Motown in-proc streaming framework used to implement media services.

services/media/framework/stages/stage_input.h

Index: services/media/framework/stages/stage_input.h
diff --git a/services/media/framework/stages/stage_input.h b/services/media/framework/stages/stage_input.h
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index 0000000000000000000000000000000000000000..2fe61c64888a2ed28951b19096b91d410a5c08a1
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@@ -0,0 +1,83 @@
+// 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.
+
+#ifndef SERVICES_MEDIA_FRAMEWORK_ENGINE_STAGE_INPUT_H_
+#define SERVICES_MEDIA_FRAMEWORK_ENGINE_STAGE_INPUT_H_
+
+#include "services/media/framework/models/demand.h"
+#include "services/media/framework/packet.h"
+
+namespace mojo {
+namespace media {
+
+class Stage;
+class Engine;
+class StageOutput;
+class LpcmStageInput;
+
+// Represents a stage's connector to an adjacent upstream stage.
+class StageInput {
+ public:
+  StageInput();
+
+  ~StageInput();
+
+  // The stage to which this input is connected.
+  Stage* upstream_stage() const { return upstream_stage_; }
+
+  // The index of the output to which this input is connected.
+  uint32_t output_index() const { return output_index_; }
+
+  // Establishes a connection. Called only by the engine.
+  void connect(Stage* upstream_stage, uint32_t output_index);
+
+  // Breaks a connection. Called only by the engine.
+  void disconnect() {
+    upstream_stage_ = nullptr;
+    output_index_ = 0;
+  }
+
+  // Determines whether the input is connected to an output.
+  bool connected() const { return upstream_stage_ != nullptr; }
+
+  // The connected output.
+  StageOutput& mate() const;
+
+  // Prepare to move packets by providing an allocator for the upstream stage
+  // if we have one and indicating whether we require its use.
+  void Prepare(Allocator* allocator, bool must_allocate);
+
+  // Returns the allocator provided by this input.
+  Allocator* allocator() const;
+
+  // Determines whether the input requires the use of its allocator.
+  bool must_allocate() const;

[jeffbrown, 2016/02/02 05:35:49]

Or else what?  Some arbitrary allocator can be used?  Is this telling the sender
of a packet whether packets can be passed by reference or must be copied, or
what?

The more I read, the more I think this allocator thing needs to be wrapped up
into a stream abstraction of some kind that can handle concerns related to
forwarding or duplicating packets as needed.

I get the sense that you're trying to ensure that whenever a copy is made that
it's being made in-place into the optimal location but it's sort of obscured by
the structure of this code.

[dalesat, 2016/02/02 21:46:40]

We should discuss. This isn't a stream attribute, if I understand what you mean.
The idea is to minimize copies.

+
+  // A packet supplied from upstream.
+  PacketPtr& packet_from_upstream() { return packet_from_upstream_; }
+
+  // Updates mate's demand. Called only by Stage::Update implementations.
+  void SetDemand(Demand demand, Engine* engine) const;
+
+  // Updates packet_from_upstream. Return value indicates whether the stage for
+  // this input should be added to the supply backlog. Called only by
+  // StageOutput instances.
+  virtual bool SupplyPacketFromOutput(PacketPtr packet);

[jeffbrown, 2016/02/02 05:35:48]

Should this type really be subclassed?  I wonder.  I mean at some level, this
thing is just a means for a Stage to get stuff from another Stage so maybe these
should bottom out into method calls on the Engine which is managing the pipeline
and the overall dataflow.

[dalesat, 2016/02/02 21:46:40]

This is virtual so that the LPCM version can run more code when this happens.
Getting stuff from stage to stage is done differently for LPCM.

+
+  // Returns the LPCM specialization if this instance is an LpcmStageInput,
+  // nullptr otherwise.
+  virtual LpcmStageInput* get_lpcm();

[jeffbrown, 2016/02/02 05:35:49]

nit: GetLpcmStageInput().

But something about this doesn't smell right...

[dalesat, 2016/02/02 21:46:40]

This is a downcast. I'd be happy to do it differently, if you can make a
suggestion.

+
+ private:
+  Stage* upstream_stage_;
+  uint32_t output_index_;
+  Allocator* allocator_;
+  bool must_allocate_;
+  PacketPtr packet_from_upstream_;
+};
+
+}  // namespace media
+}  // namespace mojo
+
+#endif  // SERVICES_MEDIA_FRAMEWORK_ENGINE_STAGE_INPUT_H_