browser/media_galleries/media_file_system_registry.cc

// Copyright (c) 2012 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.

// MediaFileSystemRegistry implementation.

#include "chrome/browser/media_galleries/media_file_system_registry.h"

#include <set>
#include <vector>

#include "base/bind.h"
#include "base/callback.h"
#include "base/files/file_path.h"
#include "base/prefs/pref_service.h"
#include "base/stl_util.h"
#include "chrome/browser/chrome_notification_types.h"
#include "chrome/browser/extensions/extension_service.h"
#include "chrome/browser/extensions/extension_system.h"
#include "chrome/browser/media_galleries/imported_media_gallery_registry.h"
#include "chrome/browser/media_galleries/media_file_system_context.h"
#include "chrome/browser/media_galleries/media_galleries_dialog_controller.h"
#include "chrome/browser/media_galleries/media_galleries_preferences_factory.h"
#include "chrome/browser/media_galleries/scoped_mtp_device_map_entry.h"
#include "chrome/browser/profiles/profile.h"
#include "chrome/browser/storage_monitor/media_storage_util.h"
#include "chrome/browser/storage_monitor/storage_monitor.h"
#include "chrome/common/chrome_paths.h"
#include "chrome/common/extensions/extension.h"
#include "chrome/common/extensions/extension_constants.h"
#include "chrome/common/extensions/extension_set.h"
#include "chrome/common/pref_names.h"
#include "content/public/browser/browser_thread.h"
#include "content/public/browser/notification_details.h"
#include "content/public/browser/notification_observer.h"
#include "content/public/browser/notification_registrar.h"
#include "content/public/browser/notification_source.h"
#include "content/public/browser/notification_types.h"
#include "content/public/browser/render_process_host.h"
#include "content/public/browser/render_view_host.h"
#include "content/public/browser/web_contents.h"
#include "webkit/browser/fileapi/isolated_context.h"
#include "webkit/common/fileapi/file_system_types.h"

using content::BrowserThread;
using content::NavigationController;
using content::RenderProcessHost;
using content::WebContents;
using fileapi::IsolatedContext;

namespace chrome {

namespace {

struct InvalidatedGalleriesInfo {
  std::set<ExtensionGalleriesHost*> extension_hosts;
  std::set<MediaGalleryPrefId> pref_ids;
};

}  // namespace

MediaFileSystemInfo::MediaFileSystemInfo(const string16& fs_name,
                                         const base::FilePath& fs_path,
                                         const std::string& filesystem_id,
                                         MediaGalleryPrefId pref_id,
                                         const std::string& transient_device_id,
                                         bool removable,
                                         bool media_device)
    : name(fs_name),
      path(fs_path),
      fsid(filesystem_id),
      pref_id(pref_id),
      transient_device_id(transient_device_id),
      removable(removable),
      media_device(media_device) {
}

MediaFileSystemInfo::MediaFileSystemInfo() {}
MediaFileSystemInfo::~MediaFileSystemInfo() {}

// Tracks the liveness of multiple RenderProcessHosts that the caller is
// interested in. Once all of the RPHs have closed or been terminated a call
// back informs the caller.
class RPHReferenceManager : public content::NotificationObserver {
 public:
  // |no_references_callback| is called when the last RenderViewHost reference
  // goes away. RenderViewHost references are added through ReferenceFromRVH().
  explicit RPHReferenceManager(const base::Closure& no_references_callback)
      : no_references_callback_(no_references_callback) {
  }

  virtual ~RPHReferenceManager() {
    Reset();
  }

  // Remove all references, but don't call |no_references_callback|.
  void Reset() {
    STLDeleteValues(&refs_);
  }

  // Returns true if there are no references;
  bool empty() const {
    return refs_.empty();
  }

  // Adds a reference to the passed |rvh|. Calling this multiple times with
  // the same |rvh| is a no-op.
  void ReferenceFromRVH(const content::RenderViewHost* rvh) {
    WebContents* contents = WebContents::FromRenderViewHost(rvh);
    RenderProcessHost* rph = contents->GetRenderProcessHost();
    RPHReferenceState* state = NULL;
    if (!ContainsKey(refs_, rph)) {
      state = new RPHReferenceState;
      refs_[rph] = state;
      state->registrar.Add(
          this, content::NOTIFICATION_RENDERER_PROCESS_TERMINATED,
          content::Source<RenderProcessHost>(rph));
    } else {
      state = refs_[rph];
    }

    if (state->web_contents_set.insert(contents).second) {
      state->registrar.Add(this, content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
          content::Source<WebContents>(contents));
      state->registrar.Add(this, content::NOTIFICATION_NAV_ENTRY_COMMITTED,
          content::Source<NavigationController>(&contents->GetController()));
    }
  }

 private:
  struct RPHReferenceState {
    content::NotificationRegistrar registrar;
    std::set<const WebContents*> web_contents_set;
  };
  typedef std::map<const RenderProcessHost*, RPHReferenceState*> RPHRefCount;

  // NotificationObserver implementation.
  virtual void Observe(int type,
                       const content::NotificationSource& source,
                       const content::NotificationDetails& details) OVERRIDE {
    switch (type) {
      case content::NOTIFICATION_RENDERER_PROCESS_TERMINATED: {
        OnRendererProcessTerminated(
            content::Source<RenderProcessHost>(source).ptr());
        break;
      }
      case content::NOTIFICATION_WEB_CONTENTS_DESTROYED: {
        OnWebContentsDestroyedOrNavigated(
            content::Source<WebContents>(source).ptr());
        break;
      }
      case content::NOTIFICATION_NAV_ENTRY_COMMITTED: {
        NavigationController* controller =
            content::Source<NavigationController>(source).ptr();
        WebContents* contents = controller->GetWebContents();
        OnWebContentsDestroyedOrNavigated(contents);
        break;
      }
      default: {
        NOTREACHED();
        break;
      }
    }
  }

  void OnRendererProcessTerminated(const RenderProcessHost* rph) {
    RPHRefCount::iterator rph_info = refs_.find(rph);
    DCHECK(rph_info != refs_.end());
    delete rph_info->second;
    refs_.erase(rph_info);
    if (refs_.empty())
      no_references_callback_.Run();
  }

  void OnWebContentsDestroyedOrNavigated(const WebContents* contents) {
    RenderProcessHost* rph = contents->GetRenderProcessHost();
    RPHRefCount::iterator rph_info = refs_.find(rph);
    DCHECK(rph_info != refs_.end());

    rph_info->second->registrar.Remove(
        this, content::NOTIFICATION_WEB_CONTENTS_DESTROYED,
        content::Source<WebContents>(contents));
    rph_info->second->registrar.Remove(
        this, content::NOTIFICATION_NAV_ENTRY_COMMITTED,
        content::Source<NavigationController>(&contents->GetController()));

    rph_info->second->web_contents_set.erase(contents);
    if (rph_info->second->web_contents_set.empty())
      OnRendererProcessTerminated(rph);
  }

  // A callback to call when the last RVH reference goes away.
  base::Closure no_references_callback_;

  // The set of render processes and web contents that may have references to
  // the file system ids this instance manages.
  RPHRefCount refs_;
};

// The main owner of this class is
// |MediaFileSystemRegistry::extension_hosts_map_|, but a callback may
// temporarily hold a reference.
class ExtensionGalleriesHost
    : public base::RefCountedThreadSafe<ExtensionGalleriesHost> {
 public:
  // |no_references_callback| is called when the last RenderViewHost reference
  // goes away. RenderViewHost references are added through ReferenceFromRVH().
  ExtensionGalleriesHost(MediaFileSystemContext* file_system_context,
                         const base::Closure& no_references_callback)
      : file_system_context_(file_system_context),
        no_references_callback_(no_references_callback),
        rph_refs_(base::Bind(&ExtensionGalleriesHost::CleanUp,
                             base::Unretained(this))) {
  }

  // For each gallery in the list of permitted |galleries|, checks if the
  // device is attached and if so looks up or creates a file system id and
  // passes the information needed for the renderer to create those file
  // system objects to the |callback|.
  void GetMediaFileSystems(const MediaGalleryPrefIdSet& galleries,
                           const MediaGalleriesPrefInfoMap& galleries_info,
                           const MediaFileSystemsCallback& callback) {
    // Extract all the device ids so we can make sure they are attached.
    MediaStorageUtil::DeviceIdSet* device_ids =
        new MediaStorageUtil::DeviceIdSet;
    for (std::set<MediaGalleryPrefId>::const_iterator id = galleries.begin();
         id != galleries.end();
         ++id) {
      device_ids->insert(galleries_info.find(*id)->second.device_id);
    }
    MediaStorageUtil::FilterAttachedDevices(device_ids, base::Bind(
        &ExtensionGalleriesHost::GetMediaFileSystemsForAttachedDevices, this,
        base::Owned(device_ids), galleries, galleries_info, callback));
  }

  void RevokeOldGalleries(const MediaGalleryPrefIdSet& new_galleries) {
    if (new_galleries.size() == pref_id_map_.size())
      return;

    MediaGalleryPrefIdSet old_galleries;
    for (PrefIdFsInfoMap::const_iterator it = pref_id_map_.begin();
         it != pref_id_map_.end();
         ++it) {
      old_galleries.insert(it->first);
    }
    MediaGalleryPrefIdSet invalid_galleries;
    std::set_difference(old_galleries.begin(), old_galleries.end(),
                        new_galleries.begin(), new_galleries.end(),
                        std::inserter(invalid_galleries,
                                      invalid_galleries.begin()));
    for (MediaGalleryPrefIdSet::const_iterator it = invalid_galleries.begin();
         it != invalid_galleries.end();
         ++it) {
      RevokeGalleryByPrefId(*it);
    }
  }

  // Revoke the file system for |id| if this extension has created one for |id|.
  void RevokeGalleryByPrefId(MediaGalleryPrefId id) {
    PrefIdFsInfoMap::iterator gallery = pref_id_map_.find(id);
    if (gallery == pref_id_map_.end())
      return;

    file_system_context_->RevokeFileSystem(gallery->second.fsid);
    pref_id_map_.erase(gallery);

    MediaDeviceEntryReferencesMap::iterator mtp_device_host =
        media_device_map_references_.find(id);
    if (mtp_device_host != media_device_map_references_.end())
      media_device_map_references_.erase(mtp_device_host);

    if (pref_id_map_.empty()) {
      rph_refs_.Reset();
      CleanUp();
    }
  }

  // Indicate that the passed |rvh| will reference the file system ids created
  // by this class.
  void ReferenceFromRVH(const content::RenderViewHost* rvh) {
    rph_refs_.ReferenceFromRVH(rvh);
  }

 private:
  typedef std::map<MediaGalleryPrefId, MediaFileSystemInfo> PrefIdFsInfoMap;
  typedef std::map<MediaGalleryPrefId, scoped_refptr<ScopedMTPDeviceMapEntry> >
      MediaDeviceEntryReferencesMap;

  // Private destructor and friend declaration for ref counted implementation.
  friend class base::RefCountedThreadSafe<ExtensionGalleriesHost>;

  virtual ~ExtensionGalleriesHost() {
    DCHECK(rph_refs_.empty());
    DCHECK(pref_id_map_.empty());

    DCHECK(media_device_map_references_.empty());
  }

  void GetMediaFileSystemsForAttachedDevices(
      const MediaStorageUtil::DeviceIdSet* attached_devices,
      const MediaGalleryPrefIdSet& galleries,
      const MediaGalleriesPrefInfoMap& galleries_info,
      const MediaFileSystemsCallback& callback) {
    std::vector<MediaFileSystemInfo> result;
    MediaGalleryPrefIdSet new_galleries;
    for (std::set<MediaGalleryPrefId>::const_iterator pref_id_it =
             galleries.begin();
         pref_id_it != galleries.end();
         ++pref_id_it) {
      const MediaGalleryPrefId& pref_id = *pref_id_it;
      const MediaGalleryPrefInfo& gallery_info =
          galleries_info.find(pref_id)->second;
      const std::string& device_id = gallery_info.device_id;
      if (!ContainsKey(*attached_devices, device_id))
        continue;

      PrefIdFsInfoMap::const_iterator existing_info =
          pref_id_map_.find(pref_id);
      if (existing_info != pref_id_map_.end()) {
        result.push_back(existing_info->second);
        new_galleries.insert(pref_id);
        continue;
      }

      base::FilePath path = gallery_info.AbsolutePath();
      if (!MediaStorageUtil::CanCreateFileSystem(device_id, path))
        continue;

      std::string fsid;
      if (StorageInfo::IsMassStorageDevice(device_id)) {
        fsid = file_system_context_->RegisterFileSystemForMassStorage(
            device_id, path);
      } else {
        scoped_refptr<ScopedMTPDeviceMapEntry> mtp_device_host;
        fsid = file_system_context_->RegisterFileSystemForMTPDevice(
            device_id, path, &mtp_device_host);
        DCHECK(mtp_device_host.get());
        media_device_map_references_[pref_id] = mtp_device_host;
      }
      if (fsid.empty())
        continue;

      MediaFileSystemInfo new_entry(
          gallery_info.GetGalleryDisplayName(),
          path,
          fsid,
          pref_id,
          GetTransientIdForRemovableDeviceId(device_id),
          StorageInfo::IsRemovableDevice(device_id),
          StorageInfo::IsMediaDevice(device_id));
      result.push_back(new_entry);
      new_galleries.insert(pref_id);
      pref_id_map_[pref_id] = new_entry;
    }

    if (result.size() == 0) {
      rph_refs_.Reset();
      CleanUp();
    } else {
      RevokeOldGalleries(new_galleries);
    }

    callback.Run(result);
  }

  std::string GetTransientIdForRemovableDeviceId(const std::string& device_id) {
    if (!StorageInfo::IsRemovableDevice(device_id))
      return std::string();

    return StorageMonitor::GetInstance()->GetTransientIdForDeviceId(device_id);
  }

  void CleanUp() {
    DCHECK(rph_refs_.empty());
    for (PrefIdFsInfoMap::const_iterator it = pref_id_map_.begin();
         it != pref_id_map_.end();
         ++it) {
      file_system_context_->RevokeFileSystem(it->second.fsid);
    }
    pref_id_map_.clear();

    media_device_map_references_.clear();

    no_references_callback_.Run();
  }

  // MediaFileSystemRegistry owns |this| and |file_system_context_|, so it's
  // safe to store a raw pointer.
  MediaFileSystemContext* file_system_context_;

  // A callback to call when the last RVH reference goes away.
  base::Closure no_references_callback_;

  // A map from the gallery preferences id to the file system information.
  PrefIdFsInfoMap pref_id_map_;

  // A map from the gallery preferences id to the corresponding media device
  // host object.
  MediaDeviceEntryReferencesMap media_device_map_references_;

  // The set of render processes and web contents that may have references to
  // the file system ids this instance manages.
  RPHReferenceManager rph_refs_;

  DISALLOW_COPY_AND_ASSIGN(ExtensionGalleriesHost);
};

/******************
 * Public methods
 ******************/

void MediaFileSystemRegistry::GetMediaFileSystemsForExtension(
    const content::RenderViewHost* rvh,
    const extensions::Extension* extension,
    const MediaFileSystemsCallback& callback) {
  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));

  Profile* profile =
      Profile::FromBrowserContext(rvh->GetProcess()->GetBrowserContext());
  MediaGalleriesPreferences* preferences = GetPreferences(profile);
  MediaGalleryPrefIdSet galleries =
      preferences->GalleriesForExtension(*extension);

  if (galleries.empty()) {
    callback.Run(std::vector<MediaFileSystemInfo>());
    return;
  }

  if (!ContainsKey(pref_change_registrar_map_, profile)) {
    PrefChangeRegistrar* pref_registrar = new PrefChangeRegistrar;
    pref_registrar->Init(profile->GetPrefs());
    pref_registrar->Add(
        prefs::kMediaGalleriesRememberedGalleries,
        base::Bind(&MediaFileSystemRegistry::OnRememberedGalleriesChanged,
                   base::Unretained(this),
                   pref_registrar->prefs()));
    pref_change_registrar_map_[profile] = pref_registrar;
  }

  ExtensionGalleriesHost* extension_host =
      extension_hosts_map_[profile][extension->id()].get();
  if (!extension_host) {
    extension_host = new ExtensionGalleriesHost(
        file_system_context_.get(),
        base::Bind(&MediaFileSystemRegistry::OnExtensionGalleriesHostEmpty,
                   base::Unretained(this), profile, extension->id()));
    extension_hosts_map_[profile][extension->id()] = extension_host;
  }
  extension_host->ReferenceFromRVH(rvh);

  extension_host->GetMediaFileSystems(galleries, preferences->known_galleries(),
                                      callback);
}

MediaGalleriesPreferences* MediaFileSystemRegistry::GetPreferences(
    Profile* profile) {
  MediaGalleriesPreferences* preferences =
      MediaGalleriesPreferencesFactory::GetForProfile(profile);
  if (ContainsKey(extension_hosts_map_, profile))
    return preferences;

  // Create an empty entry so the initialization code below only gets called
  // once per profile.
  extension_hosts_map_[profile] = ExtensionHostMap();

  // TODO(gbillock): Move this stanza to MediaGalleriesPreferences init code.
  StorageMonitor* monitor = StorageMonitor::GetInstance();
  DCHECK(monitor->IsInitialized());
  std::vector<StorageInfo> existing_devices =
      monitor->GetAllAvailableStorages();
  for (size_t i = 0; i < existing_devices.size(); i++) {
    if (!(StorageInfo::IsMediaDevice(existing_devices[i].device_id()) &&
          StorageInfo::IsRemovableDevice(existing_devices[i].device_id())))
      continue;
    preferences->AddGallery(existing_devices[i].device_id(),
                            base::FilePath(),
                            false,
                            existing_devices[i].storage_label(),
                            existing_devices[i].vendor_name(),
                            existing_devices[i].model_name(),
                            existing_devices[i].total_size_in_bytes(),
                            base::Time::Now());
  }
  return preferences;
}

void MediaFileSystemRegistry::OnRemovableStorageDetached(
    const StorageInfo& info) {
  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));

  // Since revoking a gallery in the ExtensionGalleriesHost may cause it
  // to be removed from the map and therefore invalidate any iterator pointing
  // to it, this code first copies all the invalid gallery ids and the
  // extension hosts in which they may appear (per profile) and revoked it in
  // a second step.
  std::vector<InvalidatedGalleriesInfo> invalid_galleries_info;

  for (ExtensionGalleriesHostMap::iterator profile_it =
           extension_hosts_map_.begin();
       profile_it != extension_hosts_map_.end();
       ++profile_it) {
    MediaGalleriesPreferences* preferences = GetPreferences(profile_it->first);
    InvalidatedGalleriesInfo invalid_galleries_in_profile;
    invalid_galleries_in_profile.pref_ids =
        preferences->LookUpGalleriesByDeviceId(info.device_id());

    for (ExtensionHostMap::const_iterator extension_host_it =
             profile_it->second.begin();
         extension_host_it != profile_it->second.end();
         ++extension_host_it) {
      invalid_galleries_in_profile.extension_hosts.insert(
          extension_host_it->second.get());
    }

    invalid_galleries_info.push_back(invalid_galleries_in_profile);
  }

  for (size_t i = 0; i < invalid_galleries_info.size(); i++) {
    for (std::set<ExtensionGalleriesHost*>::const_iterator extension_host_it =
             invalid_galleries_info[i].extension_hosts.begin();
         extension_host_it != invalid_galleries_info[i].extension_hosts.end();
         ++extension_host_it) {
      for (std::set<MediaGalleryPrefId>::const_iterator pref_id_it =
               invalid_galleries_info[i].pref_ids.begin();
           pref_id_it != invalid_galleries_info[i].pref_ids.end();
           ++pref_id_it) {
        (*extension_host_it)->RevokeGalleryByPrefId(*pref_id_it);
      }
    }
  }
}

/******************
 * Private methods
 ******************/

class MediaFileSystemRegistry::MediaFileSystemContextImpl
    : public MediaFileSystemContext {
 public:
  explicit MediaFileSystemContextImpl(MediaFileSystemRegistry* registry)
      : registry_(registry) {
    DCHECK(registry_);  // Suppresses unused warning on Android.
  }
  virtual ~MediaFileSystemContextImpl() {}

  // Registers and returns the file system id for the mass storage device
  // specified by |device_id| and |path|.
  virtual std::string RegisterFileSystemForMassStorage(
      const std::string& device_id, const base::FilePath& path) OVERRIDE {
    DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
    DCHECK(StorageInfo::IsMassStorageDevice(device_id));

    // Sanity checks for |path|.
    CHECK(path.IsAbsolute());
    CHECK(!path.ReferencesParent());

    std::string fsid;
    if (StorageInfo::IsITunesDevice(device_id)) {
      ImportedMediaGalleryRegistry* imported_registry =
          ImportedMediaGalleryRegistry::GetInstance();
      fsid = imported_registry->RegisterITunesFilesystemOnUIThread(path);
    } else if (StorageInfo::IsPicasaDevice(device_id)) {
      ImportedMediaGalleryRegistry* imported_registry =
          ImportedMediaGalleryRegistry::GetInstance();
      fsid = imported_registry->RegisterPicasaFilesystemOnUIThread(
          path);
    } else {
      std::string fs_name(extension_misc::kMediaFileSystemPathPart);
      fsid = IsolatedContext::GetInstance()->RegisterFileSystemForPath(
          fileapi::kFileSystemTypeNativeMedia, path, &fs_name);
    }
    return fsid;
  }

  virtual std::string RegisterFileSystemForMTPDevice(
      const std::string& device_id, const base::FilePath& path,
      scoped_refptr<ScopedMTPDeviceMapEntry>* entry) OVERRIDE {
    DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
    DCHECK(!StorageInfo::IsMassStorageDevice(device_id));

    // Sanity checks for |path|.
    CHECK(MediaStorageUtil::CanCreateFileSystem(device_id, path));
    std::string fs_name(extension_misc::kMediaFileSystemPathPart);
    const std::string fsid =
        IsolatedContext::GetInstance()->RegisterFileSystemForPath(
            fileapi::kFileSystemTypeDeviceMedia, path, &fs_name);
    CHECK(!fsid.empty());
    DCHECK(entry);
    *entry = registry_->GetOrCreateScopedMTPDeviceMapEntry(path.value());
    return fsid;
  }

  virtual void RevokeFileSystem(const std::string& fsid) OVERRIDE {
    ImportedMediaGalleryRegistry* imported_registry =
        ImportedMediaGalleryRegistry::GetInstance();
    if (imported_registry->RevokeImportedFilesystemOnUIThread(fsid))
      return;

    IsolatedContext::GetInstance()->RevokeFileSystem(fsid);
  }

 private:
  MediaFileSystemRegistry* registry_;

  DISALLOW_COPY_AND_ASSIGN(MediaFileSystemContextImpl);
};

MediaFileSystemRegistry::MediaFileSystemRegistry()
    : file_system_context_(new MediaFileSystemContextImpl(this)) {
  StorageMonitor::GetInstance()->AddObserver(this);
}

MediaFileSystemRegistry::~MediaFileSystemRegistry() {
  // TODO(gbillock): This is needed because the unit test uses the
  // g_browser_process registry. We should create one in the unit test,
  // and then can remove this.
  if (StorageMonitor::GetInstance())
    StorageMonitor::GetInstance()->RemoveObserver(this);
  DCHECK(mtp_device_delegate_map_.empty());
}

void MediaFileSystemRegistry::OnRememberedGalleriesChanged(
    PrefService* prefs) {
  // Find the Profile that contains the source PrefService.
  PrefChangeRegistrarMap::iterator pref_change_it =
      pref_change_registrar_map_.begin();
  for (; pref_change_it != pref_change_registrar_map_.end(); ++pref_change_it) {
    if (pref_change_it->first->GetPrefs() == prefs)
      break;
  }
  DCHECK(pref_change_it != pref_change_registrar_map_.end());
  Profile* profile = pref_change_it->first;

  // Get the Extensions, MediaGalleriesPreferences and ExtensionHostMap for
  // |profile|.
  const ExtensionService* extension_service =
      extensions::ExtensionSystem::Get(profile)->extension_service();
  const ExtensionSet* extensions_set = extension_service->extensions();
  const MediaGalleriesPreferences* preferences = GetPreferences(profile);
  ExtensionGalleriesHostMap::const_iterator host_map_it =
      extension_hosts_map_.find(profile);
  DCHECK(host_map_it != extension_hosts_map_.end());
  const ExtensionHostMap& extension_host_map = host_map_it->second;

  // Go through ExtensionsHosts, get the updated galleries list and use it to
  // revoke the old galleries.
  // RevokeOldGalleries() may end up deleting from |extension_host_map| and
  // even delete |extension_host_map| altogether. So do this in two loops to
  // avoid using an invalidated iterator or deleted map.
  std::vector<const extensions::Extension*> extensions;
  for (ExtensionHostMap::const_iterator it = extension_host_map.begin();
       it != extension_host_map.end();
       ++it) {
    extensions.push_back(extensions_set->GetByID(it->first));
  }
  for (size_t i = 0; i < extensions.size(); ++i) {
    if (!ContainsKey(extension_hosts_map_, profile))
      break;
    ExtensionHostMap::const_iterator gallery_host_it =
        extension_host_map.find(extensions[i]->id());
    if (gallery_host_it == extension_host_map.end())
      continue;
    gallery_host_it->second->RevokeOldGalleries(
        preferences->GalleriesForExtension(*extensions[i]));
  }
}

scoped_refptr<ScopedMTPDeviceMapEntry>
MediaFileSystemRegistry::GetOrCreateScopedMTPDeviceMapEntry(
    const base::FilePath::StringType& device_location) {
  MTPDeviceDelegateMap::iterator delegate_it =
      mtp_device_delegate_map_.find(device_location);
  if (delegate_it != mtp_device_delegate_map_.end())
    return delegate_it->second;
  scoped_refptr<ScopedMTPDeviceMapEntry> mtp_device_host =
      new ScopedMTPDeviceMapEntry(
          device_location,
          base::Bind(&MediaFileSystemRegistry::RemoveScopedMTPDeviceMapEntry,
                     base::Unretained(this),
                     device_location));
  mtp_device_host->Init();
  mtp_device_delegate_map_[device_location] = mtp_device_host.get();
  return mtp_device_host;
}

void MediaFileSystemRegistry::RemoveScopedMTPDeviceMapEntry(
    const base::FilePath::StringType& device_location) {
  MTPDeviceDelegateMap::iterator delegate_it =
      mtp_device_delegate_map_.find(device_location);
  DCHECK(delegate_it != mtp_device_delegate_map_.end());
  mtp_device_delegate_map_.erase(delegate_it);
}

void MediaFileSystemRegistry::OnExtensionGalleriesHostEmpty(
    Profile* profile, const std::string& extension_id) {
  DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));

  ExtensionGalleriesHostMap::iterator extension_hosts =
      extension_hosts_map_.find(profile);
  DCHECK(extension_hosts != extension_hosts_map_.end());
  ExtensionHostMap::size_type erase_count =
      extension_hosts->second.erase(extension_id);
  DCHECK_EQ(1U, erase_count);
  if (extension_hosts->second.empty()) {
    // When a profile has no ExtensionGalleriesHosts left, remove the
    // matching PrefChangeRegistrar since it is no longer needed. Leave the
    // |extension_hosts| entry alone, since it indicates the profile has been
    // previously used.
    PrefChangeRegistrarMap::iterator pref_it =
        pref_change_registrar_map_.find(profile);
    DCHECK(pref_it != pref_change_registrar_map_.end());
    delete pref_it->second;
    pref_change_registrar_map_.erase(pref_it);
  }
}

}  // namespace chrome