
    `glfwGetClipboardString`
diff --git a/src/x11_init.c b/src/x11_init.c
index 8dd433f1..6049904a 100644
--- a/src/x11_init.c
+++ b/src/x11_init.c
@@ -39,6 +39,7 @@
 #include <unistd.h>
 #include <fcntl.h>
 #include <errno.h>
+#include <assert.h>
 
 
 // Translate the X11 KeySyms for a key to a GLFW key code
@@ -1022,8 +1023,9 @@ static int errorHandler(Display *display, XErrorEvent* event)
 //
 void _glfwGrabErrorHandlerX11(void)
 {
+    assert(_glfw.x11.errorHandler == NULL);
     _glfw.x11.errorCode = Success;
-    XSetErrorHandler(errorHandler);
+    _glfw.x11.errorHandler = XSetErrorHandler(errorHandler);
 }
 
 // Clears the X error handler callback
@@ -1032,7 +1034,8 @@ void _glfwReleaseErrorHandlerX11(void)
 {
     // Synchronize to make sure all commands are processed
     XSync(_glfw.x11.display, False);
-    XSetErrorHandler(NULL);
+    XSetErrorHandler(_glfw.x11.errorHandler);
+    _glfw.x11.errorHandler = NULL;
 }
 
 // Reports the specified error, appending information about the last X error
diff --git a/src/x11_platform.h b/src/x11_platform.h
index 30c73a88..03ff9d24 100644
--- a/src/x11_platform.h
+++ b/src/x11_platform.h
@@ -222,6 +222,8 @@ typedef struct _GLFWlibraryX11
     XContext        context;
     // XIM input method
     XIM             im;
+    // The previous X error handler, to be restored later
+    XErrorHandler   errorHandler;
     // Most recent error code received by X error handler
     int             errorCode;
     // Primary selection string (while the primary selection is owned)