From d5c2ba04fe15f02b171fd8157550af0bfb4490cc Mon Sep 17 00:00:00 2001
From: Kleis Auke Wolthuizen <github@kleisauke.nl>
Date: Sun, 4 Aug 2024 10:36:39 +0200
Subject: [PATCH] Reduce concurrency to 1 on the main browser thread (#69)

---
 CHANGELOG.md        | 6 ++++++
 src/CMakeLists.txt  | 9 +++++----
 src/vips-library.js | 7 +++++++
 3 files changed, 18 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index c0b028dd8..29dce1aa4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,6 +15,12 @@ Uses libvips v8.15.2, compiled with Emscripten v3.1.64.
   [#70](https://github.com/kleisauke/wasm-vips/pull/70)
   [@marcosc90](https://github.com/marcosc90)
 
+### Changed
+
+- Reduce concurrency to 1 when wasm-vips was initialized on the main
+  browser thread.
+  [#69](https://github.com/kleisauke/wasm-vips/issues/69)
+
 ## [v0.0.9] - 2024-06-01
 
 Uses libvips v8.15.2, compiled with Emscripten v3.1.60.
diff --git a/src/CMakeLists.txt b/src/CMakeLists.txt
index 679c1aaa0..e297afc9a 100644
--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -131,7 +131,6 @@ set(MAIN_LINK_OPTIONS
         -sINITIAL_MEMORY=1GB
         -sSTACK_SIZE=256KB
         -sALLOW_TABLE_GROWTH
-        -sALLOW_BLOCKING_ON_MAIN_THREAD
         -sTEXTDECODER=2
         -sASSERTIONS=0
         -sFORCE_FILESYSTEM
@@ -174,12 +173,14 @@ if ("web" IN_LIST ENVIRONMENT)
             -sMIN_CHROME_VERSION=91
             )
 
-    # Build with `-sPTHREAD_POOL_SIZE="navigator.hardwareConcurrency + 2 + 1"` since `vips_sink_disc` uses two
-    # write-behind background threads, plus we need another one to run a maximum of two pipelines simultaneously.
+    # libvips requires spawning at least VIPS_CONCURRENCY threads synchronously, with a minimum of 3 threads per
+    # pipeline. This count includes the two write-behind background threads used by `vips_sink_disc`. To support up to
+    # two concurrent pipelines, we need to double that number. Therefore, build with:
+    # `-sPTHREAD_POOL_SIZE='Math.max(navigator.hardwareConcurrency, 6)'`.
     # Only needed on the web, pthreads on Node.js can be spawned synchronously without waiting for an event loop tick.
     set(WEB_LINK_OPTIONS
             --use-preload-plugins
-            -sPTHREAD_POOL_SIZE='navigator.hardwareConcurrency+2+1'
+            -sPTHREAD_POOL_SIZE='navigator.hardwareConcurrency>6?navigator.hardwareConcurrency:6'
             ${WEB_MIN_TARGETS}
             )
 
diff --git a/src/vips-library.js b/src/vips-library.js
index b7a128b24..b21548f47 100644
--- a/src/vips-library.js
+++ b/src/vips-library.js
@@ -13,6 +13,13 @@ var LibraryVips = {
       addOnPreRun(() => {
         // Enforce a fixed thread pool by default on web
         ENV['VIPS_MAX_THREADS'] = {{{ PTHREAD_POOL_SIZE }}};
+
+        // We cannot safely spawn dedicated workers when wasm-vips was initialized on the main browser thread.
+        // Therefore, to avoid any potential deadlocks, we reduce the concurrency to 1. For more details,
+        // see: https://emscripten.org/docs/porting/pthreads.html#blocking-on-the-main-browser-thread
+        if (!ENVIRONMENT_IS_WORKER) {
+          ENV['VIPS_CONCURRENCY'] = 1;
+        }
       });
 #endif