Mozart: Ensure time always runs forward.

services/gfx/compositor/graph/universe.cc

+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "services/gfx/compositor/graph/universe.h"
+
+#include "base/logging.h"
+#include "mojo/services/gfx/composition/cpp/formatting.h"
+#include "services/gfx/compositor/graph/scene_content.h"
+
+namespace compositor {
+
+Universe::Universe() {}
+
+Universe::~Universe() {}
+
+void Universe::AddScene(const SceneLabel& label) {
+  auto it = scenes_.find(label.token());

[abarth, 2016/04/08 03:13:50]

I'd move the find into the DCHECK

[jeffbrown, 2016/04/08 03:35:20]

Done.

+  DCHECK(it == scenes_.end());
+  scenes_.emplace(label.token(),
+                  std::unique_ptr<SceneInfo>(new SceneInfo(label)));
+}
+
+void Universe::PresentScene(const scoped_refptr<const SceneContent>& content) {
+  auto it = scenes_.find(content->label().token());
+  DCHECK(it != scenes_.end());
+  it->second->content_queue.emplace_front(content);
+}
+
+void Universe::RemoveScene(
+    const mojo::gfx::composition::SceneToken& scene_token) {
+  auto it = scenes_.find(scene_token.value);
+  DCHECK(it != scenes_.end());
+  scenes_.erase(it);
+}
+
+scoped_refptr<const Snapshot> Universe::SnapshotScene(
+    const mojo::gfx::composition::SceneToken& scene_token,
+    uint32_t version,
+    std::ostream* block_log) {
+  generation_++;
+  CHECK(generation_);
+
+  Snapshotter snapshotter(this, block_log);
+  scoped_refptr<const Snapshot> snapshot =
+      snapshotter.Build(scene_token, version);
+
+  // TODO(jeffbrown): Find a better way to prune unused scene versions.
+  // This logic is expensive and will break if there are multiple renderers
+  // involved.  Perhaps we should do something like say that all renderers
+  // that live in the same universe should get snapshotted simultaneously.
+  // Or maybe we could be smarter and partition scenes by reachability
+  // using some heuristics to decide which partition should be the scene's
+  // "primary" in case of conflict (so that scenes which only appear on a
+  // single display can be rate-coupled to that display whereas those that
+  // appear in multiple places might experience some resampling).
+  // There's a similar problem lurking in the scheduler mechanism.
+  for (const auto& pair : scenes_) {
+    SceneInfo* info = pair.second.get();
+    if (info->update_generation != generation_ &&
+        info->content_queue.size() > 1) {
+      info->content_queue.erase(info->content_queue.begin() + 1,
+                                info->content_queue.end());
+    }
+  }
+  return snapshot;
+}
+
+Universe::SceneInfo::SceneInfo(const SceneLabel& label) : label(label) {}
+
+Universe::SceneInfo::~SceneInfo() {}
+
+Universe::Snapshotter::Snapshotter(Universe* universe, std::ostream* block_log)
+    : SnapshotBuilder(block_log), universe_(universe) {
+  DCHECK(universe_);
+}
+
+Universe::Snapshotter::~Snapshotter() {
+  DCHECK(!cycle_);  // must have properly unwound any cycles by now
+}
+
+Snapshot::Disposition Universe::Snapshotter::ResolveAndSnapshotScene(
+    const mojo::gfx::composition::SceneToken& scene_token,
+    uint32_t version,
+    scoped_refptr<const SceneContent>* out_content) {
+  auto it = universe_->scenes_.find(scene_token.value);
+  if (it == universe_->scenes_.end()) {
+    if (block_log()) {
+      *block_log() << "Scene not available: " << scene_token << std::endl;
+    }
+    return Snapshot::Disposition::kBlocked;
+  }
+
+  // TODO(jeffbrown): Ok, this logic is downright terrible.  It will end
+  // up doing N^2 work when things are blocked.  Replace this with some kind
+  // of sane invalidation mechanism before the system explodes.
+  SceneInfo* info = it->second.get();
+  if (info->update_generation == universe_->generation_) {
+    if (info->disposition == Snapshot::Disposition::kCycle) {
+      cycle_ = info;  // start unwinding, remember where to stop
+      return Snapshot::Disposition::kCycle;
+    }
+    if (info->disposition == Snapshot::Disposition::kBlocked) {
+      if (block_log()) {
+        *block_log() << "Scene blocked (cached prior disposition): "
+                     << info->label.FormattedLabel() << std::endl;
+      }
+      return Snapshot::Disposition::kBlocked;
+    }
+  } else {
+    info->update_generation = universe_->generation_;
+    if (info->content_queue.empty()) {
+      if (block_log()) {
+        *block_log() << "Scene has not presented any content: "
+                     << info->label.FormattedLabel() << std::endl;
+      }
+      info->disposition = Snapshot::Disposition::kBlocked;
+      return Snapshot::Disposition::kBlocked;
+    }
+
+    auto it = info->content_queue.begin();
+    for (;;) {
+      info->disposition = Snapshot::Disposition::kCycle;
+      info->disposition = SnapshotSceneContent((*it).get());
+      if (info->disposition == Snapshot::Disposition::kSuccess)
+        break;
+      if (info->disposition == Snapshot::Disposition::kCycle) {
+        DCHECK(cycle_);
+        if (block_log()) {
+          *block_log() << "Scene is part of a cycle: "
+                       << (*it)->FormattedLabel() << std::endl;
+        }
+        if (cycle_ == info)
+          cycle_ = nullptr;  // found the ouroboros tail, stop unwinding
+        info->disposition = Snapshot::Disposition::kBlocked;
+        return Snapshot::Disposition::kCycle;
+      }
+      if (++it == info->content_queue.end())
+        return Snapshot::Disposition::kBlocked;
+    }
+    if (it + 1 != info->content_queue.end())
+      info->content_queue.erase(it + 1, info->content_queue.end());
+  }
+
+  DCHECK(info->disposition == Snapshot::Disposition::kSuccess);
+  DCHECK(!info->content_queue.empty());
+  const scoped_refptr<const SceneContent>& content = info->content_queue.back();
+  if (!content->MatchesVersion(version)) {
+    if (block_log()) {
+      *block_log() << "Scene version mismatch: " << info->label.FormattedLabel()
+                   << ", requested version " << version
+                   << ", available version " << content->version() << std::endl;
+    }
+    return Snapshot::Disposition::kBlocked;
+  }
+
+  *out_content = content;
+  return Snapshot::Disposition::kSuccess;
+}
+
+}  // namespace compositor