Wasm: enable thread support

configure.json: Make the “thread” feature be allowed
for wasm but disabled by default.

Change qmake.conf and wasm.prf to enable Emscripten
pthreads mode:
- Add USE_PTHREADS=1 linker flag
- Add PTHREAD_POOL_SIZE linker flag with a default pool size (4).
- Add TOTAL_MEMORY linker flag to set available memory (1GB)

It is possible to override options such as PTHREAD_POOL_SIZE
from the application .pro file using QMAKE_WASM_PTHREAD_POOL_SIZE
To change TOTAL_MEMORY, use QMAKE_WASM_TOTAL_MEMORY

Make qtloader.js work in pthreads mode:
- The Module.instantiateWasm callback must provide the module
  in addition to the instance to Emscripten.
- Set Module.mainScriptUrlOrBlob so that the pthreads web workers
  can access the main script

Task-number: QTBUG-64625
Change-Id: I1ab5a559ec97c27c5fc24500ba5f863bcd275141
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

configure.json

@@ -1199,7 +1199,7 @@
             "label": "Thread support",
             "purpose": "Provides QThread and related classes.",
             "section": "Kernel",
-            "condition": "!config.wasm",
+            "autoDetect": "!config.wasm",
             "output": [ "publicFeature" ]
         },
         "future": {

mkspecs/features/qt.prf

@@ -2,7 +2,7 @@
 # due to required Qt modules being missing.
 !isEmpty(QMAKE_FAILED_REQUIREMENTS): return()
 
-CONFIG *= thread
+qtConfig(thread): CONFIG *= thread
 
 #handle defines
 win32 {

mkspecs/features/wasm/wasm.prf

@@ -2,6 +2,39 @@
 # DESTDIR will be empty if not set in the app .pro file; make sure it has a value
 isEmpty(DESTDIR): DESTDIR = $$OUT_PWD
 
+exists($$QMAKE_QT_CONFIG) {
+    qtConfig(thread) {
+
+        EMCC_THREAD_LFLAGS += -s USE_PTHREADS=1
+        # Hardcode wasm memory size. Emscripten does not currently support memory growth
+        # (ALLOW_MEMORY_GROWTH) in pthreads mode, and requires specifying the memory size
+        # at build time. Further, browsers limit the maximum initial memory size to 1GB.
+        TOTAL_MEMORY = 1GB
+        !isEmpty(QMAKE_WASM_TOTAL_MEMORY) {
+            TOTAL_MEMORY = $$QMAKE_WASM_TOTAL_MEMORY
+        }
+
+        message("Setting TOTAL_MEMORY to" $$TOTAL_MEMORY)
+        EMCC_THREAD_LFLAGS += -s TOTAL_MEMORY=$$TOTAL_MEMORY
+
+        # Create worker threads at startup. This is supposed to be an optimization,
+        # however exceeding the pool size has been obesverved to hang the application.
+        POOL_SIZE = 4
+        !isEmpty(QMAKE_WASM_PTHREAD_POOL_SIZE) {
+            POOL_SIZE = $$QMAKE_WASM_PTHREAD_POOL_SIZE
+        }
+
+        message("Setting PTHREAD_POOL_SIZE to" $$POOL_SIZE)
+        EMCC_THREAD_LFLAGS += -s PTHREAD_POOL_SIZE=$$POOL_SIZE
+    } else {
+        EMCC_THREAD_LFLAGS += -s ALLOW_MEMORY_GROWTH=1
+    }
+    QMAKE_LFLAGS       += $$EMCC_THREAD_LFLAGS
+    QMAKE_LFLAGS_DEBUG += $$EMCC_THREAD_LFLAGS
+    QMAKE_CFLAGS += $$EMCC_THREAD_LFLAGS
+    QMAKE_CXXFLAGS += $$EMCC_THREAD_LFLAGS
+}
+
 # Create js and wasm files for applications
 contains(TEMPLATE, .*app) {
     TARGET_BASE = $${TARGET}

mkspecs/wasm-emscripten/qmake.conf

@@ -15,7 +15,6 @@ EMTERP_FLAGS = \
 EMCC_COMMON_LFLAGS = \
     -s WASM=1 \
     -s FULL_ES2=1 \
-    -s ALLOW_MEMORY_GROWTH=1 \
     -s USE_WEBGL2=1 \
     -s NO_EXIT_RUNTIME=0 \
     -s ERROR_ON_UNDEFINED_SYMBOLS=1 \

src/plugins/platforms/wasm/qtloader.js

@@ -312,13 +312,13 @@ function QtLoader(config)
         // and is ready to be instantiated. Define the instantiateWasm callback which
         // emscripten will call to create the instance.
         Module.instantiateWasm = function(imports, successCallback) {
-            return WebAssembly.instantiate(wasmModule, imports).then(function(instance) {
-                successCallback(instance);
-                return instance;
+            WebAssembly.instantiate(wasmModule, imports).then(function(instance) {
+                successCallback(instance, wasmModule);
             }, function(error) {
                 self.error = error;
                 setStatus("Error");
             });
+            return {};
         };
 
         Module.locateFile = Module.locateFile || function(filename) {
@@ -382,6 +382,8 @@ function QtLoader(config)
             }
         });
 
+        Module.mainScriptUrlOrBlob = new Blob([emscriptenModuleSource], {type: 'text/javascript'});
+
         config.restart = function() {
 
             // Restart by reloading the page. This will wipe all state which means