From 6d4f66b48cc0378ee9c85a1ae698cf77f4c49add Mon Sep 17 00:00:00 2001
From: Sam Clegg <sbc@chromium.org>
Date: Thu, 21 May 2026 14:32:00 -0700
Subject: [PATCH] [pthreads] Simplify message relaying code

Historically, `targetThread` relaying was used to pass `objectTransfer`
messages (containing `OffscreenCanvas` objects) back to parent threads
on worker exit. This was introduced in commit 1b278917cd (2018/2019)
and required supporting `transferList` in the relaying code.

However, automatic canvas return on exit was later disabled in
6dcf7298e8 (2019) and the `objectTransfer` command was fully removed
in 0f536c60bb (2021).

These days, the relaying mechanism is only used by `CMD_CHECK_MAILBOX`
(via `_emscripten_notify_mailbox_postmessage`) to send simple mailbox
notifications, which do not require a transfer list.
---
 src/lib/libpthread.js                          | 18 +++++++++++-------
 .../test_codesize_minimal_pthreads.json        |  8 ++++----
 ...st_codesize_minimal_pthreads_memgrowth.json |  8 ++++----
 3 files changed, 19 insertions(+), 15 deletions(-)

diff --git a/src/lib/libpthread.js b/src/lib/libpthread.js
index c9ba01e4820d6..ac112b6a6dc80 100644
--- a/src/lib/libpthread.js
+++ b/src/lib/libpthread.js
@@ -278,14 +278,18 @@ var LibraryPThread = {
 #endif
 
         // If this message is intended to a recipient that is not the main
-        // thread, forward it to the target thread.
-        if (d.targetThread && d.targetThread != _pthread_self()) {
+        // thread, forward it to the target thread. This is currently only
+        // used by `CMD_CHECK_MAILBOX`.
+        if (d.targetThread) {
+#if ASSERTIONS
+          // pthreads should not be relaying messages to themselves.
+          assert(d.targetThread != _pthread_self());
+#endif
           var targetWorker = PThread.pthreads[d.targetThread];
-          if (targetWorker) {
-            targetWorker.postMessage(d, d.transferList);
-          } else {
-            err(`worker sent message (${cmd}) to pthread (${d.targetThread}) that no longer exists`);
-          }
+#if ASSERTIONS
+          if (!targetWorker) err(`worker sent message (${cmd}) to pthread (${d.targetThread}) that no longer exists`);
+#endif
+          targetWorker?.postMessage(d);
           return;
         }
 
diff --git a/test/codesize/test_codesize_minimal_pthreads.json b/test/codesize/test_codesize_minimal_pthreads.json
index 851b17340ee79..7843e557c59d8 100644
--- a/test/codesize/test_codesize_minimal_pthreads.json
+++ b/test/codesize/test_codesize_minimal_pthreads.json
@@ -1,10 +1,10 @@
 {
-  "a.out.js": 7110,
-  "a.out.js.gz": 3524,
+  "a.out.js": 7012,
+  "a.out.js.gz": 3472,
   "a.out.nodebug.wasm": 19037,
   "a.out.nodebug.wasm.gz": 8787,
-  "total": 26147,
-  "total_gz": 12311,
+  "total": 26049,
+  "total_gz": 12259,
   "sent": [
     "a (memory)",
     "b (exit)",
diff --git a/test/codesize/test_codesize_minimal_pthreads_memgrowth.json b/test/codesize/test_codesize_minimal_pthreads_memgrowth.json
index 9dcfca60775f6..079165820445c 100644
--- a/test/codesize/test_codesize_minimal_pthreads_memgrowth.json
+++ b/test/codesize/test_codesize_minimal_pthreads_memgrowth.json
@@ -1,10 +1,10 @@
 {
-  "a.out.js": 7518,
-  "a.out.js.gz": 3728,
+  "a.out.js": 7420,
+  "a.out.js.gz": 3672,
   "a.out.nodebug.wasm": 19038,
   "a.out.nodebug.wasm.gz": 8788,
-  "total": 26556,
-  "total_gz": 12516,
+  "total": 26458,
+  "total_gz": 12460,
   "sent": [
     "a (memory)",
     "b (exit)",