wasm: disable run-time check for specialHTMLTargets

Recent versions of Emscripten may abort if module_property()
is used to look up specialHTMLTargets, which makes run-time
checking impossible. The abort is implemented on the JS property
getter on the Module object, which means that it's not possible
to bypass this by using e.g. EM_ASM instead of the C++ API.

Disable usage of specialHTMLTargets on emsdk > 3.1.14 for now,
until we can add API (or find some other means) for opting in.

This commit also adds functions for comparing Emscripten
versions.

Pick-to: 6.4
Change-Id: Ibe5d1a82f267fa95dd62cdfc66595bc23036933f
Reviewed-by: Aleksandr Reviakin <aleksandr.reviakin@qt.io>
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>

src/plugins/platforms/wasm/qwasmscreen.cpp

@@ -16,6 +16,8 @@
 #include <QtGui/qguiapplication.h>
 #include <private/qhighdpiscaling_p.h>
 
+#include <tuple>
+
 QT_BEGIN_NAMESPACE
 
 using namespace emscripten;
@@ -174,10 +176,46 @@ std::string QWasmScreen::canvasSpecialHtmlTargetId() const
     return std::string("!qtcanvas_") + std::to_string(uintptr_t(this));
 }
 
+namespace {
+
+// Compare Emscripten versions, returns > 0 if a is greater than b.
+
+int compareVersionComponents(int a, int b)
+{
+    return a >= 0 && b >= 0 ? a - b : 0;
+}
+
+int compareEmscriptenVersions(std::tuple<int, int, int> a, std::tuple<int, int, int> b)
+{
+    if (std::get<0>(a) == std::get<0>(b)) {
+        if (std::get<1>(a) == std::get<1>(b)) {
+            return compareVersionComponents(std::get<2>(a), std::get<2>(b));
+        }
+        return compareVersionComponents(std::get<1>(a), std::get<1>(b));
+    }
+    return compareVersionComponents(std::get<0>(a), std::get<0>(b));
+}
+
+bool isEmsdkVersionGreaterThan(std::tuple<int, int, int> test)
+{
+    return compareEmscriptenVersions(
+        std::make_tuple(__EMSCRIPTEN_major__, __EMSCRIPTEN_minor__, __EMSCRIPTEN_tiny__), test) > 0;
+}
+
+} // namespace
+
 bool QWasmScreen::hasSpecialHtmlTargets() const
 {
     static bool gotIt = []{
         // Enable use of specialHTMLTargets, if available
+
+        // On Emscripten > 3.1.14 (exact version not known), emscripten::val::module_property()
+        // aborts instead of returning undefined when attempting to resolve the specialHTMLTargets
+        // property, in the case where it is not defined. Disable the availability test in this case.
+        // FIXME: Add alternative way to enable.
+        if (isEmsdkVersionGreaterThan(std::make_tuple(3, 1, 14)))
+            return false;
+
         emscripten::val htmlTargets = emscripten::val::module_property("specialHTMLTargets");
         if (htmlTargets.isUndefined())
             return false;