fix a possible leak when editing a material with matdbg

A MaterialParser could be leaked if several edits happened before they
were latched -- this was because the MaterialParser was stored as
a raw pointer instead of a unique_ptr<>, this was done as an attempt
to avoid to use a lock around accessing mPendingEdits.

filament/src/details/Material.cpp

@@ -815,10 +815,7 @@ void FMaterial::applyPendingEdits() noexcept {
     const char* name = mName.c_str();
     slog.d << "Applying edits to " << (name ? name : "(untitled)") << io::endl;
     destroyPrograms(mEngine); // FIXME: this will not destroy the shared variants
-
-    std::unique_ptr<MaterialParser> pending{ mPendingEdits };
-    std::swap(pending, mMaterialParser);
-    mPendingEdits = nullptr;
+    latchPendingEdits();
 }
 
 /**
@@ -837,7 +834,7 @@ void FMaterial::onEditCallback(void* userdata, const utils::CString&, const void
     // and swapping out the MaterialParser until the next getProgram call.
     std::unique_ptr<MaterialParser> pending = createParser(
             engine.getBackend(), engine.getShaderLanguage(), packageData, packageSize);
-    material->mPendingEdits = pending.release();
+    material->setPendingEdits(std::move(pending));
 }
 
 void FMaterial::onQueryCallback(void* userdata, VariantList* pVariants) {

filament/src/details/Material.h

@@ -219,7 +219,7 @@ class FMaterial : public Material {
     static void onQueryCallback(void* userdata, VariantList* pActiveVariants);
 
     void checkProgramEdits() noexcept {
-        if (UTILS_UNLIKELY(mPendingEdits.load())) {
+        if (UTILS_UNLIKELY(hasPendingEdits())) {
             applyPendingEdits();
         }
     }
@@ -302,8 +302,21 @@ class FMaterial : public Material {
     matdbg::MaterialKey mDebuggerId;
     mutable utils::Mutex mActiveProgramsLock;
     mutable VariantList mActivePrograms;
-    // FIXME: multi-threaded access to mPendingEdits is very broken
-    std::atomic<MaterialParser*> mPendingEdits = {};
+    mutable utils::Mutex mPendingEditsLock;
+    std::unique_ptr<MaterialParser> mPendingEdits;
+    void setPendingEdits(std::unique_ptr<MaterialParser> pendingEdits) noexcept {
+        std::lock_guard lock(mPendingEditsLock);
+        std::swap(pendingEdits, mPendingEdits);
+    }
+    bool hasPendingEdits() noexcept {
+        std::lock_guard lock(mPendingEditsLock);
+        return (bool)mPendingEdits;
+    }
+    void latchPendingEdits() noexcept {
+        std::lock_guard lock(mPendingEditsLock);
+        std::swap(mPendingEdits, mMaterialParser);
+    }
+
 #endif
 
     utils::CString mName;