Adding upstream version 124.0.1.upstream/124.0.1

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 775 insertions, 0 deletions

diff --git a/gfx/layers/PersistentBufferProvider.cpp b/gfx/layers/PersistentBufferProvider.cpp
new file mode 100644
index 0000000000..0e02501a4a
--- /dev/null
+++ b/gfx/layers/PersistentBufferProvider.cpp
@@ -0,0 +1,775 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "PersistentBufferProvider.h"
+
+#include "mozilla/layers/KnowsCompositor.h"
+#include "mozilla/layers/RemoteTextureMap.h"
+#include "mozilla/layers/TextureClient.h"
+#include "mozilla/layers/TextureForwarder.h"
+#include "mozilla/gfx/gfxVars.h"
+#include "mozilla/gfx/CanvasManagerChild.h"
+#include "mozilla/gfx/DrawTargetWebgl.h"
+#include "mozilla/gfx/Logging.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/StaticPrefs_layers.h"
+#include "pratom.h"
+#include "gfxPlatform.h"
+
+namespace mozilla {
+
+using namespace gfx;
+
+namespace layers {
+
+PersistentBufferProviderBasic::PersistentBufferProviderBasic(DrawTarget* aDt)
+    : mDrawTarget(aDt) {
+  MOZ_COUNT_CTOR(PersistentBufferProviderBasic);
+}
+
+PersistentBufferProviderBasic::~PersistentBufferProviderBasic() {
+  MOZ_COUNT_DTOR(PersistentBufferProviderBasic);
+  Destroy();
+}
+
+already_AddRefed<gfx::DrawTarget>
+PersistentBufferProviderBasic::BorrowDrawTarget(
+    const gfx::IntRect& aPersistedRect) {
+  MOZ_ASSERT(!mSnapshot);
+  RefPtr<gfx::DrawTarget> dt(mDrawTarget);
+  return dt.forget();
+}
+
+bool PersistentBufferProviderBasic::ReturnDrawTarget(
+    already_AddRefed<gfx::DrawTarget> aDT) {
+  RefPtr<gfx::DrawTarget> dt(aDT);
+  MOZ_ASSERT(mDrawTarget == dt);
+  if (dt) {
+    // Since SkiaGL default to storing drawing command until flush
+    // we have to flush it before present.
+    dt->Flush();
+  }
+  return true;
+}
+
+already_AddRefed<gfx::SourceSurface>
+PersistentBufferProviderBasic::BorrowSnapshot(gfx::DrawTarget* aTarget) {
+  mSnapshot = mDrawTarget->Snapshot();
+  RefPtr<SourceSurface> snapshot = mSnapshot;
+  return snapshot.forget();
+}
+
+void PersistentBufferProviderBasic::ReturnSnapshot(
+    already_AddRefed<gfx::SourceSurface> aSnapshot) {
+  RefPtr<SourceSurface> snapshot = aSnapshot;
+  MOZ_ASSERT(!snapshot || snapshot == mSnapshot);
+  mSnapshot = nullptr;
+}
+
+void PersistentBufferProviderBasic::Destroy() {
+  mSnapshot = nullptr;
+  mDrawTarget = nullptr;
+}
+
+// static
+already_AddRefed<PersistentBufferProviderBasic>
+PersistentBufferProviderBasic::Create(gfx::IntSize aSize,
+                                      gfx::SurfaceFormat aFormat,
+                                      gfx::BackendType aBackend) {
+  RefPtr<DrawTarget> dt =
+      gfxPlatform::GetPlatform()->CreateDrawTargetForBackend(aBackend, aSize,
+                                                             aFormat);
+
+  if (dt) {
+    // This is simply to ensure the DrawTarget gets initialized, and will detect
+    // a device reset, even if we're on the main thread.
+    dt->ClearRect(Rect(0, 0, 0, 0));
+  }
+
+  if (!dt || !dt->IsValid()) {
+    return nullptr;
+  }
+
+  RefPtr<PersistentBufferProviderBasic> provider =
+      new PersistentBufferProviderBasic(dt);
+
+  return provider.forget();
+}
+
+static already_AddRefed<TextureClient> CreateTexture(
+    KnowsCompositor* aKnowsCompositor, gfx::SurfaceFormat aFormat,
+    gfx::IntSize aSize, bool aWillReadFrequently = false,
+    Maybe<RemoteTextureOwnerId> aRemoteTextureOwnerId = {}) {
+  TextureAllocationFlags flags = ALLOC_DEFAULT;
+  if (aWillReadFrequently) {
+    flags = TextureAllocationFlags(flags | ALLOC_DO_NOT_ACCELERATE);
+  }
+  if (aRemoteTextureOwnerId) {
+    flags = TextureAllocationFlags(flags | ALLOC_FORCE_REMOTE);
+  }
+  RefPtr<TextureClient> tc = TextureClient::CreateForDrawing(
+      aKnowsCompositor, aFormat, aSize, BackendSelector::Canvas,
+      TextureFlags::DEFAULT | TextureFlags::NON_BLOCKING_READ_LOCK, flags);
+  if (tc && aRemoteTextureOwnerId) {
+    if (TextureData* td = tc->GetInternalData()) {
+      td->SetRemoteTextureOwnerId(*aRemoteTextureOwnerId);
+    }
+  }
+  return tc.forget();
+}
+
+// static
+already_AddRefed<PersistentBufferProviderAccelerated>
+PersistentBufferProviderAccelerated::Create(gfx::IntSize aSize,
+                                            gfx::SurfaceFormat aFormat,
+                                            KnowsCompositor* aKnowsCompositor) {
+  if (!aKnowsCompositor || !aKnowsCompositor->GetTextureForwarder() ||
+      !aKnowsCompositor->GetTextureForwarder()->IPCOpen()) {
+    return nullptr;
+  }
+
+  if (!DrawTargetWebgl::CanCreate(aSize, aFormat)) {
+#ifdef XP_WIN
+    // Direct2D acceleration does not require DrawTargetWebgl, but still
+    // requires PersistentBufferProviderAccelerated.
+    if (!TextureData::IsRemote(aKnowsCompositor, BackendSelector::Canvas,
+                               aFormat, aSize)) {
+      return nullptr;
+    }
+#else
+    return nullptr;
+#endif
+  }
+
+  auto remoteTextureOwnerId = RemoteTextureOwnerId::GetNext();
+
+  RefPtr<TextureClient> texture = CreateTexture(
+      aKnowsCompositor, aFormat, aSize, false, Some(remoteTextureOwnerId));
+  if (!texture) {
+    return nullptr;
+  }
+
+  RefPtr<PersistentBufferProviderAccelerated> provider =
+      new PersistentBufferProviderAccelerated(texture);
+  return provider.forget();
+}
+
+PersistentBufferProviderAccelerated::PersistentBufferProviderAccelerated(
+    const RefPtr<TextureClient>& aTexture)
+    : mTexture(aTexture) {
+  MOZ_COUNT_CTOR(PersistentBufferProviderAccelerated);
+}
+
+PersistentBufferProviderAccelerated::~PersistentBufferProviderAccelerated() {
+  MOZ_COUNT_DTOR(PersistentBufferProviderAccelerated);
+  Destroy();
+}
+
+void PersistentBufferProviderAccelerated::Destroy() {
+  mSnapshot = nullptr;
+  mDrawTarget = nullptr;
+
+  if (mTexture) {
+    if (mTexture->IsLocked()) {
+      MOZ_ASSERT(false);
+      mTexture->Unlock();
+    }
+    mTexture = nullptr;
+  }
+}
+
+already_AddRefed<gfx::DrawTarget>
+PersistentBufferProviderAccelerated::BorrowDrawTarget(
+    const gfx::IntRect& aPersistedRect) {
+  if (!mDrawTarget) {
+    if (aPersistedRect.IsEmpty()) {
+      mTexture->GetInternalData()->InvalidateContents();
+    }
+    if (!mTexture->Lock(OpenMode::OPEN_READ_WRITE)) {
+      return nullptr;
+    }
+    mDrawTarget = mTexture->BorrowDrawTarget();
+    if (!mDrawTarget || !mDrawTarget->IsValid()) {
+      mDrawTarget = nullptr;
+      mTexture->Unlock();
+      return nullptr;
+    }
+  }
+  return do_AddRef(mDrawTarget);
+}
+
+bool PersistentBufferProviderAccelerated::ReturnDrawTarget(
+    already_AddRefed<gfx::DrawTarget> aDT) {
+  {
+    RefPtr<gfx::DrawTarget> dt(aDT);
+    MOZ_ASSERT(mDrawTarget == dt);
+    if (!mDrawTarget) {
+      return false;
+    }
+    mDrawTarget = nullptr;
+  }
+  mTexture->Unlock();
+  return true;
+}
+
+already_AddRefed<gfx::SourceSurface>
+PersistentBufferProviderAccelerated::BorrowSnapshot(gfx::DrawTarget* aTarget) {
+  if (mDrawTarget) {
+    MOZ_ASSERT(mTexture->IsLocked());
+  } else {
+    if (mTexture->IsLocked()) {
+      MOZ_ASSERT(false);
+      return nullptr;
+    }
+    if (!mTexture->Lock(OpenMode::OPEN_READ)) {
+      return nullptr;
+    }
+  }
+  mSnapshot = mTexture->BorrowSnapshot();
+  return do_AddRef(mSnapshot);
+}
+
+void PersistentBufferProviderAccelerated::ReturnSnapshot(
+    already_AddRefed<gfx::SourceSurface> aSnapshot) {
+  RefPtr<SourceSurface> snapshot = aSnapshot;
+  MOZ_ASSERT(!snapshot || snapshot == mSnapshot);
+  snapshot = nullptr;
+  mTexture->ReturnSnapshot(mSnapshot.forget());
+  if (!mDrawTarget) {
+    mTexture->Unlock();
+  }
+}
+
+Maybe<SurfaceDescriptor> PersistentBufferProviderAccelerated::GetFrontBuffer() {
+  SurfaceDescriptor desc;
+  if (mTexture->GetInternalData()->Serialize(desc)) {
+    return Some(desc);
+  }
+  return Nothing();
+}
+
+bool PersistentBufferProviderAccelerated::RequiresRefresh() const {
+  return mTexture->GetInternalData()->RequiresRefresh();
+}
+
+already_AddRefed<FwdTransactionTracker>
+PersistentBufferProviderAccelerated::UseCompositableForwarder(
+    CompositableForwarder* aForwarder) {
+  return mTexture->GetInternalData()->UseCompositableForwarder(aForwarder);
+}
+
+// static
+already_AddRefed<PersistentBufferProviderShared>
+PersistentBufferProviderShared::Create(gfx::IntSize aSize,
+                                       gfx::SurfaceFormat aFormat,
+                                       KnowsCompositor* aKnowsCompositor,
+                                       bool aWillReadFrequently,
+                                       const Maybe<uint64_t>& aWindowID) {
+  if (!aKnowsCompositor || !aKnowsCompositor->GetTextureForwarder() ||
+      !aKnowsCompositor->GetTextureForwarder()->IPCOpen()) {
+    return nullptr;
+  }
+
+  if (!StaticPrefs::layers_shared_buffer_provider_enabled()) {
+    return nullptr;
+  }
+
+#ifdef XP_WIN
+  // Bug 1285271 - Disable shared buffer provider on Windows with D2D due to
+  // instability.
+  aWillReadFrequently = true;
+#endif
+
+  RefPtr<TextureClient> texture =
+      CreateTexture(aKnowsCompositor, aFormat, aSize, aWillReadFrequently);
+  if (!texture) {
+    return nullptr;
+  }
+
+  RefPtr<PersistentBufferProviderShared> provider =
+      new PersistentBufferProviderShared(aSize, aFormat, aKnowsCompositor,
+                                         texture, aWillReadFrequently,
+                                         aWindowID);
+  return provider.forget();
+}
+
+PersistentBufferProviderShared::PersistentBufferProviderShared(
+    gfx::IntSize aSize, gfx::SurfaceFormat aFormat,
+    KnowsCompositor* aKnowsCompositor, RefPtr<TextureClient>& aTexture,
+    bool aWillReadFrequently, const Maybe<uint64_t>& aWindowID)
+    : mSize(aSize),
+      mFormat(aFormat),
+      mKnowsCompositor(aKnowsCompositor),
+      mFront(Nothing()),
+      mWillReadFrequently(aWillReadFrequently),
+      mWindowID(aWindowID) {
+  MOZ_ASSERT(aKnowsCompositor);
+  if (mTextures.append(aTexture)) {
+    mBack = Some<uint32_t>(0);
+  }
+
+  // XXX KnowsCompositor could be used for mMaxAllowedTextures
+  if (gfxVars::UseWebRenderTripleBufferingWin()) {
+    ++mMaxAllowedTextures;
+  }
+
+  MOZ_COUNT_CTOR(PersistentBufferProviderShared);
+}
+
+PersistentBufferProviderShared::~PersistentBufferProviderShared() {
+  MOZ_COUNT_DTOR(PersistentBufferProviderShared);
+
+  if (IsActivityTracked()) {
+    if (auto* cm = CanvasManagerChild::Get()) {
+      cm->GetActiveResourceTracker()->RemoveObject(this);
+    } else {
+      MOZ_ASSERT_UNREACHABLE("Tracked but no CanvasManagerChild!");
+    }
+  }
+
+  Destroy();
+}
+
+bool PersistentBufferProviderShared::SetKnowsCompositor(
+    KnowsCompositor* aKnowsCompositor, bool& aOutLostFrontTexture) {
+  MOZ_ASSERT(aKnowsCompositor);
+  MOZ_ASSERT(!aOutLostFrontTexture);
+  if (!aKnowsCompositor) {
+    return false;
+  }
+
+  if (mKnowsCompositor == aKnowsCompositor) {
+    // The forwarder should not change most of the time.
+    return true;
+  }
+
+  if (IsActivityTracked()) {
+    if (auto* cm = CanvasManagerChild::Get()) {
+      cm->GetActiveResourceTracker()->RemoveObject(this);
+    } else {
+      MOZ_ASSERT_UNREACHABLE("Tracked but no CanvasManagerChild!");
+    }
+  }
+
+  if (mKnowsCompositor->GetTextureForwarder() !=
+          aKnowsCompositor->GetTextureForwarder() ||
+      mKnowsCompositor->GetCompositorBackendType() !=
+          aKnowsCompositor->GetCompositorBackendType()) {
+    // We are going to be used with an different and/or incompatible forwarder.
+    // This should be extremely rare. We have to copy the front buffer into a
+    // texture that is compatible with the new forwarder.
+
+    // Grab the current front buffer.
+    RefPtr<TextureClient> prevTexture = GetTexture(mFront);
+
+    // Get rid of everything else
+    Destroy();
+
+    if (prevTexture && !prevTexture->IsValid()) {
+      aOutLostFrontTexture = true;
+    } else if (prevTexture && prevTexture->IsValid()) {
+      RefPtr<TextureClient> newTexture =
+          CreateTexture(aKnowsCompositor, mFormat, mSize, mWillReadFrequently);
+
+      MOZ_ASSERT(newTexture);
+      if (!newTexture) {
+        return false;
+      }
+
+      // If we early-return in one of the following branches, we will
+      // leave the buffer provider in an empty state, since we called
+      // Destroy. Not ideal but at least we won't try to use it with a
+      // an incompatible ipc channel.
+
+      if (!newTexture->Lock(OpenMode::OPEN_WRITE)) {
+        return false;
+      }
+
+      if (!prevTexture->Lock(OpenMode::OPEN_READ)) {
+        newTexture->Unlock();
+        return false;
+      }
+
+      bool success =
+          prevTexture->CopyToTextureClient(newTexture, nullptr, nullptr);
+
+      prevTexture->Unlock();
+      newTexture->Unlock();
+
+      if (!success) {
+        return false;
+      }
+
+      if (!mTextures.append(newTexture)) {
+        return false;
+      }
+      mFront = Some<uint32_t>(mTextures.length() - 1);
+      mBack = mFront;
+    }
+  }
+
+  mKnowsCompositor = aKnowsCompositor;
+
+  return true;
+}
+
+TextureClient* PersistentBufferProviderShared::GetTexture(
+    const Maybe<uint32_t>& aIndex) {
+  if (aIndex.isNothing() || !CheckIndex(aIndex.value())) {
+    return nullptr;
+  }
+  return mTextures[aIndex.value()];
+}
+
+already_AddRefed<gfx::DrawTarget>
+PersistentBufferProviderShared::BorrowDrawTarget(
+    const gfx::IntRect& aPersistedRect) {
+  if (!mKnowsCompositor->GetTextureForwarder() ||
+      !mKnowsCompositor->GetTextureForwarder()->IPCOpen()) {
+    return nullptr;
+  }
+
+  auto* cm = CanvasManagerChild::Get();
+  if (NS_WARN_IF(!cm)) {
+    return nullptr;
+  }
+
+  MOZ_ASSERT(!mSnapshot);
+
+  if (IsActivityTracked()) {
+    cm->GetActiveResourceTracker()->MarkUsed(this);
+  } else {
+    cm->GetActiveResourceTracker()->AddObject(this);
+  }
+
+  if (mDrawTarget) {
+    RefPtr<gfx::DrawTarget> dt(mDrawTarget);
+    return dt.forget();
+  }
+
+  auto previousBackBuffer = mBack;
+
+  TextureClient* tex = GetTexture(mBack);
+
+  // First try to reuse the current back buffer. If we can do that it means
+  // we can skip copying its content to the new back buffer.
+  if (tex && tex->IsReadLocked()) {
+    // The back buffer is currently used by the compositor, we can't draw
+    // into it.
+    tex = nullptr;
+  }
+
+  if (!tex) {
+    // Try to grab an already allocated texture if any is available.
+    for (uint32_t i = 0; i < mTextures.length(); ++i) {
+      if (!mTextures[i]->IsReadLocked()) {
+        mBack = Some(i);
+        tex = mTextures[i];
+        break;
+      }
+    }
+  }
+
+  if (!tex) {
+    // We have to allocate a new texture.
+    if (mTextures.length() >= mMaxAllowedTextures) {
+      // We should never need to buffer that many textures, something's wrong.
+      // In theory we throttle the main thread when the compositor can't keep
+      // up, so we shoud never get in a situation where we sent 4 textures to
+      // the compositor and the latter has not released any of them. In
+      // practice, though, the throttling mechanism appears to have some issues,
+      // especially when switching between layer managers (during tab-switch).
+      // To make sure we don't get too far ahead of the compositor, we send a
+      // sync ping to the compositor thread...
+      mKnowsCompositor->SyncWithCompositor(mWindowID);
+      // ...and try again.
+      for (uint32_t i = 0; i < mTextures.length(); ++i) {
+        if (!mTextures[i]->IsReadLocked()) {
+          gfxCriticalNote << "Managed to allocate after flush.";
+          mBack = Some(i);
+          tex = mTextures[i];
+          break;
+        }
+      }
+
+      if (!tex) {
+        gfxCriticalNote << "Unexpected BufferProvider over-production.";
+        // It would be pretty bad to keep piling textures up at this point so we
+        // call NotifyInactive to remove some of our textures.
+        NotifyInactive();
+        // Give up now. The caller can fall-back to a non-shared buffer
+        // provider.
+        return nullptr;
+      }
+    }
+
+    RefPtr<TextureClient> newTexture =
+        CreateTexture(mKnowsCompositor, mFormat, mSize, mWillReadFrequently);
+
+    MOZ_ASSERT(newTexture);
+    if (newTexture) {
+      if (mTextures.append(newTexture)) {
+        tex = newTexture;
+        mBack = Some<uint32_t>(mTextures.length() - 1);
+      }
+    }
+  }
+
+  if (!tex) {
+    return nullptr;
+  }
+
+  if (mPermanentBackBuffer) {
+    // If we have a permanent back buffer lock the selected one and switch to
+    // the permanent one before borrowing the DrawTarget. We will copy back into
+    // the selected one when ReturnDrawTarget is called, before we make it the
+    // new front buffer.
+    if (!tex->Lock(OpenMode::OPEN_WRITE)) {
+      return nullptr;
+    }
+    tex = mPermanentBackBuffer;
+  } else {
+    // Copy from the previous back buffer if required.
+    Maybe<TextureClientAutoLock> autoReadLock;
+    TextureClient* previous = nullptr;
+    if (mBack != previousBackBuffer && !aPersistedRect.IsEmpty()) {
+      if (tex->HasSynchronization()) {
+        // We are about to read lock a texture that is in use by the compositor
+        // and has synchronization. To prevent possible future contention we
+        // switch to using a permanent back buffer.
+        mPermanentBackBuffer = CreateTexture(mKnowsCompositor, mFormat, mSize,
+                                             mWillReadFrequently);
+        if (!mPermanentBackBuffer) {
+          return nullptr;
+        }
+        if (!tex->Lock(OpenMode::OPEN_WRITE)) {
+          return nullptr;
+        }
+        tex = mPermanentBackBuffer;
+      }
+
+      previous = GetTexture(previousBackBuffer);
+      if (previous) {
+        autoReadLock.emplace(previous, OpenMode::OPEN_READ);
+      }
+    }
+
+    if (!tex->Lock(OpenMode::OPEN_READ_WRITE)) {
+      return nullptr;
+    }
+
+    if (autoReadLock.isSome() && autoReadLock->Succeeded() && previous) {
+      DebugOnly<bool> success =
+          previous->CopyToTextureClient(tex, &aPersistedRect, nullptr);
+      MOZ_ASSERT(success);
+    }
+  }
+
+  mDrawTarget = tex->BorrowDrawTarget();
+  if (mDrawTarget) {
+    // This is simply to ensure the DrawTarget gets initialized, and will detect
+    // a device reset, even if we're on the main thread.
+    mDrawTarget->ClearRect(Rect(0, 0, 0, 0));
+
+    if (!mDrawTarget->IsValid()) {
+      mDrawTarget = nullptr;
+    }
+  }
+
+  RefPtr<gfx::DrawTarget> dt(mDrawTarget);
+  return dt.forget();
+}
+
+bool PersistentBufferProviderShared::ReturnDrawTarget(
+    already_AddRefed<gfx::DrawTarget> aDT) {
+  RefPtr<gfx::DrawTarget> dt(aDT);
+  MOZ_ASSERT(mDrawTarget == dt);
+  // Can't change the current front buffer while its snapshot is borrowed!
+  MOZ_ASSERT(!mSnapshot);
+
+  TextureClient* back = GetTexture(mBack);
+  MOZ_ASSERT(back);
+
+  mDrawTarget = nullptr;
+  dt = nullptr;
+
+  // If we have a permanent back buffer we have actually been drawing to that,
+  // so now we must copy to the shared one.
+  if (mPermanentBackBuffer && back) {
+    DebugOnly<bool> success =
+        mPermanentBackBuffer->CopyToTextureClient(back, nullptr, nullptr);
+    MOZ_ASSERT(success);
+
+    // Let our permanent back buffer know that we have finished drawing.
+    mPermanentBackBuffer->EndDraw();
+  }
+
+  if (back) {
+    back->Unlock();
+    mFront = mBack;
+  }
+
+  return !!back;
+}
+
+TextureClient* PersistentBufferProviderShared::GetTextureClient() {
+  // Can't access the front buffer while drawing.
+  MOZ_ASSERT(!mDrawTarget);
+  TextureClient* texture = GetTexture(mFront);
+  if (!texture) {
+    gfxCriticalNote
+        << "PersistentBufferProviderShared: front buffer unavailable";
+    return nullptr;
+  }
+
+  // Sometimes, for example on tab switch, we re-forward our texture. So if we
+  // are and it is still read locked, then borrow and return our DrawTarget to
+  // force it to be copied to a texture that we will safely read lock.
+  if (texture->IsReadLocked()) {
+    RefPtr<DrawTarget> dt =
+        BorrowDrawTarget(IntRect(0, 0, mSize.width, mSize.height));
+
+    // If we failed to borrow a DrawTarget then all our textures must be read
+    // locked or we failed to create one, so we'll just return the current front
+    // buffer even though that might lead to contention.
+    if (dt) {
+      ReturnDrawTarget(dt.forget());
+      texture = GetTexture(mFront);
+      if (!texture) {
+        gfxCriticalNote
+            << "PersistentBufferProviderShared: front buffer unavailable";
+        return nullptr;
+      }
+    }
+  } else {
+    // If it isn't read locked then make sure it is set as updated, so that we
+    // will always read lock even if we've forwarded the texture before.
+    texture->SetUpdated();
+  }
+
+  return texture;
+}
+
+already_AddRefed<gfx::SourceSurface>
+PersistentBufferProviderShared::BorrowSnapshot(gfx::DrawTarget* aTarget) {
+  // If we have a permanent back buffer we can always use that to snapshot.
+  if (mPermanentBackBuffer) {
+    mSnapshot = mPermanentBackBuffer->BorrowSnapshot();
+    return do_AddRef(mSnapshot);
+  }
+
+  if (mDrawTarget) {
+    auto back = GetTexture(mBack);
+    MOZ_ASSERT(back && back->IsLocked());
+    mSnapshot = back->BorrowSnapshot();
+    return do_AddRef(mSnapshot);
+  }
+
+  auto front = GetTexture(mFront);
+  if (!front || front->IsLocked()) {
+    MOZ_ASSERT(false);
+    return nullptr;
+  }
+
+  if (front->IsReadLocked() && front->HasSynchronization()) {
+    // We are about to read lock a texture that is in use by the compositor and
+    // has synchronization. To prevent possible future contention we switch to
+    // using a permanent back buffer.
+    mPermanentBackBuffer =
+        CreateTexture(mKnowsCompositor, mFormat, mSize, mWillReadFrequently);
+    if (!mPermanentBackBuffer ||
+        !mPermanentBackBuffer->Lock(OpenMode::OPEN_READ_WRITE)) {
+      return nullptr;
+    }
+
+    if (!front->Lock(OpenMode::OPEN_READ)) {
+      return nullptr;
+    }
+
+    DebugOnly<bool> success =
+        front->CopyToTextureClient(mPermanentBackBuffer, nullptr, nullptr);
+    MOZ_ASSERT(success);
+    front->Unlock();
+    mSnapshot = mPermanentBackBuffer->BorrowSnapshot();
+    return do_AddRef(mSnapshot);
+  }
+
+  if (!front->Lock(OpenMode::OPEN_READ)) {
+    return nullptr;
+  }
+
+  mSnapshot = front->BorrowSnapshot();
+
+  return do_AddRef(mSnapshot);
+}
+
+void PersistentBufferProviderShared::ReturnSnapshot(
+    already_AddRefed<gfx::SourceSurface> aSnapshot) {
+  RefPtr<SourceSurface> snapshot = aSnapshot;
+  MOZ_ASSERT(!snapshot || snapshot == mSnapshot);
+
+  mSnapshot = nullptr;
+  snapshot = nullptr;
+
+  if (mDrawTarget || mPermanentBackBuffer) {
+    return;
+  }
+
+  auto front = GetTexture(mFront);
+  if (front) {
+    front->Unlock();
+  }
+}
+
+void PersistentBufferProviderShared::NotifyInactive() {
+  ClearCachedResources();
+}
+
+void PersistentBufferProviderShared::ClearCachedResources() {
+  RefPtr<TextureClient> front = GetTexture(mFront);
+  RefPtr<TextureClient> back = GetTexture(mBack);
+
+  // Clear all textures (except the front and back ones that we just kept).
+  mTextures.clear();
+  mPermanentBackBuffer = nullptr;
+
+  if (back) {
+    if (mTextures.append(back)) {
+      mBack = Some<uint32_t>(0);
+    }
+    if (front == back) {
+      mFront = mBack;
+    }
+  }
+
+  if (front && front != back) {
+    if (mTextures.append(front)) {
+      mFront = Some<uint32_t>(mTextures.length() - 1);
+    }
+  }
+}
+
+void PersistentBufferProviderShared::Destroy() {
+  mSnapshot = nullptr;
+  mDrawTarget = nullptr;
+
+  if (mPermanentBackBuffer) {
+    mPermanentBackBuffer->Unlock();
+    mPermanentBackBuffer = nullptr;
+  }
+
+  for (auto& texture : mTextures) {
+    if (texture && texture->IsLocked()) {
+      MOZ_ASSERT(false);
+      texture->Unlock();
+    }
+  }
+
+  mTextures.clear();
+}
+
+}  // namespace layers
+}  // namespace mozilla