Bug 1478815 part 3 - Add a DualTextureClientLock for texture clients used for component alpha. r=nical

This commit adds a RAII class for the common operation of attempting
to lock one or two TextureClients and then maybe constructing a
DrawTargetDual from them.

MozReview-Commit-ID: ECQkDSgpyuL

--HG--
extra : rebase_source : abad14bfee32ea2fd1626069f8229487d1f05015

gfx/layers/client/TextureClient.h

@@ -829,6 +829,80 @@ private:
   bool mSucceeded;
 };
 
+// Automatically locks and unlocks two texture clients, and exposes them as a
+// a single draw target dual. Since texture locking is fallible, Succeeded()
+// must be checked on the guard object before proceeding.
+class MOZ_RAII DualTextureClientAutoLock
+{
+public:
+  DualTextureClientAutoLock(TextureClient* aTexture, TextureClient* aTextureOnWhite, OpenMode aMode)
+    : mTarget(nullptr)
+    , mTexture(aTexture)
+    , mTextureOnWhite(aTextureOnWhite)
+  {
+    if (!mTexture->Lock(aMode)) {
+      return;
+    }
+
+    mTarget = mTexture->BorrowDrawTarget();
+
+    if (!mTarget) {
+      mTexture->Unlock();
+      return;
+    }
+
+    if (!mTextureOnWhite) {
+      return;
+    }
+
+    if (!mTextureOnWhite->Lock(aMode)) {
+      mTarget = nullptr;
+      mTexture->Unlock();
+      return;
+    }
+
+    RefPtr<gfx::DrawTarget> targetOnWhite = mTextureOnWhite->BorrowDrawTarget();
+
+    if (!targetOnWhite) {
+      mTarget = nullptr;
+      mTexture->Unlock();
+      mTextureOnWhite->Unlock();
+      return;
+    }
+
+    mTarget = gfx::Factory::CreateDualDrawTarget(mTarget, targetOnWhite);
+
+    if (!mTarget) {
+      mTarget = nullptr;
+      mTexture->Unlock();
+      mTextureOnWhite->Unlock();
+    }
+  }
+
+  ~DualTextureClientAutoLock()
+  {
+    if (Succeeded()) {
+      mTarget = nullptr;
+
+      mTexture->Unlock();
+      if (mTextureOnWhite) {
+        mTextureOnWhite->Unlock();
+      }
+    }
+  }
+
+  bool Succeeded() const { return !!mTarget; }
+
+  operator gfx::DrawTarget*() const { return mTarget; }
+  gfx::DrawTarget* operator->() const { return mTarget; }
+
+  RefPtr<gfx::DrawTarget> mTarget;
+
+private:
+  RefPtr<TextureClient> mTexture;
+  RefPtr<TextureClient> mTextureOnWhite;
+};
+
 class KeepAlive
 {
 public: