From 2cde4482aa15cea7ec8c80e596586151640f2a0d Mon Sep 17 00:00:00 2001
From: ramen-bully <248799758+ramen-bully@users.noreply.github.com>
Date: Wed, 13 May 2026 14:48:29 -0500
Subject: [PATCH] druntime: defer rt_term to atexit on Emscripten
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Emscripten programs that register an async event loop (sokol_app,
emscripten_set_main_loop, emscripten_request_animation_frame_loop,
etc.) return from main while user code is still scheduled to run via
JS callbacks. The unmodified `_d_run_main2` epilogue runs `rt_term`
immediately, which tears down the GC (`gc_term` → `Gcx.Dtor` → unmap
every pool). Subsequent malloc()s from those callbacks reuse the
addresses and clobber any live D objects allocated before main
returned.

Register `rt_term` via `atexit` on Emscripten instead of calling it
inline. This handles all three cases correctly:

  - `EXIT_RUNTIME=0` async programs (browser tabs): atexit never
    fires, the page lives until the tab closes, the GC stays alive
    while rAF callbacks run.
  - `EXIT_RUNTIME=1` sync programs: emscripten runs atexit handlers
    when main returns, rt_term fires at the right time, module
    destructors and `gc_term` run normally.
  - `EXIT_RUNTIME=1` programs that call `exit()` after cancelling
    their event loop: same as above — atexit fires on `exit()`.

The deferral preserves D-side teardown semantics for programs that
genuinely want them, while preventing the heap-clobber for programs
whose "main returned" is only a JS-scheduling artifact.

Gated on `version (Emscripten)`; other targets unchanged.
---
 runtime/druntime/src/rt/dmain2.d | 15 ++++++++++++++-
 1 file changed, 14 insertions(+), 1 deletion(-)

diff --git a/runtime/druntime/src/rt/dmain2.d b/runtime/druntime/src/rt/dmain2.d
index a669c7fb29..526fb04fac 100644
--- a/runtime/druntime/src/rt/dmain2.d
+++ b/runtime/druntime/src/rt/dmain2.d
@@ -144,6 +144,14 @@ extern (C) int rt_init()
     return 0;
 }
 
+version (Emscripten)
+{
+    private extern (C) void rt_term_atexit()
+    {
+        rt_term();
+    }
+}
+
 /**********************************************
  * Terminate use of druntime.
  */
@@ -547,7 +555,12 @@ private extern (C) int _d_run_main2(char[][] args, size_t totalArgsLength, MainF
         else
             result = EXIT_FAILURE;
 
-        if (!rt_term())
+        version (Emscripten)
+        {
+            import core.stdc.stdlib : atexit;
+            atexit(&rt_term_atexit);
+        }
+        else if (!rt_term())
             result = (result == EXIT_SUCCESS) ? EXIT_FAILURE : result;
     }