[skwasm] Dynamic Threading (#164748)

This switches skwasm over from the emscripten pthreads implementation to
emscripten's "wasm workers" threading implementation. The pthreads
implementation simply will not run at all in a non-crossOriginIsolated
context, but the wasm workers implementation only fails if we actually
attempt to spawn a thread. This means we can actually choose whether to
use a single-threaded or multi-threaded strategy at runtime, which means
we don't have to build two variants of skwasm for single- vs
multi-threaded.

Author: eyebrowsoffire

engine/src/build/config/compiler/BUILD.gn

@@ -277,9 +277,9 @@ config("compiler") {
   }
 
   if (is_wasm) {
-    if (wasm_use_pthreads) {
-      cflags += [ "-pthread" ]
-      ldflags += [ "-pthread" ]
+    if (wasm_use_workers) {
+      cflags += [ "-sWASM_WORKERS=1" ]
+      ldflags += [ "-sWASM_WORKERS=1" ]
     } if (wasm_shared_memory) {
       cflags += [ "-sSHARED_MEMORY=1" ]
       ldflags += [ "-sSHARED_MEMORY=1" ]
@@ -396,12 +396,12 @@ config("compiler") {
     # to say that it does. Define them here instead.
     defines += [ "HAVE_SYS_UIO_H" ]
 
-    # When Android requires new flags consider also editing the flags in 
-    # the following locations. 
+    # When Android requires new flags consider also editing the flags in
+    # the following locations.
     # Framework plugin_ffi template: packages/flutter_tools/templates/plugin_ffi/src.tmpl/CMakeLists.txt.tmpl
     # Example PR: https://github.com/flutter/flutter/pull/155508
-    # Dart Lang JNI package: pkgs/jni/src/CMakeLists.txt 
-    # Example PR: https://github.com/dart-lang/native/pull/1615 
+    # Dart Lang JNI package: pkgs/jni/src/CMakeLists.txt
+    # Example PR: https://github.com/dart-lang/native/pull/1615
     ldflags += [
       "-Wl,--no-undefined",
       "-Wl,--exclude-libs,ALL",

engine/src/build/toolchain/wasm.gni

@@ -11,7 +11,7 @@ declare_args() {
   # The location of an activated embedded emsdk.
   emsdk_dir = rebase_path("//flutter/prebuilts/emsdk")
 
-  wasm_use_pthreads = false
+  wasm_use_workers = false
   wasm_shared_memory = false
   wasm_use_dwarf = false
 }

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -42726,8 +42726,6 @@ ORIGIN: ../../../flutter/lib/web_ui/skwasm/filters.cpp + ../../../flutter/LICENS
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/fonts.cpp + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/helpers.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/image.cpp + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/lib/web_ui/skwasm/library_skwasm_multi_threaded.js + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/lib/web_ui/skwasm/library_skwasm_single_threaded.js + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/library_skwasm_support.js + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/paint.cpp + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/path.cpp + ../../../flutter/LICENSE
@@ -42737,8 +42735,6 @@ ORIGIN: ../../../flutter/lib/web_ui/skwasm/skwasm_support.h + ../../../flutter/L
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/string.cpp + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/surface.cpp + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/surface.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/lib/web_ui/skwasm/surface_mt.cpp + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/lib/web_ui/skwasm/surface_st.cpp + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/text/line_metrics.cpp + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/text/paragraph.cpp + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/skwasm/text/paragraph_builder.cpp + ../../../flutter/LICENSE
@@ -45696,8 +45692,6 @@ FILE: ../../../flutter/lib/web_ui/skwasm/filters.cpp
 FILE: ../../../flutter/lib/web_ui/skwasm/fonts.cpp
 FILE: ../../../flutter/lib/web_ui/skwasm/helpers.h
 FILE: ../../../flutter/lib/web_ui/skwasm/image.cpp
-FILE: ../../../flutter/lib/web_ui/skwasm/library_skwasm_multi_threaded.js
-FILE: ../../../flutter/lib/web_ui/skwasm/library_skwasm_single_threaded.js
 FILE: ../../../flutter/lib/web_ui/skwasm/library_skwasm_support.js
 FILE: ../../../flutter/lib/web_ui/skwasm/paint.cpp
 FILE: ../../../flutter/lib/web_ui/skwasm/path.cpp
@@ -45707,8 +45701,6 @@ FILE: ../../../flutter/lib/web_ui/skwasm/skwasm_support.h
 FILE: ../../../flutter/lib/web_ui/skwasm/string.cpp
 FILE: ../../../flutter/lib/web_ui/skwasm/surface.cpp
 FILE: ../../../flutter/lib/web_ui/skwasm/surface.h
-FILE: ../../../flutter/lib/web_ui/skwasm/surface_mt.cpp
-FILE: ../../../flutter/lib/web_ui/skwasm/surface_st.cpp
 FILE: ../../../flutter/lib/web_ui/skwasm/text/line_metrics.cpp
 FILE: ../../../flutter/lib/web_ui/skwasm/text/paragraph.cpp
 FILE: ../../../flutter/lib/web_ui/skwasm/text/paragraph_builder.cpp

engine/src/flutter/lib/web_ui/dev/steps/copy_artifacts_step.dart

@@ -76,15 +76,13 @@ class CopyArtifactsStep implements PipelineStep {
     final String canvaskitSourceDirectory;
     final String canvaskitChromiumSourceDirectory;
     final String skwasmSourceDirectory;
-    final String skwasmStSourceDirectory;
     switch (source) {
       case LocalArtifactSource(:final mode):
         final buildDirectory = getBuildDirectoryForRuntimeMode(mode).path;
         flutterJsSourceDirectory = pathlib.join(buildDirectory, 'flutter_web_sdk', 'flutter_js');
         canvaskitSourceDirectory = pathlib.join(buildDirectory, 'canvaskit');
         canvaskitChromiumSourceDirectory = pathlib.join(buildDirectory, 'canvaskit_chromium');
         skwasmSourceDirectory = pathlib.join(buildDirectory, 'skwasm');
-        skwasmStSourceDirectory = pathlib.join(buildDirectory, 'skwasm_st');
 
       case GcsArtifactSource(:final realm):
         final artifactsDirectory = (await _downloadArtifacts(realm)).path;
@@ -96,7 +94,6 @@ class CopyArtifactsStep implements PipelineStep {
           'chromium',
         );
         skwasmSourceDirectory = pathlib.join(artifactsDirectory, 'canvaskit');
-        skwasmStSourceDirectory = pathlib.join(artifactsDirectory, 'canvaskit');
     }
 
     await environment.webTestsArtifactsDir.create(recursive: true);
@@ -116,7 +113,6 @@ class CopyArtifactsStep implements PipelineStep {
     if (artifactDeps.skwasm) {
       copied.add('Skwasm');
       await copyWasmLibrary('skwasm', skwasmSourceDirectory, 'canvaskit');
-      await copyWasmLibrary('skwasm_st', skwasmStSourceDirectory, 'canvaskit');
     }
     print('Copied artifacts: ${copied.join(', ')}');
   }

engine/src/flutter/lib/web_ui/flutter_js/src/skwasm_loader.js

@@ -6,24 +6,64 @@ import { createWasmInstantiator } from "./instantiate_wasm.js";
 import { resolveUrlWithSegments } from "./utils.js";
 
 export const loadSkwasm = async (deps, config, browserEnvironment, baseUrl) => {
-  const fileStem = (browserEnvironment.crossOriginIsolated && !config.forceSingleThreadedSkwasm) ? "skwasm" : "skwasm_st";
-  const rawSkwasmUrl = resolveUrlWithSegments(baseUrl, `${fileStem}.js`)
+  const rawSkwasmUrl = resolveUrlWithSegments(baseUrl, 'skwasm.js')
   let skwasmUrl = rawSkwasmUrl;
   if (deps.flutterTT.policy) {
     skwasmUrl = deps.flutterTT.policy.createScriptURL(skwasmUrl);
   }
-  const wasmInstantiator = createWasmInstantiator(resolveUrlWithSegments(baseUrl, `${fileStem}.wasm`));
+  const wasmInstantiator = createWasmInstantiator(resolveUrlWithSegments(baseUrl, 'skwasm.wasm'));
   const skwasm = await import(skwasmUrl);
   return await skwasm.default({
+    skwasmSingleThreaded: !browserEnvironment.crossOriginIsolated || config.forceSingleThreadedSkwasm,
     instantiateWasm: wasmInstantiator,
-    // When hosted via a CDN or some other url that is not the same
-    // origin as the main script of the page, we will fail to create
-    // a web worker with the bootstrapping script. This workaround will
-    // make sure that the worker JS can be loaded regardless of where
-    // it is hosted.
-    mainScriptUrlOrBlob: new Blob(
-      [`import '${skwasmUrl}'`],
-      { 'type': 'application/javascript' },
-    ),
+    locateFile: (filename, scriptDirectory) => {
+      // The wasm workers API has a separate .ww.js file that bootstraps the
+      // web worker. However, it turns out this worker bootstrapper doesn't
+      // actually work with ES6 modules, which we have enabled. So we instead
+      // pass our own bootstrapper that loads skwasm.js as an ES6 module, and
+      // queues/flushes pending messages that were received during the
+      // asynchronous load.
+      if (filename.endsWith('.ww.js')) {
+        const url = resolveUrlWithSegments(baseUrl, filename);
+        return URL.createObjectURL(new Blob(
+          [`
+"use strict";
+
+let eventListener;
+eventListener = (message) => {
+    const pendingMessages = [];
+    const data = message.data;
+    data["instantiateWasm"] = (info,receiveInstance) => {
+        const instance = new WebAssembly.Instance(data["wasm"], info);
+        return receiveInstance(instance, data["wasm"])
+    };
+    import(data.js).then(async (skwasm) => {
+        await skwasm.default(data);
+
+        removeEventListener("message", eventListener);
+        for (const message of pendingMessages) {
+            dispatchEvent(message);
+        }
+    });
+    console.log("removing initial listener");
+    removeEventListener("message", eventListener);
+    eventListener = (message) => {
+
+        pendingMessages.push(message);
+    };
+
+    addEventListener("message", eventListener);
+};
+addEventListener("message", eventListener);
+`
+          ],
+          { 'type': 'application/javascript' }));
+      }
+      return url;
+    },
+    // Because of the above workaround, the worker is just a blob and
+    // can't locate the main script using a relative path to itself,
+    // so we pass the main script location in.
+    mainScriptUrlOrBlob: rawSkwasmUrl,
   });
 }

engine/src/flutter/lib/web_ui/skwasm/BUILD.gn

@@ -4,96 +4,69 @@
 
 import("//build/toolchain/wasm.gni")
 
-template("skwasm_variant") {
-  wasm_lib(target_name) {
-    public_configs = [ "//flutter:config" ]
+wasm_lib("skwasm") {
+  public_configs = [ "//flutter:config" ]
 
-    sources = [
-      "canvas.cpp",
-      "contour_measure.cpp",
-      "data.cpp",
-      "export.h",
-      "filters.cpp",
-      "fonts.cpp",
-      "helpers.h",
-      "image.cpp",
-      "paint.cpp",
-      "path.cpp",
-      "picture.cpp",
-      "shaders.cpp",
-      "skwasm_support.h",
-      "string.cpp",
-      "surface.cpp",
-      "text/line_metrics.cpp",
-      "text/paragraph.cpp",
-      "text/paragraph_builder.cpp",
-      "text/paragraph_style.cpp",
-      "text/strut_style.cpp",
-      "text/text_style.cpp",
-      "vertices.cpp",
-      "wrappers.h",
-    ]
-
-    cflags = [ "-mreference-types" ]
-
-    ldflags = [
-      "-std=c++20",
-      "-lGL",
-      "-sUSE_WEBGL2=1",
-      "-sMAX_WEBGL_VERSION=2",
-      "-sOFFSCREENCANVAS_SUPPORT",
-      "-sALLOW_MEMORY_GROWTH",
-      "-sALLOW_TABLE_GROWTH",
-      "-lexports.js",
-      "-sEXPORTED_FUNCTIONS=[stackAlloc]",
-      "-sEXPORTED_RUNTIME_METHODS=[addFunction,wasmExports,wasmMemory,stackAlloc]",
-      "-sINCOMING_MODULE_JS_API=[instantiateWasm,noExitRuntime,mainScriptUrlOrBlob]",
-      "-sUSE_ES6_IMPORT_META=0",
-      "--js-library",
-      rebase_path("library_skwasm_support.js"),
-    ]
+  sources = [
+    "canvas.cpp",
+    "contour_measure.cpp",
+    "data.cpp",
+    "export.h",
+    "filters.cpp",
+    "fonts.cpp",
+    "helpers.h",
+    "image.cpp",
+    "paint.cpp",
+    "path.cpp",
+    "picture.cpp",
+    "shaders.cpp",
+    "skwasm_support.h",
+    "string.cpp",
+    "surface.cpp",
+    "text/line_metrics.cpp",
+    "text/paragraph.cpp",
+    "text/paragraph_builder.cpp",
+    "text/paragraph_style.cpp",
+    "text/strut_style.cpp",
+    "text/text_style.cpp",
+    "vertices.cpp",
+    "wrappers.h",
+  ]
 
-    inputs = [ rebase_path("library_skwasm_support.js") ]
+  cflags = [ "-mreference-types" ]
 
-    if (invoker.multi_threaded) {
-      sources += [ "surface_mt.cpp" ]
-      ldflags += [
-        "-sPTHREAD_POOL_SIZE=1",
-        "-Wno-pthreads-mem-growth",
-        "--js-library",
-        rebase_path("library_skwasm_multi_threaded.js"),
-      ]
-      inputs += [ rebase_path("library_skwasm_multi_threaded.js") ]
-    } else {
-      sources += [ "surface_st.cpp" ]
-      ldflags += [
-        "--js-library",
-        rebase_path("library_skwasm_single_threaded.js"),
-      ]
-      inputs += [ rebase_path("library_skwasm_single_threaded.js") ]
-    }
+  ldflags = [
+    "-std=c++20",
+    "-lGL",
+    "-sUSE_WEBGL2=1",
+    "-sMAX_WEBGL_VERSION=2",
+    "-sOFFSCREENCANVAS_SUPPORT",
+    "-sALLOW_MEMORY_GROWTH",
+    "-sALLOW_TABLE_GROWTH",
+    "-lexports.js",
+    "-sEXPORTED_FUNCTIONS=[stackAlloc]",
+    "-sEXPORTED_RUNTIME_METHODS=[addFunction,wasmExports,wasmMemory,stackAlloc]",
+    "-sINCOMING_MODULE_JS_API=[instantiateWasm,locateFile,noExitRuntime,mainScriptUrlOrBlob,wasmMemory,wasm,skwasmSingleThreaded]",
+    "-sUSE_ES6_IMPORT_META=0",
+    "--js-library",
+    rebase_path("library_skwasm_support.js"),
+  ]
 
-    if (is_debug) {
-      ldflags += [
-        "-sASSERTIONS=1",
-        "-sGL_ASSERTIONS=1",
-      ]
-    } else {
-      ldflags += [ "--closure=1" ]
-    }
+  inputs = [ rebase_path("library_skwasm_support.js") ]
 
-    deps = [
-      "//flutter/skia",
-      "//flutter/skia/modules/skparagraph",
-      "//flutter/skia/modules/skunicode",
+  if (is_debug) {
+    ldflags += [
+      "-sASSERTIONS=1",
+      "-sGL_ASSERTIONS=1",
+      "-sSTACK_OVERFLOW_CHECK=2",
     ]
+  } else {
+    ldflags += [ "--closure=1" ]
   }
-}
-
-skwasm_variant("skwasm") {
-  multi_threaded = true
-}
 
-skwasm_variant("skwasm_st") {
-  multi_threaded = false
+  deps = [
+    "//flutter/skia",
+    "//flutter/skia/modules/skparagraph",
+    "//flutter/skia/modules/skunicode",
+  ]
 }