Queue commands to the Os to start a video device.

content/browser/renderer_host/media/video_capture_manager.h

Index: content/browser/renderer_host/media/video_capture_manager.h
diff --git a/content/browser/renderer_host/media/video_capture_manager.h b/content/browser/renderer_host/media/video_capture_manager.h
index 58ed04b6be5e8040f65fca8f6a3ca006a32ff19a..f8bc4b35bde24acd3a85178f2f5d86f8987a6834 100644
--- a/content/browser/renderer_host/media/video_capture_manager.h
+++ b/content/browser/renderer_host/media/video_capture_manager.h
@@ -12,11 +12,13 @@
 #ifndef CONTENT_BROWSER_RENDERER_HOST_MEDIA_VIDEO_CAPTURE_MANAGER_H_
 #define CONTENT_BROWSER_RENDERER_HOST_MEDIA_VIDEO_CAPTURE_MANAGER_H_
 
+#include <list>
 #include <map>
 #include <set>
 #include <string>
 
 #include "base/memory/ref_counted.h"
+#include "base/memory/scoped_vector.h"
 #include "base/memory/weak_ptr.h"
 #include "base/message_loop/message_loop.h"
 #include "base/process/process_handle.h"
@@ -182,18 +184,34 @@ class CONTENT_EXPORT VideoCaptureManager : public MediaStreamProvider {
       const media::VideoCaptureDeviceInfos& old_device_info_cache,
       scoped_ptr<media::VideoCaptureDevice::Names> names_snapshot);
 
-  // Creates and Starts a new VideoCaptureDevice, storing the result in
-  // |entry->video_capture_device|. Ownership of |client| passes to
+  // Starting a capture device can take 1-2 seconds.
+  // To avoid multiple unnecessary start/stop commands to the OS, each start
+  // request is queued in |device_start_queue_|.
+  // QueueStartDevice creates a new entry in |device_start_queue_| and posts a
+  // request to start the device on the device thread unless there is
+  // another request pending start.
+  void QueueStartDevice(media::VideoCaptureSessionId session_id,
+                        DeviceEntry* entry,
+                        const media::VideoCaptureParams& params);
+  void OnDeviceStarted(DeviceEntry* device_entry,
+                       scoped_ptr<media::VideoCaptureDevice> device);
+  void DoStopDevice(DeviceEntry* entry);
+  void HandleQueuedStartRequest();
+
+  // Creates and Starts a new VideoCaptureDevice. The resulting
+  // VideoCaptureDevice is returned to the IO-thread and stored in
+  // a DeviceEntry in |devices_|. Ownership of |client| passes to
   // the device.
-  void DoStartDeviceOnDeviceThread(
+  scoped_ptr<media::VideoCaptureDevice> DoStartDeviceOnDeviceThread(
       media::VideoCaptureSessionId session_id,
-      DeviceEntry* entry,
+      const std::string& device_id,
+      MediaStreamType stream_type,
       const media::VideoCaptureParams& params,
       scoped_ptr<media::VideoCaptureDevice::Client> client);
 
   // Stops and destroys the VideoCaptureDevice held in
-  // |entry->video_capture_device|.
-  void DoStopDeviceOnDeviceThread(DeviceEntry* entry);
+  // |device|.
+  void DoStopDeviceOnDeviceThread(scoped_ptr<media::VideoCaptureDevice> device);
 
   media::VideoCaptureDeviceInfo* FindDeviceInfoById(
       const std::string& id,
@@ -226,9 +244,7 @@ class CONTENT_EXPORT VideoCaptureManager : public MediaStreamProvider {
   // when they are not used any longer.
   //
   // The set of currently started VideoCaptureDevice and VideoCaptureController
-  // objects is only accessed from IO thread, though the DeviceEntry instances
-  // themselves may visit to the device thread for device creation and
-  // destruction.
+  // objects is only accessed from IO thread.
   struct DeviceEntry {
     DeviceEntry(MediaStreamType stream_type,
                 const std::string& id,
@@ -244,9 +260,24 @@ class CONTENT_EXPORT VideoCaptureManager : public MediaStreamProvider {
     // The capture device. Only used from the device thread.
     scoped_ptr<media::VideoCaptureDevice> video_capture_device;
   };
-  typedef std::set<DeviceEntry*> DeviceEntries;
+  typedef ScopedVector<DeviceEntry> DeviceEntries;

[tommi (sloooow) - chröme, 2014/12/16 22:31:25]

nit: empty line before this one

[perkj_chrome, 2014/12/17 10:14:10]

Done.

+  // Currently opened devices. The device may or may not be started.
   DeviceEntries devices_;
 
+  // Struct used for queing request for starting a device.
+  struct CaptureDeviceStartRequest {
+    CaptureDeviceStartRequest(
+        DeviceEntry* entry,
+        media::VideoCaptureSessionId session_id,
+        const media::VideoCaptureParams& params);
+
+    DeviceEntry* entry; // Owned by |devices_|

[tommi (sloooow) - chröme, 2014/12/16 22:31:25]

do we use this pointer? (If it's only used for lookup, then it would be good to
add a note on that)

[perkj_chrome, 2014/12/17 10:14:10]

Well, its used in HandleQueuedStartRequest() but we do dcheck that it exists. 

[tommi (sloooow) - chröme, 2014/12/17 14:46:12]

What about making this pointer type a void* to make sure that it never gets used
for anything except lookups?  Alternatively, we could generate a serial number
per instance and use that as an id.

+    media::VideoCaptureSessionId session_id;
+    media::VideoCaptureParams params;
+  };
+  typedef std::list<CaptureDeviceStartRequest> DeviceStartQueue;

[tommi (sloooow) - chröme, 2014/12/16 22:31:25]

nit: empty line above

[perkj_chrome, 2014/12/17 10:14:10]

Done.

[perkj_chrome, 2014/12/17 10:14:10]

Done.

+  DeviceStartQueue device_start_queue_;
+
   // Device creation factory injected on construction from MediaStreamManager or
   // from the test harness.
   scoped_ptr<media::VideoCaptureDeviceFactory> video_capture_device_factory_;