Fix on-disk structure for persistent storage in webview tags.

content/browser/storage_partition_impl_map.cc

Index: content/browser/storage_partition_impl_map.cc
diff --git a/content/browser/storage_partition_impl_map.cc b/content/browser/storage_partition_impl_map.cc
index 8f17aed0beebfe706e589f8e954b5c8b45a7a9f3..06ff3768b5eb649615389d1c82f91b634945372b 100644
--- a/content/browser/storage_partition_impl_map.cc
+++ b/content/browser/storage_partition_impl_map.cc
@@ -8,6 +8,7 @@
 #include "base/callback.h"
 #include "base/file_path.h"
 #include "base/stl_util.h"
+#include "base/string_number_conversions.h"
 #include "base/string_util.h"
 #include "content/browser/appcache/chrome_appcache_service.h"
 #include "content/browser/fileapi/browser_file_system_helper.h"
@@ -24,6 +25,7 @@
 #include "content/public/browser/storage_partition.h"
 #include "content/public/common/content_constants.h"
 #include "content/public/common/url_constants.h"
+#include "crypto/sha2.h"
 #include "net/url_request/url_request_context_getter.h"
 #include "net/url_request/url_request_context.h"
 #include "webkit/appcache/view_appcache_internals_job.h"
@@ -183,8 +185,81 @@ void InitializeURLRequestContext(
   // TODO(jam): Add the ProtocolHandlerRegistryIntercepter here!
 }
 
+// These constants are used to create the directory structure under the profile
+// where renderers with a non-default storage partition keep their persistent
+// state. This will contain a set of directories that partially mirror the
+// directory structure of BrowserContext::GetPath().
+//
+// The kStoragePartitionDirname is contains an extensions directory which is

[nasko, 2012/11/08 05:03:54]

No need for "is" after the constant name.

+// further partitioned by extension id, followed by another level of directories
+// for the "default" extension storage partition and one directory for each
+// persistent partition used by a webview tags. Example:
+//
+//   Storage/ext/ABCDEF/def
+//   Storage/ext/ABCDEF/hash(partition_name)
+//
+// The code in GetPartitionPath() constructs these path names.
+const FilePath::CharType kStoragePartitionDirname[] =
+    FILE_PATH_LITERAL("Storage");
+const FilePath::CharType kExtensionsDirname[] =
+    FILE_PATH_LITERAL("ext");
+const FilePath::CharType kDefaultPartitionDirname[] =
+    FILE_PATH_LITERAL("def");
+
+// Because partition names are user specified, they can be arbitrarily long
+// which makes them unsuitable for paths names. We use a truncation of a
+// SHA256 hash to perform a deterministic shortening of the string. The
+// kPartitionNameHashBytes constant controls the length of the truncation.
+// We use 6 bytes, which gives us 99.999% reliability against collisions over
+// 1 million partition domains.
+//
+// Analysis:
+// We assume that all partition names within one partition domain are
+// controlled by the the same entity. Thus there is no chance for adverserial
+// attack and all we care about is accidental collision. To get 5 9s over
+// 1 million domains, we need the probability of a collision in any one domain
+// to be
+//
+//    p < nroot(1000000, .99999) ~= 10^-11.
+//
+// We the following birthday attack approximation to caculate the max number
+// of unique names for this probability:
+//
+//    n(p,H) = sqrt(2*H * ln(1/(1-p)))
+//
+// For a 6-byte hash, H = 2^(6*8).  n(10^-11, H) ~= 75
+//
+// An average partition domain is likely to have less than 10 unique
+// partition names which is far lower than 75.
+//
+// Note, that for 4 9s of reliability, the limit is 237 partition names per
+// partition domain.

[nasko, 2012/11/08 05:03:54]

Awesome comment!

+const int kPartitionNameHashBytes = 6;
+
 }  // namespace
 
+// static
+FilePath StoragePartitionImplMap::GetStoragePartitionPath(
+    const StoragePartitionDescriptor& descriptor) {
+  if (descriptor.partition_domain.empty())
+    return FilePath();
+
+  FilePath path = FilePath(kStoragePartitionDirname).Append(kExtensionsDirname)
+      .AppendASCII(descriptor.partition_domain);
+
+  if (!descriptor.partition_name.empty()) {
+    // For analysis of why this is safe, see the comment on
+    // kPartitionNameHashBytes.
+    char buffer[kPartitionNameHashBytes];
+    crypto::SHA256HashString(descriptor.partition_name, &buffer[0],
+                             sizeof(buffer));
+    return path.AppendASCII(base::HexEncode(buffer, sizeof(buffer)));
+  }
+
+  return path.Append(kDefaultPartitionDirname);
+}
+
+
 StoragePartitionImplMap::StoragePartitionImplMap(
     BrowserContext* browser_context)
     : browser_context_(browser_context),
@@ -210,17 +285,19 @@ StoragePartitionImpl* StoragePartitionImplMap::Get(
   }
 
   // Find the previously created partition if it's available.
-  StoragePartitionImpl::StoragePartitionDescriptor partition_descriptor(
+  StoragePartitionDescriptor partition_descriptor(
       partition_domain, partition_name, in_memory);
 
   PartitionsMap::const_iterator it = partitions_.find(partition_descriptor);
   if (it != partitions_.end())
     return it->second;
 
-  // There was no previous partition, so let's make a new one.
+  FilePath partition_path =
+      browser_context_->GetPath().Append(
+          GetStoragePartitionPath(partition_descriptor));
   StoragePartitionImpl* partition =
-      StoragePartitionImpl::Create(browser_context_, partition_descriptor,
-                                   browser_context_->GetPath());
+      StoragePartitionImpl::Create(browser_context_, in_memory,
+                                   partition_path);
   partitions_[partition_descriptor] = partition;
 
   // These calls must happen after StoragePartitionImpl::Create().