content/browser/storage_partition_impl_map.cc

   1 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "content/browser/storage_partition_impl_map.h"
   6
   7 #include "base/bind.h"
   8 #include "base/callback.h"
   9 #include "base/file_util.h"
  10 #include "base/files/file_enumerator.h"
  11 #include "base/files/file_path.h"
  12 #include "base/stl_util.h"
  13 #include "base/strings/string_number_conversions.h"
  14 #include "base/strings/string_util.h"
  15 #include "base/strings/stringprintf.h"
  16 #include "base/threading/sequenced_worker_pool.h"
  17 #include "content/browser/appcache/chrome_appcache_service.h"
  18 #include "content/browser/fileapi/browser_file_system_helper.h"
  19 #include "content/browser/fileapi/chrome_blob_storage_context.h"
  20 #include "content/browser/loader/resource_request_info_impl.h"
  21 #include "content/browser/resource_context_impl.h"
  22 #include "content/browser/storage_partition_impl.h"
  23 #include "content/browser/streams/stream.h"
  24 #include "content/browser/streams/stream_context.h"
  25 #include "content/browser/streams/stream_registry.h"
  26 #include "content/browser/streams/stream_url_request_job.h"
  27 #include "content/browser/webui/url_data_manager_backend.h"
  28 #include "content/public/browser/browser_context.h"
  29 #include "content/public/browser/browser_thread.h"
  30 #include "content/public/browser/content_browser_client.h"
  31 #include "content/public/browser/storage_partition.h"
  32 #include "content/public/common/content_constants.h"
  33 #include "content/public/common/url_constants.h"
  34 #include "crypto/sha2.h"
  35 #include "net/url_request/url_request_context.h"
  36 #include "net/url_request/url_request_context_getter.h"
  37 #include "webkit/browser/blob/blob_storage_context.h"
  38 #include "webkit/browser/blob/blob_url_request_job_factory.h"
  39 #include "webkit/browser/fileapi/file_system_url_request_job_factory.h"
  40 #include "webkit/common/blob/blob_data.h"
  41
  42 using appcache::AppCacheService;
  43 using fileapi::FileSystemContext;
  44 using webkit_blob::BlobStorageContext;
  45
  46 namespace content {
  47
  48 namespace {
  49
  50 // A derivative that knows about Streams too.
  51 class BlobProtocolHandler : public net::URLRequestJobFactory::ProtocolHandler {
  52  public:
  53   BlobProtocolHandler(ChromeBlobStorageContext* blob_storage_context,
  54                       StreamContext* stream_context,
  55                       fileapi::FileSystemContext* file_system_context)
  56       : blob_storage_context_(blob_storage_context),
  57         stream_context_(stream_context),
  58         file_system_context_(file_system_context) {
  59   }
  60
  61   virtual ~BlobProtocolHandler() {
  62   }
  63
  64   virtual net::URLRequestJob* MaybeCreateJob(
  65       net::URLRequest* request,
  66       net::NetworkDelegate* network_delegate) const OVERRIDE {
  67     scoped_refptr<Stream> stream =
  68         stream_context_->registry()->GetStream(request->url());
  69     if (stream.get())
  70       return new StreamURLRequestJob(request, network_delegate, stream);
  71
  72     if (!blob_protocol_handler_) {
  73       // Construction is deferred because 'this' is constructed on
  74       // the main thread but we want blob_protocol_handler_ constructed
  75       // on the IO thread.
  76       blob_protocol_handler_.reset(
  77           new webkit_blob::BlobProtocolHandler(
  78               blob_storage_context_->context(),
  79               file_system_context_,
  80               BrowserThread::GetMessageLoopProxyForThread(
  81                   BrowserThread::FILE).get()));
  82     }
  83     return blob_protocol_handler_->MaybeCreateJob(request, network_delegate);
  84   }
  85
  86  private:
  87   const scoped_refptr<ChromeBlobStorageContext> blob_storage_context_;
  88   const scoped_refptr<StreamContext> stream_context_;
  89   const scoped_refptr<fileapi::FileSystemContext> file_system_context_;
  90   mutable scoped_ptr<webkit_blob::BlobProtocolHandler> blob_protocol_handler_;
  91   DISALLOW_COPY_AND_ASSIGN(BlobProtocolHandler);
  92 };
  93
  94 // These constants are used to create the directory structure under the profile
  95 // where renderers with a non-default storage partition keep their persistent
  96 // state. This will contain a set of directories that partially mirror the
  97 // directory structure of BrowserContext::GetPath().
  98 //
  99 // The kStoragePartitionDirname contains an extensions directory which is
 100 // further partitioned by extension id, followed by another level of directories
 101 // for the "default" extension storage partition and one directory for each
 102 // persistent partition used by a webview tag. Example:
 103 //
 104 //   Storage/ext/ABCDEF/def
 105 //   Storage/ext/ABCDEF/hash(partition name)
 106 //
 107 // The code in GetStoragePartitionPath() constructs these path names.
 108 //
 109 // TODO(nasko): Move extension related path code out of content.
 110 const base::FilePath::CharType kStoragePartitionDirname[] =
 111     FILE_PATH_LITERAL("Storage");
 112 const base::FilePath::CharType kExtensionsDirname[] =
 113     FILE_PATH_LITERAL("ext");
 114 const base::FilePath::CharType kDefaultPartitionDirname[] =
 115     FILE_PATH_LITERAL("def");
 116 const base::FilePath::CharType kTrashDirname[] =
 117     FILE_PATH_LITERAL("trash");
 118
 119 // Because partition names are user specified, they can be arbitrarily long
 120 // which makes them unsuitable for paths names. We use a truncation of a
 121 // SHA256 hash to perform a deterministic shortening of the string. The
 122 // kPartitionNameHashBytes constant controls the length of the truncation.
 123 // We use 6 bytes, which gives us 99.999% reliability against collisions over
 124 // 1 million partition domains.
 125 //
 126 // Analysis:
 127 // We assume that all partition names within one partition domain are
 128 // controlled by the the same entity. Thus there is no chance for adverserial
 129 // attack and all we care about is accidental collision. To get 5 9s over
 130 // 1 million domains, we need the probability of a collision in any one domain
 131 // to be
 132 //
 133 //    p < nroot(1000000, .99999) ~= 10^-11
 134 //
 135 // We use the following birthday attack approximation to calculate the max
 136 // number of unique names for this probability:
 137 //
 138 //    n(p,H) = sqrt(2*H * ln(1/(1-p)))
 139 //
 140 // For a 6-byte hash, H = 2^(6*8).  n(10^-11, H) ~= 75
 141 //
 142 // An average partition domain is likely to have less than 10 unique
 143 // partition names which is far lower than 75.
 144 //
 145 // Note, that for 4 9s of reliability, the limit is 237 partition names per
 146 // partition domain.
 147 const int kPartitionNameHashBytes = 6;
 148
 149 // Needed for selecting all files in ObliterateOneDirectory() below.
 150 #if defined(OS_POSIX)
 151 const int kAllFileTypes = base::FileEnumerator::FILES |
 152                           base::FileEnumerator::DIRECTORIES |
 153                           base::FileEnumerator::SHOW_SYM_LINKS;
 154 #else
 155 const int kAllFileTypes = base::FileEnumerator::FILES |
 156                           base::FileEnumerator::DIRECTORIES;
 157 #endif
 158
 159 base::FilePath GetStoragePartitionDomainPath(
 160     const std::string& partition_domain) {
 161   CHECK(IsStringUTF8(partition_domain));
 162
 163   return base::FilePath(kStoragePartitionDirname).Append(kExtensionsDirname)
 164       .Append(base::FilePath::FromUTF8Unsafe(partition_domain));
 165 }
 166
 167 // Helper function for doing a depth-first deletion of the data on disk.
 168 // Examines paths directly in |current_dir| (no recursion) and tries to
 169 // delete from disk anything that is in, or isn't a parent of something in
 170 // |paths_to_keep|. Paths that need further expansion are added to
 171 // |paths_to_consider|.
 172 void ObliterateOneDirectory(const base::FilePath& current_dir,
 173                             const std::vector<base::FilePath>& paths_to_keep,
 174                             std::vector<base::FilePath>* paths_to_consider) {
 175   CHECK(current_dir.IsAbsolute());
 176
 177   base::FileEnumerator enumerator(current_dir, false, kAllFileTypes);
 178   for (base::FilePath to_delete = enumerator.Next(); !to_delete.empty();
 179        to_delete = enumerator.Next()) {
 180     // Enum tracking which of the 3 possible actions to take for |to_delete|.
 181     enum { kSkip, kEnqueue, kDelete } action = kDelete;
 182
 183     for (std::vector<base::FilePath>::const_iterator to_keep =
 184              paths_to_keep.begin();
 185          to_keep != paths_to_keep.end();
 186          ++to_keep) {
 187       if (to_delete == *to_keep) {
 188         action = kSkip;
 189         break;
 190       } else if (to_delete.IsParent(*to_keep)) {
 191         // |to_delete| contains a path to keep. Add to stack for further
 192         // processing.
 193         action = kEnqueue;
 194         break;
 195       }
 196     }
 197
 198     switch (action) {
 199       case kDelete:
 200         base::DeleteFile(to_delete, true);
 201         break;
 202
 203       case kEnqueue:
 204         paths_to_consider->push_back(to_delete);
 205         break;
 206
 207       case kSkip:
 208         break;
 209     }
 210   }
 211 }
 212
 213 // Synchronously attempts to delete |unnormalized_root|, preserving only
 214 // entries in |paths_to_keep|. If there are no entries in |paths_to_keep| on
 215 // disk, then it completely removes |unnormalized_root|. All paths must be
 216 // absolute paths.
 217 void BlockingObliteratePath(
 218     const base::FilePath& unnormalized_browser_context_root,
 219     const base::FilePath& unnormalized_root,
 220     const std::vector<base::FilePath>& paths_to_keep,
 221     const scoped_refptr<base::TaskRunner>& closure_runner,
 222     const base::Closure& on_gc_required) {
 223   // Early exit required because MakeAbsoluteFilePath() will fail on POSIX
 224   // if |unnormalized_root| does not exist. This is safe because there is
 225   // nothing to do in this situation anwyays.
 226   if (!base::PathExists(unnormalized_root)) {
 227     return;
 228   }
 229
 230   // Never try to obliterate things outside of the browser context root or the
 231   // browser context root itself. Die hard.
 232   base::FilePath root = base::MakeAbsoluteFilePath(unnormalized_root);
 233   base::FilePath browser_context_root =
 234       base::MakeAbsoluteFilePath(unnormalized_browser_context_root);
 235   CHECK(!root.empty());
 236   CHECK(!browser_context_root.empty());
 237   CHECK(browser_context_root.IsParent(root) && browser_context_root != root);
 238
 239   // Reduce |paths_to_keep| set to those under the root and actually on disk.
 240   std::vector<base::FilePath> valid_paths_to_keep;
 241   for (std::vector<base::FilePath>::const_iterator it = paths_to_keep.begin();
 242        it != paths_to_keep.end();
 243        ++it) {
 244     if (root.IsParent(*it) && base::PathExists(*it))
 245       valid_paths_to_keep.push_back(*it);
 246   }
 247
 248   // If none of the |paths_to_keep| are valid anymore then we just whack the
 249   // root and be done with it.  Otherwise, signal garbage collection and do
 250   // a best-effort delete of the on-disk structures.
 251   if (valid_paths_to_keep.empty()) {
 252     base::DeleteFile(root, true);
 253     return;
 254   }
 255   closure_runner->PostTask(FROM_HERE, on_gc_required);
 256
 257   // Otherwise, start at the root and delete everything that is not in
 258   // |valid_paths_to_keep|.
 259   std::vector<base::FilePath> paths_to_consider;
 260   paths_to_consider.push_back(root);
 261   while(!paths_to_consider.empty()) {
 262     base::FilePath path = paths_to_consider.back();
 263     paths_to_consider.pop_back();
 264     ObliterateOneDirectory(path, valid_paths_to_keep, &paths_to_consider);
 265   }
 266 }
 267
 268 // Deletes all entries inside the |storage_root| that are not in the
 269 // |active_paths|.  Deletion is done in 2 steps:
 270 //
 271 //   (1) Moving all garbage collected paths into a trash directory.
 272 //   (2) Asynchronously deleting the trash directory.
 273 //
 274 // The deletion is asynchronous because after (1) completes, calling code can
 275 // safely continue to use the paths that had just been garbage collected
 276 // without fear of race conditions.
 277 //
 278 // This code also ignores failed moves rather than attempting a smarter retry.
 279 // Moves shouldn't fail here unless there is some out-of-band error (eg.,
 280 // FS corruption). Retry logic is dangerous in the general case because
 281 // there is not necessarily a guaranteed case where the logic may succeed.
 282 //
 283 // This function is still named BlockingGarbageCollect() because it does
 284 // execute a few filesystem operations synchronously.
 285 void BlockingGarbageCollect(
 286     const base::FilePath& storage_root,
 287     const scoped_refptr<base::TaskRunner>& file_access_runner,
 288     scoped_ptr<base::hash_set<base::FilePath> > active_paths) {
 289   CHECK(storage_root.IsAbsolute());
 290
 291   base::FileEnumerator enumerator(storage_root, false, kAllFileTypes);
 292   base::FilePath trash_directory;
 293   if (!base::CreateTemporaryDirInDir(storage_root, kTrashDirname,
 294                                      &trash_directory)) {
 295     // Unable to continue without creating the trash directory so give up.
 296     return;
 297   }
 298   for (base::FilePath path = enumerator.Next(); !path.empty();
 299        path = enumerator.Next()) {
 300     if (active_paths->find(path) == active_paths->end() &&
 301         path != trash_directory) {
 302       // Since |trash_directory| is unique for each run of this function there
 303       // can be no colllisions on the move.
 304       base::Move(path, trash_directory.Append(path.BaseName()));
 305     }
 306   }
 307
 308   file_access_runner->PostTask(
 309       FROM_HERE,
 310       base::Bind(base::IgnoreResult(&base::DeleteFile), trash_directory, true));
 311 }
 312
 313 }  // namespace
 314
 315 // static
 316 base::FilePath StoragePartitionImplMap::GetStoragePartitionPath(
 317     const std::string& partition_domain,
 318     const std::string& partition_name) {
 319   if (partition_domain.empty())
 320     return base::FilePath();
 321
 322   base::FilePath path = GetStoragePartitionDomainPath(partition_domain);
 323
 324   // TODO(ajwong): Mangle in-memory into this somehow, either by putting
 325   // it into the partition_name, or by manually adding another path component
 326   // here.  Otherwise, it's possible to have an in-memory StoragePartition and
 327   // a persistent one that return the same FilePath for GetPath().
 328   if (!partition_name.empty()) {
 329     // For analysis of why we can ignore collisions, see the comment above
 330     // kPartitionNameHashBytes.
 331     char buffer[kPartitionNameHashBytes];
 332     crypto::SHA256HashString(partition_name, &buffer[0],
 333                              sizeof(buffer));
 334     return path.AppendASCII(base::HexEncode(buffer, sizeof(buffer)));
 335   }
 336
 337   return path.Append(kDefaultPartitionDirname);
 338 }
 339
 340 StoragePartitionImplMap::StoragePartitionImplMap(
 341     BrowserContext* browser_context)
 342     : browser_context_(browser_context),
 343       resource_context_initialized_(false) {
 344   // Doing here instead of initializer list cause it's just too ugly to read.
 345   base::SequencedWorkerPool* blocking_pool = BrowserThread::GetBlockingPool();
 346   file_access_runner_ =
 347       blocking_pool->GetSequencedTaskRunner(blocking_pool->GetSequenceToken());
 348 }
 349
 350 StoragePartitionImplMap::~StoragePartitionImplMap() {
 351   STLDeleteContainerPairSecondPointers(partitions_.begin(),
 352                                        partitions_.end());
 353 }
 354
 355 StoragePartitionImpl* StoragePartitionImplMap::Get(
 356     const std::string& partition_domain,
 357     const std::string& partition_name,
 358     bool in_memory) {
 359   // Find the previously created partition if it's available.
 360   StoragePartitionConfig partition_config(
 361       partition_domain, partition_name, in_memory);
 362
 363   PartitionMap::const_iterator it = partitions_.find(partition_config);
 364   if (it != partitions_.end())
 365     return it->second;
 366
 367   base::FilePath partition_path =
 368       browser_context_->GetPath().Append(
 369           GetStoragePartitionPath(partition_domain, partition_name));
 370   StoragePartitionImpl* partition =
 371       StoragePartitionImpl::Create(browser_context_, in_memory,
 372                                    partition_path);
 373   partitions_[partition_config] = partition;
 374
 375   ChromeBlobStorageContext* blob_storage_context =
 376       ChromeBlobStorageContext::GetFor(browser_context_);
 377   StreamContext* stream_context = StreamContext::GetFor(browser_context_);
 378   ProtocolHandlerMap protocol_handlers;
 379   protocol_handlers[chrome::kBlobScheme] =
 380       linked_ptr<net::URLRequestJobFactory::ProtocolHandler>(
 381           new BlobProtocolHandler(blob_storage_context,
 382                                   stream_context,
 383                                   partition->GetFileSystemContext()));
 384   protocol_handlers[chrome::kFileSystemScheme] =
 385       linked_ptr<net::URLRequestJobFactory::ProtocolHandler>(
 386           CreateFileSystemProtocolHandler(partition->GetFileSystemContext()));
 387   protocol_handlers[chrome::kChromeUIScheme] =
 388       linked_ptr<net::URLRequestJobFactory::ProtocolHandler>(
 389           URLDataManagerBackend::CreateProtocolHandler(
 390               browser_context_->GetResourceContext(),
 391               browser_context_->IsOffTheRecord(),
 392               partition->GetAppCacheService(),
 393               blob_storage_context));
 394   std::vector<std::string> additional_webui_schemes;
 395   GetContentClient()->browser()->GetAdditionalWebUISchemes(
 396       &additional_webui_schemes);
 397   for (std::vector<std::string>::const_iterator it =
 398            additional_webui_schemes.begin();
 399        it != additional_webui_schemes.end();
 400        ++it) {
 401     protocol_handlers[*it] =
 402         linked_ptr<net::URLRequestJobFactory::ProtocolHandler>(
 403             URLDataManagerBackend::CreateProtocolHandler(
 404                 browser_context_->GetResourceContext(),
 405                 browser_context_->IsOffTheRecord(),
 406                 partition->GetAppCacheService(),
 407                 blob_storage_context));
 408   }
 409   protocol_handlers[chrome::kChromeDevToolsScheme] =
 410       linked_ptr<net::URLRequestJobFactory::ProtocolHandler>(
 411           CreateDevToolsProtocolHandler(browser_context_->GetResourceContext(),
 412                                         browser_context_->IsOffTheRecord()));
 413
 414   // These calls must happen after StoragePartitionImpl::Create().
 415   if (partition_domain.empty()) {
 416     partition->SetURLRequestContext(
 417         GetContentClient()->browser()->CreateRequestContext(
 418             browser_context_,
 419             &protocol_handlers));
 420   } else {
 421     partition->SetURLRequestContext(
 422         GetContentClient()->browser()->CreateRequestContextForStoragePartition(
 423             browser_context_, partition->GetPath(), in_memory,
 424             &protocol_handlers));
 425   }
 426   partition->SetMediaURLRequestContext(
 427       partition_domain.empty() ?
 428       browser_context_->GetMediaRequestContext() :
 429       browser_context_->GetMediaRequestContextForStoragePartition(
 430           partition->GetPath(), in_memory));
 431
 432   PostCreateInitialization(partition, in_memory);
 433
 434   return partition;
 435 }
 436
 437 void StoragePartitionImplMap::AsyncObliterate(
 438     const GURL& site,
 439     const base::Closure& on_gc_required) {
 440   // This method should avoid creating any StoragePartition (which would
 441   // create more open file handles) so that it can delete as much of the
 442   // data off disk as possible.
 443   std::string partition_domain;
 444   std::string partition_name;
 445   bool in_memory = false;
 446   GetContentClient()->browser()->GetStoragePartitionConfigForSite(
 447       browser_context_, site, false, &partition_domain,
 448       &partition_name, &in_memory);
 449
 450   // Find the active partitions for the domain. Because these partitions are
 451   // active, it is not possible to just delete the directories that contain
 452   // the backing data structures without causing the browser to crash. Instead,
 453   // of deleteing the directory, we tell each storage context later to
 454   // remove any data they have saved. This will leave the directory structure
 455   // intact but it will only contain empty databases.
 456   std::vector<StoragePartitionImpl*> active_partitions;
 457   std::vector<base::FilePath> paths_to_keep;
 458   for (PartitionMap::const_iterator it = partitions_.begin();
 459        it != partitions_.end();
 460        ++it) {
 461     const StoragePartitionConfig& config = it->first;
 462     if (config.partition_domain == partition_domain) {
 463       it->second->ClearData(
 464           // All except shader cache.
 465           StoragePartition::REMOVE_DATA_MASK_ALL &
 466             (~StoragePartition::REMOVE_DATA_MASK_SHADER_CACHE),
 467           StoragePartition::QUOTA_MANAGED_STORAGE_MASK_ALL,
 468           NULL,
 469           StoragePartition::OriginMatcherFunction(),
 470           base::Time(), base::Time::Max(),
 471           base::Bind(&base::DoNothing));
 472       if (!config.in_memory) {
 473         paths_to_keep.push_back(it->second->GetPath());
 474       }
 475     }
 476   }
 477
 478   // Start a best-effort delete of the on-disk storage excluding paths that are
 479   // known to still be in use. This is to delete any previously created
 480   // StoragePartition state that just happens to not have been used during this
 481   // run of the browser.
 482   base::FilePath domain_root = browser_context_->GetPath().Append(
 483       GetStoragePartitionDomainPath(partition_domain));
 484
 485   BrowserThread::PostBlockingPoolTask(
 486       FROM_HERE,
 487       base::Bind(&BlockingObliteratePath, browser_context_->GetPath(),
 488                  domain_root, paths_to_keep,
 489                  base::MessageLoopProxy::current(), on_gc_required));
 490 }
 491
 492 void StoragePartitionImplMap::GarbageCollect(
 493     scoped_ptr<base::hash_set<base::FilePath> > active_paths,
 494     const base::Closure& done) {
 495   // Include all paths for current StoragePartitions in the active_paths since
 496   // they cannot be deleted safely.
 497   for (PartitionMap::const_iterator it = partitions_.begin();
 498        it != partitions_.end();
 499        ++it) {
 500     const StoragePartitionConfig& config = it->first;
 501     if (!config.in_memory)
 502       active_paths->insert(it->second->GetPath());
 503   }
 504
 505   // Find the directory holding the StoragePartitions and delete everything in
 506   // there that isn't considered active.
 507   base::FilePath storage_root = browser_context_->GetPath().Append(
 508       GetStoragePartitionDomainPath(std::string()));
 509   file_access_runner_->PostTaskAndReply(
 510       FROM_HERE,
 511       base::Bind(&BlockingGarbageCollect, storage_root,
 512                  file_access_runner_,
 513                  base::Passed(&active_paths)),
 514       done);
 515 }
 516
 517 void StoragePartitionImplMap::ForEach(
 518     const BrowserContext::StoragePartitionCallback& callback) {
 519   for (PartitionMap::const_iterator it = partitions_.begin();
 520        it != partitions_.end();
 521        ++it) {
 522     callback.Run(it->second);
 523   }
 524 }
 525
 526 void StoragePartitionImplMap::PostCreateInitialization(
 527     StoragePartitionImpl* partition,
 528     bool in_memory) {
 529   // TODO(ajwong): ResourceContexts no longer have any storage related state.
 530   // We should move this into a place where it is called once per
 531   // BrowserContext creation rather than piggybacking off the default context
 532   // creation.
 533   // Note: moving this into Get() before partitions_[] is set causes reentrency.
 534   if (!resource_context_initialized_) {
 535     resource_context_initialized_ = true;
 536     InitializeResourceContext(browser_context_);
 537   }
 538
 539   // Check first to avoid memory leak in unittests.
 540   if (BrowserThread::IsMessageLoopValid(BrowserThread::IO)) {
 541     BrowserThread::PostTask(
 542         BrowserThread::IO, FROM_HERE,
 543         base::Bind(&ChromeAppCacheService::InitializeOnIOThread,
 544                    partition->GetAppCacheService(),
 545                    in_memory ? base::FilePath() :
 546                        partition->GetPath().Append(kAppCacheDirname),
 547                    browser_context_->GetResourceContext(),
 548                    make_scoped_refptr(partition->GetURLRequestContext()),
 549                    make_scoped_refptr(
 550                        browser_context_->GetSpecialStoragePolicy())));
 551
 552     // We do not call InitializeURLRequestContext() for media contexts because,
 553     // other than the HTTP cache, the media contexts share the same backing
 554     // objects as their associated "normal" request context.  Thus, the previous
 555     // call serves to initialize the media request context for this storage
 556     // partition as well.
 557   }
 558 }
 559
 560 }  // namespace content