Reland of "Remove void** from disk_cache interface."

Enumeration and iteration were passing around void**. With this CL, we
instead use an Iterator object.

TBR=clamy@chromium.org,jkarlin@chromium.org,jsbell@chromium.org
BUG=413644

Review URL: https://codereview.chromium.org/583283002

Cr-Commit-Position: refs/heads/master@{#295870}

13 files changed, 322 insertions, 298 deletions

diff --git a/net/disk_cache/backend_unittest.cc b/net/disk_cache/backend_unittest.cc
index c3f2cfa..572691c 100644
--- a/net/disk_cache/backend_unittest.cc
+++ b/net/disk_cache/backend_unittest.cc
@@ -81,7 +81,7 @@ class DiskCacheBackendTest : public DiskCacheTestWithCache {
 
   bool CreateSetOfRandomEntries(std::set<std::string>* key_pool);
   bool EnumerateAndMatchKeys(int max_to_open,
-                             void** iter,
+                             TestIterator* iter,
                              std::set<std::string>* keys_to_match,
                              size_t* count);
 
@@ -256,12 +256,14 @@ bool DiskCacheBackendTest::CreateSetOfRandomEntries(
 // OpenNextEntry stops returning net::OK.
 bool DiskCacheBackendTest::EnumerateAndMatchKeys(
     int max_to_open,
-    void** iter,
+    TestIterator* iter,
     std::set<std::string>* keys_to_match,
     size_t* count) {
   disk_cache::Entry* entry;
 
-  while (OpenNextEntry(iter, &entry) == net::OK) {
+  if (!iter)
+    return false;
+  while (iter->OpenNextEntry(&entry) == net::OK) {
     if (!entry)
       return false;
     EXPECT_EQ(1U, keys_to_match->erase(entry->GetKey()));
@@ -1237,11 +1239,11 @@ void DiskCacheBackendTest::BackendEnumerations() {
   Time final = Time::Now();
 
   disk_cache::Entry* entry;
-  void* iter = NULL;
+  scoped_ptr<TestIterator> iter = CreateIterator();
   int count = 0;
   Time last_modified[kNumEntries];
   Time last_used[kNumEntries];
-  while (OpenNextEntry(&iter, &entry) == net::OK) {
+  while (iter->OpenNextEntry(&entry) == net::OK) {
     ASSERT_TRUE(NULL != entry);
     if (count < kNumEntries) {
       last_modified[count] = entry->GetLastModified();
@@ -1255,10 +1257,10 @@ void DiskCacheBackendTest::BackendEnumerations() {
   };
   EXPECT_EQ(kNumEntries, count);
 
-  iter = NULL;
+  iter = CreateIterator();
   count = 0;
   // The previous enumeration should not have changed the timestamps.
-  while (OpenNextEntry(&iter, &entry) == net::OK) {
+  while (iter->OpenNextEntry(&entry) == net::OK) {
     ASSERT_TRUE(NULL != entry);
     if (count < kNumEntries) {
       EXPECT_TRUE(last_modified[count] == entry->GetLastModified());
@@ -1309,8 +1311,8 @@ void DiskCacheBackendTest::BackendEnumerations2() {
   // Make sure that the timestamp is not the same.
   AddDelay();
   ASSERT_EQ(net::OK, OpenEntry(second, &entry1));
-  void* iter = NULL;
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry2));
+  scoped_ptr<TestIterator> iter = CreateIterator();
+  ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry2));
   EXPECT_EQ(entry2->GetKey(), second);
 
   // Two entries and the iterator pointing at "first".
@@ -1318,15 +1320,15 @@ void DiskCacheBackendTest::BackendEnumerations2() {
   entry2->Close();
 
   // The iterator should still be valid, so we should not crash.
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry2));
+  ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry2));
   EXPECT_EQ(entry2->GetKey(), first);
   entry2->Close();
-  cache_->EndEnumeration(&iter);
+  iter = CreateIterator();
 
   // Modify the oldest entry and get the newest element.
   ASSERT_EQ(net::OK, OpenEntry(first, &entry1));
   EXPECT_EQ(0, WriteData(entry1, 0, 200, NULL, 0, false));
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry2));
+  ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry2));
   if (type_ == net::APP_CACHE) {
     // The list is not updated.
     EXPECT_EQ(entry2->GetKey(), second);
@@ -1336,7 +1338,6 @@ void DiskCacheBackendTest::BackendEnumerations2() {
 
   entry1->Close();
   entry2->Close();
-  cache_->EndEnumeration(&iter);
 }
 
 TEST_F(DiskCacheBackendTest, Enumerations2) {
@@ -1391,11 +1392,10 @@ TEST_F(DiskCacheBackendTest, ShaderCacheEnumerationReadData) {
   EXPECT_EQ(kSize, ReadData(entry1, 0, 0, buffer1.get(), kSize));
   entry1->Close();
 
-  void* iter = NULL;
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry2));
+  scoped_ptr<TestIterator> iter = CreateIterator();
+  ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry2));
   EXPECT_EQ(entry2->GetKey(), second);
   entry2->Close();
-  cache_->EndEnumeration(&iter);
 }
 
 #if !defined(LEAK_SANITIZER)
@@ -1424,9 +1424,9 @@ void DiskCacheBackendTest::BackendInvalidEntryEnumeration() {
 
   SimulateCrash();
 
-  void* iter = NULL;
+  scoped_ptr<TestIterator> iter = CreateIterator();
   int count = 0;
-  while (OpenNextEntry(&iter, &entry) == net::OK) {
+  while (iter->OpenNextEntry(&entry) == net::OK) {
     ASSERT_TRUE(NULL != entry);
     EXPECT_EQ(key2, entry->GetKey());
     entry->Close();
@@ -1466,9 +1466,8 @@ void DiskCacheBackendTest::BackendFixEnumerators() {
   EXPECT_EQ(kNumEntries, cache_->GetEntryCount());
 
   disk_cache::Entry *entry1, *entry2;
-  void* iter1 = NULL;
-  void* iter2 = NULL;
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter1, &entry1));
+  scoped_ptr<TestIterator> iter1 = CreateIterator(), iter2 = CreateIterator();
+  ASSERT_EQ(net::OK, iter1->OpenNextEntry(&entry1));
   ASSERT_TRUE(NULL != entry1);
   entry1->Close();
   entry1 = NULL;
@@ -1477,17 +1476,17 @@ void DiskCacheBackendTest::BackendFixEnumerators() {
   for (int i = 0; i < kNumEntries / 2; i++) {
     if (entry1)
       entry1->Close();
-    ASSERT_EQ(net::OK, OpenNextEntry(&iter1, &entry1));
+    ASSERT_EQ(net::OK, iter1->OpenNextEntry(&entry1));
     ASSERT_TRUE(NULL != entry1);
 
-    ASSERT_EQ(net::OK, OpenNextEntry(&iter2, &entry2));
+    ASSERT_EQ(net::OK, iter2->OpenNextEntry(&entry2));
     ASSERT_TRUE(NULL != entry2);
     entry2->Close();
   }
 
   // Messing up with entry1 will modify entry2->next.
   entry1->Doom();
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter2, &entry2));
+  ASSERT_EQ(net::OK, iter2->OpenNextEntry(&entry2));
   ASSERT_TRUE(NULL != entry2);
 
   // The link entry2->entry1 should be broken.
@@ -1496,12 +1495,9 @@ void DiskCacheBackendTest::BackendFixEnumerators() {
   entry2->Close();
 
   // And the second iterator should keep working.
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter2, &entry2));
+  ASSERT_EQ(net::OK, iter2->OpenNextEntry(&entry2));
   ASSERT_TRUE(NULL != entry2);
   entry2->Close();
-
-  cache_->EndEnumeration(&iter1);
-  cache_->EndEnumeration(&iter2);
 }
 
 TEST_F(DiskCacheBackendTest, FixEnumerators) {
@@ -2027,8 +2023,8 @@ void DiskCacheBackendTest::BackendInvalidEntry3() {
   InitCache();
 
   disk_cache::Entry* entry;
-  void* iter = NULL;
-  while (OpenNextEntry(&iter, &entry) == net::OK) {
+  scoped_ptr<TestIterator> iter = CreateIterator();
+  while (iter->OpenNextEntry(&entry) == net::OK) {
     entry->Close();
   }
 }
@@ -2172,8 +2168,8 @@ void DiskCacheBackendTest::BackendInvalidEntry7() {
   EXPECT_EQ(1, cache_->GetEntryCount());
 
   // We should delete the cache. The list still has a corrupt node.
-  void* iter = NULL;
-  EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry));
+  scoped_ptr<TestIterator> iter = CreateIterator();
+  EXPECT_NE(net::OK, iter->OpenNextEntry(&entry));
   FlushQueueForTest();
   EXPECT_EQ(0, cache_->GetEntryCount());
 }
@@ -2216,10 +2212,10 @@ void DiskCacheBackendTest::BackendInvalidEntry8() {
   EXPECT_EQ(1, cache_->GetEntryCount());
 
   // We should not delete the cache.
-  void* iter = NULL;
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry));
+  scoped_ptr<TestIterator> iter = CreateIterator();
+  ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry));
   entry->Close();
-  EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry));
+  EXPECT_NE(net::OK, iter->OpenNextEntry(&entry));
   EXPECT_EQ(1, cache_->GetEntryCount());
 }
 
@@ -2266,13 +2262,13 @@ void DiskCacheBackendTest::BackendInvalidEntry9(bool eviction) {
   } else {
     // We should detect the problem through the list, but we should not delete
     // the entry, just fail the iteration.
-    void* iter = NULL;
-    EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry));
+    scoped_ptr<TestIterator> iter = CreateIterator();
+    EXPECT_NE(net::OK, iter->OpenNextEntry(&entry));
 
     // Now a full iteration will work, and return one entry.
-    ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry));
+    ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry));
     entry->Close();
-    EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry));
+    EXPECT_NE(net::OK, iter->OpenNextEntry(&entry));
 
     // This should detect what's left of the bad entry.
     EXPECT_NE(net::OK, OpenEntry(second, &entry));
@@ -2343,13 +2339,13 @@ void DiskCacheBackendTest::BackendInvalidEntry10(bool eviction) {
     // Detection order: third -> second -> first.
     // We should detect the problem through the list, but we should not delete
     // the entry.
-    void* iter = NULL;
-    ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry));
+    scoped_ptr<TestIterator> iter = CreateIterator();
+    ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry));
     entry->Close();
-    ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry));
+    ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry));
     EXPECT_EQ(first, entry->GetKey());
     entry->Close();
-    EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry));
+    EXPECT_NE(net::OK, iter->OpenNextEntry(&entry));
   }
   DisableIntegrityCheck();
 }
@@ -2410,17 +2406,17 @@ void DiskCacheBackendTest::BackendInvalidEntry11(bool eviction) {
     // Detection order: third -> second.
     // We should detect the problem through the list, but we should not delete
     // the entry, just fail the iteration.
-    void* iter = NULL;
-    ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry));
+    scoped_ptr<TestIterator> iter = CreateIterator();
+    ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry));
     entry->Close();
-    EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry));
+    EXPECT_NE(net::OK, iter->OpenNextEntry(&entry));
 
     // Now a full iteration will work, and return two entries.
-    ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry));
+    ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry));
     entry->Close();
-    ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry));
+    ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry));
     entry->Close();
-    EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry));
+    EXPECT_NE(net::OK, iter->OpenNextEntry(&entry));
   }
   DisableIntegrityCheck();
 }
@@ -2498,12 +2494,12 @@ TEST_F(DiskCacheBackendTest, NewEvictionInvalidRankings2) {
 // If the LRU is corrupt, we delete the cache.
 void DiskCacheBackendTest::BackendInvalidRankings() {
   disk_cache::Entry* entry;
-  void* iter = NULL;
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry));
+  scoped_ptr<TestIterator> iter = CreateIterator();
+  ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry));
   entry->Close();
   EXPECT_EQ(2, cache_->GetEntryCount());
 
-  EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry));
+  EXPECT_NE(net::OK, iter->OpenNextEntry(&entry));
   FlushQueueForTest();  // Allow the restart to finish.
   EXPECT_EQ(0, cache_->GetEntryCount());
 }
@@ -2543,10 +2539,10 @@ TEST_F(DiskCacheBackendTest, NewEvictionInvalidRankingsFailure) {
 // If the LRU is corrupt and we have open entries, we disable the cache.
 void DiskCacheBackendTest::BackendDisable() {
   disk_cache::Entry *entry1, *entry2;
-  void* iter = NULL;
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry1));
+  scoped_ptr<TestIterator> iter = CreateIterator();
+  ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry1));
 
-  EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry2));
+  EXPECT_NE(net::OK, iter->OpenNextEntry(&entry2));
   EXPECT_EQ(0, cache_->GetEntryCount());
   EXPECT_NE(net::OK, CreateEntry("Something new", &entry2));
 
@@ -2594,9 +2590,9 @@ void DiskCacheBackendTest::BackendDisable2() {
   EXPECT_EQ(8, cache_->GetEntryCount());
 
   disk_cache::Entry* entry;
-  void* iter = NULL;
+  scoped_ptr<TestIterator> iter = CreateIterator();
   int count = 0;
-  while (OpenNextEntry(&iter, &entry) == net::OK) {
+  while (iter->OpenNextEntry(&entry) == net::OK) {
     ASSERT_TRUE(NULL != entry);
     entry->Close();
     count++;
@@ -2642,12 +2638,12 @@ TEST_F(DiskCacheBackendTest, NewEvictionDisableFailure2) {
 // If the index size changes when we disable the cache, we should not crash.
 void DiskCacheBackendTest::BackendDisable3() {
   disk_cache::Entry *entry1, *entry2;
-  void* iter = NULL;
+  scoped_ptr<TestIterator> iter = CreateIterator();
   EXPECT_EQ(2, cache_->GetEntryCount());
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry1));
+  ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry1));
   entry1->Close();
 
-  EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry2));
+  EXPECT_NE(net::OK, iter->OpenNextEntry(&entry2));
   FlushQueueForTest();
 
   ASSERT_EQ(net::OK, CreateEntry("Something new", &entry2));
@@ -2676,8 +2672,8 @@ TEST_F(DiskCacheBackendTest, NewEvictionDisableSuccess3) {
 // If we disable the cache, already open entries should work as far as possible.
 void DiskCacheBackendTest::BackendDisable4() {
   disk_cache::Entry *entry1, *entry2, *entry3, *entry4;
-  void* iter = NULL;
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry1));
+  scoped_ptr<TestIterator> iter = CreateIterator();
+  ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry1));
 
   char key2[2000];
   char key3[20000];
@@ -2695,7 +2691,7 @@ void DiskCacheBackendTest::BackendDisable4() {
   EXPECT_EQ(kBufSize, WriteData(entry3, 0, 0, buf.get(), kBufSize, false));
 
   // This line should disable the cache but not delete it.
-  EXPECT_NE(net::OK, OpenNextEntry(&iter, &entry4));
+  EXPECT_NE(net::OK, iter->OpenNextEntry(&entry4));
   EXPECT_EQ(0, cache_->GetEntryCount());
 
   EXPECT_NE(net::OK, CreateEntry("cache is disabled", &entry4));
@@ -3376,29 +3372,29 @@ TEST_F(DiskCacheBackendTest, SimpleCacheEnumerationBasics) {
 
   // Check that enumeration returns all entries.
   std::set<std::string> keys_to_match(key_pool);
-  void* iter = NULL;
+  scoped_ptr<TestIterator> iter = CreateIterator();
   size_t count = 0;
-  ASSERT_TRUE(EnumerateAndMatchKeys(-1, &iter, &keys_to_match, &count));
-  cache_->EndEnumeration(&iter);
+  ASSERT_TRUE(EnumerateAndMatchKeys(-1, iter.get(), &keys_to_match, &count));
+  iter.reset();
   EXPECT_EQ(key_pool.size(), count);
   EXPECT_TRUE(keys_to_match.empty());
 
   // Check that opening entries does not affect enumeration.
   keys_to_match = key_pool;
-  iter = NULL;
+  iter = CreateIterator();
   count = 0;
   disk_cache::Entry* entry_opened_before;
   ASSERT_EQ(net::OK, OpenEntry(*(key_pool.begin()), &entry_opened_before));
   ASSERT_TRUE(EnumerateAndMatchKeys(key_pool.size()/2,
-                                    &iter,
+                                    iter.get(),
                                     &keys_to_match,
                                     &count));
 
   disk_cache::Entry* entry_opened_middle;
   ASSERT_EQ(net::OK,
             OpenEntry(*(keys_to_match.begin()), &entry_opened_middle));
-  ASSERT_TRUE(EnumerateAndMatchKeys(-1, &iter, &keys_to_match, &count));
-  cache_->EndEnumeration(&iter);
+  ASSERT_TRUE(EnumerateAndMatchKeys(-1, iter.get(), &keys_to_match, &count));
+  iter.reset();
   entry_opened_before->Close();
   entry_opened_middle->Close();
 
@@ -3416,10 +3412,10 @@ TEST_F(DiskCacheBackendTest, SimpleCacheEnumerationWhileDoomed) {
 
   // Check that enumeration returns all entries but the doomed one.
   std::set<std::string> keys_to_match(key_pool);
-  void* iter = NULL;
+  scoped_ptr<TestIterator> iter = CreateIterator();
   size_t count = 0;
   ASSERT_TRUE(EnumerateAndMatchKeys(key_pool.size()/2,
-                                    &iter,
+                                    iter.get(),
                                     &keys_to_match,
                                     &count));
 
@@ -3427,8 +3423,8 @@ TEST_F(DiskCacheBackendTest, SimpleCacheEnumerationWhileDoomed) {
   DoomEntry(key_to_delete);
   keys_to_match.erase(key_to_delete);
   key_pool.erase(key_to_delete);
-  ASSERT_TRUE(EnumerateAndMatchKeys(-1, &iter, &keys_to_match, &count));
-  cache_->EndEnumeration(&iter);
+  ASSERT_TRUE(EnumerateAndMatchKeys(-1, iter.get(), &keys_to_match, &count));
+  iter.reset();
 
   EXPECT_EQ(key_pool.size(), count);
   EXPECT_TRUE(keys_to_match.empty());
@@ -3464,10 +3460,10 @@ TEST_F(DiskCacheBackendTest, SimpleCacheEnumerationCorruption) {
 
   // Check that enumeration returns all entries but the corrupt one.
   std::set<std::string> keys_to_match(key_pool);
-  void* iter = NULL;
+  scoped_ptr<TestIterator> iter = CreateIterator();
   size_t count = 0;
-  ASSERT_TRUE(EnumerateAndMatchKeys(-1, &iter, &keys_to_match, &count));
-  cache_->EndEnumeration(&iter);
+  ASSERT_TRUE(EnumerateAndMatchKeys(-1, iter.get(), &keys_to_match, &count));
+  iter.reset();
 
   EXPECT_EQ(key_pool.size(), count);
   EXPECT_TRUE(keys_to_match.empty());
@@ -3481,9 +3477,9 @@ TEST_F(DiskCacheBackendTest, SimpleCacheEnumerationDestruction) {
   std::set<std::string> key_pool;
   ASSERT_TRUE(CreateSetOfRandomEntries(&key_pool));
 
-  void* iter = NULL;
+  scoped_ptr<TestIterator> iter = CreateIterator();
   disk_cache::Entry* entry = NULL;
-  ASSERT_EQ(net::OK, OpenNextEntry(&iter, &entry));
+  ASSERT_EQ(net::OK, iter->OpenNextEntry(&entry));
   EXPECT_TRUE(entry);
   disk_cache::ScopedEntryPtr entry_closer(entry);
 
diff --git a/net/disk_cache/blockfile/backend_impl.cc b/net/disk_cache/blockfile/backend_impl.cc
index c38b2bb..b278543 100644
--- a/net/disk_cache/blockfile/backend_impl.cc
+++ b/net/disk_cache/blockfile/backend_impl.cc
@@ -1250,16 +1250,32 @@ int BackendImpl::DoomEntriesSince(const base::Time initial_time,
   return net::ERR_IO_PENDING;
 }
 
-int BackendImpl::OpenNextEntry(void** iter, Entry** next_entry,
-                               const CompletionCallback& callback) {
-  DCHECK(!callback.is_null());
-  background_queue_.OpenNextEntry(iter, next_entry, callback);
-  return net::ERR_IO_PENDING;
-}
+class BackendImpl::IteratorImpl : public Backend::Iterator {
+ public:
+  explicit IteratorImpl(base::WeakPtr<InFlightBackendIO> background_queue)
+      : background_queue_(background_queue), data_(NULL) {
+  }
 
-void BackendImpl::EndEnumeration(void** iter) {
-  background_queue_.EndEnumeration(*iter);
-  *iter = NULL;
+  virtual ~IteratorImpl() {
+    if (background_queue_)
+      background_queue_->EndEnumeration(data_);
+  }
+
+  virtual int OpenNextEntry(Entry** next_entry,
+                            const net::CompletionCallback& callback) OVERRIDE {
+    if (!background_queue_)
+      return net::ERR_FAILED;
+    background_queue_->OpenNextEntry(&data_, next_entry, callback);
+    return net::ERR_IO_PENDING;
+  }
+
+ private:
+  const base::WeakPtr<InFlightBackendIO> background_queue_;
+  void* data_;
+};
+
+scoped_ptr<Backend::Iterator> BackendImpl::CreateIterator() {
+  return scoped_ptr<Backend::Iterator>(new IteratorImpl(GetBackgroundQueue()));
 }
 
 void BackendImpl::GetStats(StatsItems* stats) {
diff --git a/net/disk_cache/blockfile/backend_impl.h b/net/disk_cache/blockfile/backend_impl.h
index e2304ff..775374e 100644
--- a/net/disk_cache/blockfile/backend_impl.h
+++ b/net/disk_cache/blockfile/backend_impl.h
@@ -273,14 +273,21 @@ class NET_EXPORT_PRIVATE BackendImpl : public Backend {
                                  const CompletionCallback& callback) OVERRIDE;
   virtual int DoomEntriesSince(base::Time initial_time,
                                const CompletionCallback& callback) OVERRIDE;
-  virtual int OpenNextEntry(void** iter, Entry** next_entry,
-                            const CompletionCallback& callback) OVERRIDE;
-  virtual void EndEnumeration(void** iter) OVERRIDE;
+  // NOTE: The blockfile Backend::Iterator::OpenNextEntry method does not modify
+  // the last_used field of the entry, and therefore it does not impact the
+  // eviction ranking of the entry. However, an enumeration will go through all
+  // entries on the cache only if the cache is not modified while the
+  // enumeration is taking place. Significantly altering the entry pointed by
+  // the iterator (for example, deleting the entry) will invalidate the
+  // iterator. Performing operations on an entry that modify the entry may
+  // result in loops in the iteration, skipped entries or similar.
+  virtual scoped_ptr<Iterator> CreateIterator() OVERRIDE;
   virtual void GetStats(StatsItems* stats) OVERRIDE;
   virtual void OnExternalCacheHit(const std::string& key) OVERRIDE;
 
  private:
   typedef base::hash_map<CacheAddr, EntryImpl*> EntriesMap;
+  class IteratorImpl;
 
   // Creates a new backing file for the cache index.
   bool CreateBackingStore(disk_cache::File* file);
diff --git a/net/disk_cache/blockfile/backend_impl_v3.cc b/net/disk_cache/blockfile/backend_impl_v3.cc
index 9a7cff1..b44b195 100644
--- a/net/disk_cache/blockfile/backend_impl_v3.cc
+++ b/net/disk_cache/blockfile/backend_impl_v3.cc
@@ -660,17 +660,27 @@ int BackendImplV3::DoomEntriesSince(base::Time initial_time,
   }
 }
 
-int BackendImplV3::OpenNextEntry(void** iter, Entry** next_entry,
-                                 const CompletionCallback& callback) {
-  DCHECK(!callback.is_null());
-  background_queue_.OpenNextEntry(iter, next_entry, callback);
-  return net::ERR_IO_PENDING;
-}
+class BackendImplV3::IteratorImpl : public Backend::Iterator {
+ public:
+  explicit IteratorImpl(base::WeakPtr<InFlightBackendIO> background_queue)
+      : background_queue_(background_queue), data_(NULL) {
+  }
+
+  virtual int OpenNextEntry(Entry** next_entry,
+                            const net::CompletionCallback& callback) OVERRIDE {
+    if (!background_queue_)
+      return net::ERR_FAILED;
+    background_queue_->OpenNextEntry(&data_, next_entry, callback);
+    return net::ERR_IO_PENDING;
+  }
 
-void BackendImplV3::EndEnumeration(void** iter) {
-  scoped_ptr<IndexIterator> iterator(
-      reinterpret_cast<IndexIterator*>(*iter));
-  *iter = NULL;
+ private:
+  const base::WeakPtr<InFlightBackendIO> background_queue_;
+  void* data_;
+};
+
+scoped_ptr<Backend::Iterator> BackendImplV3::CreateIterator() {
+  return scoped_ptr<Backend::Iterator>(new IteratorImpl(GetBackgroundQueue()));
 }
 
 void BackendImplV3::GetStats(StatsItems* stats) {
@@ -1497,13 +1507,17 @@ int BackendImplV3::DoomEntriesSince(base::Time initial_time,
   return net::ERR_FAILED;
 }
 
-int BackendImplV3::OpenNextEntry(void** iter, Entry** next_entry,
-                                 const CompletionCallback& callback) {
-  return net::ERR_FAILED;
-}
 
-void BackendImplV3::EndEnumeration(void** iter) {
-  NOTIMPLEMENTED();
+class BackendImplV3::NotImplementedIterator : public Backend::Iterator {
+ public:
+  virtual int OpenNextEntry(disk_cache::Entry** next_entry,
+                            const net::CompletionCallback& callback) OVERRIDE {
+    return net::ERR_NOT_IMPLEMENTED;
+  }
+};
+
+scoped_ptr<Backend::Iterator> BackendImplV3::CreateIterator() {
+  return scoped_ptr<Iterator>(new NotImplementedIterator());
 }
 
 void BackendImplV3::GetStats(StatsItems* stats) {
diff --git a/net/disk_cache/blockfile/backend_impl_v3.h b/net/disk_cache/blockfile/backend_impl_v3.h
index b57bb89..ca64997 100644
--- a/net/disk_cache/blockfile/backend_impl_v3.h
+++ b/net/disk_cache/blockfile/backend_impl_v3.h
@@ -186,15 +186,15 @@ class NET_EXPORT_PRIVATE BackendImplV3 : public Backend {
                                  const CompletionCallback& callback) OVERRIDE;
   virtual int DoomEntriesSince(base::Time initial_time,
                                const CompletionCallback& callback) OVERRIDE;
-  virtual int OpenNextEntry(void** iter, Entry** next_entry,
-                            const CompletionCallback& callback) OVERRIDE;
-  virtual void EndEnumeration(void** iter) OVERRIDE;
+  virtual scoped_ptr<Iterator> CreateIterator() OVERRIDE;
   virtual void GetStats(StatsItems* stats) OVERRIDE;
   virtual void OnExternalCacheHit(const std::string& key) OVERRIDE;
 
  private:
   friend class EvictionV3;
   typedef base::hash_map<CacheAddr, EntryImplV3*> EntriesMap;
+  class IteratorImpl;
+  class NotImplementedIterator;
   class Worker;
 
   void AdjustMaxCacheSize();
diff --git a/net/disk_cache/disk_cache.h b/net/disk_cache/disk_cache.h
index 4f64b9a..769ab36 100644
--- a/net/disk_cache/disk_cache.h
+++ b/net/disk_cache/disk_cache.h
@@ -13,6 +13,7 @@
 
 #include "base/basictypes.h"
 #include "base/memory/ref_counted.h"
+#include "base/memory/scoped_ptr.h"
 #include "base/time/time.h"
 #include "net/base/cache_type.h"
 #include "net/base/completion_callback.h"
@@ -64,6 +65,26 @@ class NET_EXPORT Backend {
  public:
   typedef net::CompletionCallback CompletionCallback;
 
+  class Iterator {
+   public:
+    virtual ~Iterator() {}
+
+    // OpenNextEntry returns |net::OK| and provides |next_entry| if there is an
+    // entry to enumerate. It returns |net::ERR_FAILED| at the end of
+    // enumeration. If the function returns |net::ERR_IO_PENDING|, then the
+    // final result will be passed to the provided |callback|, otherwise
+    // |callback| will not be called. If any entry in the cache is modified
+    // during iteration, the result of this function is thereafter undefined.
+    //
+    // Calling OpenNextEntry after the backend which created it is destroyed
+    // may fail with |net::ERR_FAILED|; however it should not crash.
+    //
+    // Some cache backends make stronger guarantees about mutation during
+    // iteration, see top comment in simple_backend_impl.h for details.
+    virtual int OpenNextEntry(Entry** next_entry,
+                              const CompletionCallback& callback) = 0;
+  };
+
   // If the backend is destroyed when there are operations in progress (any
   // callback that has not been invoked yet), this method cancels said
   // operations so the callbacks are not invoked, possibly leaving the work
@@ -123,31 +144,9 @@ class NET_EXPORT Backend {
   virtual int DoomEntriesSince(base::Time initial_time,
                                const CompletionCallback& callback) = 0;
 
-  // Enumerates the cache. Initialize |iter| to NULL before calling this method
-  // the first time. That will cause the enumeration to start at the head of
-  // the cache. For subsequent calls, pass the same |iter| pointer again without
-  // changing its value. This method returns ERR_FAILED when there are no more
-  // entries to enumerate. When the entry pointer is no longer needed, its
-  // Close method should be called. The return value is a net error code. If
-  // this method returns ERR_IO_PENDING, the |callback| will be invoked when the
-  // |next_entry| is available. The pointer to receive the |next_entry| must
-  // remain valid until the operation completes.
-  //
-  // NOTE: This method does not modify the last_used field of the entry, and
-  // therefore it does not impact the eviction ranking of the entry. However,
-  // an enumeration will go through all entries on the cache only if the cache
-  // is not modified while the enumeration is taking place. Significantly
-  // altering the entry pointed by |iter| (for example, deleting the entry) will
-  // invalidate |iter|. Performing operations on an entry that modify the entry
-  // may result in loops in the iteration, skipped entries or similar.
-  virtual int OpenNextEntry(void** iter, Entry** next_entry,
-                            const CompletionCallback& callback) = 0;
-
-  // Releases iter without returning the next entry. Whenever OpenNextEntry()
-  // returns true, but the caller is not interested in continuing the
-  // enumeration by calling OpenNextEntry() again, the enumeration must be
-  // ended by calling this method with iter returned by OpenNextEntry().
-  virtual void EndEnumeration(void** iter) = 0;
+  // Returns an iterator which will enumerate all entries of the cache in an
+  // undefined order.
+  virtual scoped_ptr<Iterator> CreateIterator() = 0;
 
   // Return a list of cache statistics.
   virtual void GetStats(
diff --git a/net/disk_cache/disk_cache_test_base.cc b/net/disk_cache/disk_cache_test_base.cc
index d0f9842..c3d94e6 100644
--- a/net/disk_cache/disk_cache_test_base.cc
+++ b/net/disk_cache/disk_cache_test_base.cc
@@ -52,6 +52,20 @@ void DiskCacheTest::TearDown() {
   base::RunLoop().RunUntilIdle();
 }
 
+DiskCacheTestWithCache::TestIterator::TestIterator(
+    scoped_ptr<disk_cache::Backend::Iterator> iterator)
+    : iterator_(iterator.Pass()) {
+}
+
+DiskCacheTestWithCache::TestIterator::~TestIterator() {}
+
+int DiskCacheTestWithCache::TestIterator::OpenNextEntry(
+    disk_cache::Entry** next_entry) {
+  net::TestCompletionCallback cb;
+  int rv = iterator_->OpenNextEntry(next_entry, cb.callback());
+  return cb.GetResult(rv);
+}
+
 DiskCacheTestWithCache::DiskCacheTestWithCache()
     : cache_impl_(NULL),
       simple_cache_impl_(NULL),
@@ -153,11 +167,9 @@ int DiskCacheTestWithCache::DoomEntriesSince(const base::Time initial_time) {
   return cb.GetResult(rv);
 }
 
-int DiskCacheTestWithCache::OpenNextEntry(void** iter,
-                                          disk_cache::Entry** next_entry) {
-  net::TestCompletionCallback cb;
-  int rv = cache_->OpenNextEntry(iter, next_entry, cb.callback());
-  return cb.GetResult(rv);
+scoped_ptr<DiskCacheTestWithCache::TestIterator>
+    DiskCacheTestWithCache::CreateIterator() {
+  return scoped_ptr<TestIterator>(new TestIterator(cache_->CreateIterator()));
 }
 
 void DiskCacheTestWithCache::FlushQueueForTest() {
diff --git a/net/disk_cache/disk_cache_test_base.h b/net/disk_cache/disk_cache_test_base.h
index b724906..225d37f 100644
--- a/net/disk_cache/disk_cache_test_base.h
+++ b/net/disk_cache/disk_cache_test_base.h
@@ -11,6 +11,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/threading/thread.h"
 #include "net/base/cache_type.h"
+#include "net/disk_cache/disk_cache.h"
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/platform_test.h"
 
@@ -57,6 +58,17 @@ class DiskCacheTest : public PlatformTest {
 // Provides basic support for cache related tests.
 class DiskCacheTestWithCache : public DiskCacheTest {
  protected:
+  class TestIterator {
+   public:
+    explicit TestIterator(scoped_ptr<disk_cache::Backend::Iterator> iterator);
+    ~TestIterator();
+
+    int OpenNextEntry(disk_cache::Entry** next_entry);
+
+   private:
+    scoped_ptr<disk_cache::Backend::Iterator> iterator_;
+  };
+
   DiskCacheTestWithCache();
   virtual ~DiskCacheTestWithCache();
 
@@ -117,7 +129,7 @@ class DiskCacheTestWithCache : public DiskCacheTest {
   int DoomEntriesBetween(const base::Time initial_time,
                          const base::Time end_time);
   int DoomEntriesSince(const base::Time initial_time);
-  int OpenNextEntry(void** iter, disk_cache::Entry** next_entry);
+  scoped_ptr<TestIterator> CreateIterator();
   void FlushQueueForTest();
   void RunTaskForTest(const base::Closure& closure);
   int ReadData(disk_cache::Entry* entry, int index, int offset,
diff --git a/net/disk_cache/entry_unittest.cc b/net/disk_cache/entry_unittest.cc
index 803f98f..afe93ac 100644
--- a/net/disk_cache/entry_unittest.cc
+++ b/net/disk_cache/entry_unittest.cc
@@ -1617,10 +1617,10 @@ TEST_F(DiskCacheEntryTest, MemoryOnlyEnumerationWithSparseEntries) {
   parent_entry->Close();
 
   // Perform the enumerations.
-  void* iter = NULL;
+  scoped_ptr<TestIterator> iter = CreateIterator();
   disk_cache::Entry* entry = NULL;
   int count = 0;
-  while (OpenNextEntry(&iter, &entry) == net::OK) {
+  while (iter->OpenNextEntry(&entry) == net::OK) {
     ASSERT_TRUE(entry != NULL);
     ++count;
     disk_cache::MemEntryImpl* mem_entry =
@@ -2217,10 +2217,10 @@ TEST_F(DiskCacheEntryTest, CleanupSparseEntry) {
   entry->Close();
   EXPECT_EQ(4, cache_->GetEntryCount());
 
-  void* iter = NULL;
+  scoped_ptr<TestIterator> iter = CreateIterator();
   int count = 0;
   std::string child_key[2];
-  while (OpenNextEntry(&iter, &entry) == net::OK) {
+  while (iter->OpenNextEntry(&entry) == net::OK) {
     ASSERT_TRUE(entry != NULL);
     // Writing to an entry will alter the LRU list and invalidate the iterator.
     if (entry->GetKey() != key && count < 2)
diff --git a/net/disk_cache/memory/mem_backend_impl.cc b/net/disk_cache/memory/mem_backend_impl.cc
index e69c00e..848ef98 100644
--- a/net/disk_cache/memory/mem_backend_impl.cc
+++ b/net/disk_cache/memory/mem_backend_impl.cc
@@ -29,7 +29,8 @@ int LowWaterAdjust(int high_water) {
 namespace disk_cache {
 
 MemBackendImpl::MemBackendImpl(net::NetLog* net_log)
-    : max_size_(0), current_size_(0), net_log_(net_log) {}
+    : max_size_(0), current_size_(0), net_log_(net_log), weak_factory_(this) {
+}
 
 MemBackendImpl::~MemBackendImpl() {
   EntryMap::iterator it = entries_.begin();
@@ -180,16 +181,40 @@ int MemBackendImpl::DoomEntriesSince(const base::Time initial_time,
   return net::ERR_FAILED;
 }
 
-int MemBackendImpl::OpenNextEntry(void** iter, Entry** next_entry,
-                                  const CompletionCallback& callback) {
-  if (OpenNextEntry(iter, next_entry))
-    return net::OK;
+class MemBackendImpl::MemIterator : public Backend::Iterator {
+ public:
+  explicit MemIterator(base::WeakPtr<MemBackendImpl> backend)
+      : backend_(backend), current_(NULL) {
+  }
 
-  return net::ERR_FAILED;
-}
+  virtual int OpenNextEntry(Entry** next_entry,
+                            const CompletionCallback& callback) OVERRIDE {
+    if (!backend_)
+      return net::ERR_FAILED;
+
+    MemEntryImpl* node = backend_->rankings_.GetNext(current_);
+    // We should never return a child entry so iterate until we hit a parent
+    // entry.
+    while (node && node->type() != MemEntryImpl::kParentEntry)
+      node = backend_->rankings_.GetNext(node);
+    *next_entry = node;
+    current_ = node;
+
+    if (node) {
+      node->Open();
+      return net::OK;
+    }
+    return net::ERR_FAILED;
+  }
 
-void MemBackendImpl::EndEnumeration(void** iter) {
-  *iter = NULL;
+ private:
+  base::WeakPtr<MemBackendImpl> backend_;
+  MemEntryImpl* current_;
+};
+
+scoped_ptr<Backend::Iterator> MemBackendImpl::CreateIterator() {
+  return scoped_ptr<Backend::Iterator>(
+      new MemIterator(weak_factory_.GetWeakPtr()));
 }
 
 void MemBackendImpl::OnExternalCacheHit(const std::string& key) {
@@ -287,23 +312,6 @@ bool MemBackendImpl::DoomEntriesSince(const Time initial_time) {
   }
 }
 
-bool MemBackendImpl::OpenNextEntry(void** iter, Entry** next_entry) {
-  MemEntryImpl* current = reinterpret_cast<MemEntryImpl*>(*iter);
-  MemEntryImpl* node = rankings_.GetNext(current);
-  // We should never return a child entry so iterate until we hit a parent
-  // entry.
-  while (node && node->type() != MemEntryImpl::kParentEntry) {
-    node = rankings_.GetNext(node);
-  }
-  *next_entry = node;
-  *iter = node;
-
-  if (node)
-    node->Open();
-
-  return NULL != node;
-}
-
 void MemBackendImpl::TrimCache(bool empty) {
   MemEntryImpl* next = rankings_.GetPrev(NULL);
   if (!next)
diff --git a/net/disk_cache/memory/mem_backend_impl.h b/net/disk_cache/memory/mem_backend_impl.h
index 5f31be5..10946c5 100644
--- a/net/disk_cache/memory/mem_backend_impl.h
+++ b/net/disk_cache/memory/mem_backend_impl.h
@@ -9,6 +9,7 @@
 
 #include "base/compiler_specific.h"
 #include "base/containers/hash_tables.h"
+#include "base/memory/weak_ptr.h"
 #include "net/disk_cache/disk_cache.h"
 #include "net/disk_cache/memory/mem_rankings.h"
 
@@ -76,14 +77,15 @@ class NET_EXPORT_PRIVATE MemBackendImpl : public Backend {
                                  const CompletionCallback& callback) OVERRIDE;
   virtual int DoomEntriesSince(base::Time initial_time,
                                const CompletionCallback& callback) OVERRIDE;
-  virtual int OpenNextEntry(void** iter, Entry** next_entry,
-                            const CompletionCallback& callback) OVERRIDE;
-  virtual void EndEnumeration(void** iter) OVERRIDE;
+  virtual scoped_ptr<Iterator> CreateIterator() OVERRIDE;
   virtual void GetStats(
       std::vector<std::pair<std::string, std::string> >* stats) OVERRIDE {}
   virtual void OnExternalCacheHit(const std::string& key) OVERRIDE;
 
  private:
+  class MemIterator;
+  friend class MemIterator;
+
   typedef base::hash_map<std::string, MemEntryImpl*> EntryMap;
 
   // Old Backend interface.
@@ -94,7 +96,6 @@ class NET_EXPORT_PRIVATE MemBackendImpl : public Backend {
   bool DoomEntriesBetween(const base::Time initial_time,
                           const base::Time end_time);
   bool DoomEntriesSince(const base::Time initial_time);
-  bool OpenNextEntry(void** iter, Entry** next_entry);
 
   // Deletes entries from the cache until the current size is below the limit.
   // If empty is true, the whole cache will be trimmed, regardless of being in
@@ -112,6 +113,8 @@ class NET_EXPORT_PRIVATE MemBackendImpl : public Backend {
 
   net::NetLog* net_log_;
 
+  base::WeakPtrFactory<MemBackendImpl> weak_factory_;
+
   DISALLOW_COPY_AND_ASSIGN(MemBackendImpl);
 };
 
diff --git a/net/disk_cache/simple/simple_backend_impl.cc b/net/disk_cache/simple/simple_backend_impl.cc
index 2e16576..e84211b 100644
--- a/net/disk_cache/simple/simple_backend_impl.cc
+++ b/net/disk_cache/simple/simple_backend_impl.cc
@@ -473,18 +473,78 @@ int SimpleBackendImpl::DoomEntriesSince(
   return DoomEntriesBetween(initial_time, Time(), callback);
 }
 
-int SimpleBackendImpl::OpenNextEntry(void** iter,
-                                     Entry** next_entry,
-                                     const CompletionCallback& callback) {
-  CompletionCallback get_next_entry =
-      base::Bind(&SimpleBackendImpl::GetNextEntryInIterator, AsWeakPtr(), iter,
-                 next_entry, callback);
-  return index_->ExecuteWhenReady(get_next_entry);
-}
+class SimpleBackendImpl::SimpleIterator FINAL : public Iterator {
+ public:
+  explicit SimpleIterator(base::WeakPtr<SimpleBackendImpl> backend)
+      : backend_(backend),
+        weak_factory_(this) {
+  }
+
+  // From Backend::Iterator:
+  virtual int OpenNextEntry(Entry** next_entry,
+                            const CompletionCallback& callback) OVERRIDE {
+    CompletionCallback open_next_entry_impl =
+        base::Bind(&SimpleIterator::OpenNextEntryImpl,
+                   weak_factory_.GetWeakPtr(), next_entry, callback);
+    return backend_->index_->ExecuteWhenReady(open_next_entry_impl);
+  }
+
+  void OpenNextEntryImpl(Entry** next_entry,
+                         const CompletionCallback& callback,
+                         int index_initialization_error_code) {
+    if (!backend_) {
+      callback.Run(net::ERR_FAILED);
+      return;
+    }
+    if (index_initialization_error_code != net::OK) {
+      callback.Run(index_initialization_error_code);
+      return;
+    }
+    if (!hashes_to_enumerate_)
+      hashes_to_enumerate_ = backend_->index()->GetAllHashes().Pass();
+
+    while (!hashes_to_enumerate_->empty()) {
+      uint64 entry_hash = hashes_to_enumerate_->back();
+      hashes_to_enumerate_->pop_back();
+      if (backend_->index()->Has(entry_hash)) {
+        *next_entry = NULL;
+        CompletionCallback continue_iteration = base::Bind(
+            &SimpleIterator::CheckIterationReturnValue,
+            weak_factory_.GetWeakPtr(),
+            next_entry,
+            callback);
+        int error_code_open = backend_->OpenEntryFromHash(entry_hash,
+                                                          next_entry,
+                                                          continue_iteration);
+        if (error_code_open == net::ERR_IO_PENDING)
+          return;
+        if (error_code_open != net::ERR_FAILED) {
+          callback.Run(error_code_open);
+          return;
+        }
+      }
+    }
+    callback.Run(net::ERR_FAILED);
+  }
 
-void SimpleBackendImpl::EndEnumeration(void** iter) {
-  active_enumerations_.Remove(IteratorToEnumerationId(iter));
-  *iter = NULL;
+  void CheckIterationReturnValue(Entry** entry,
+                                 const CompletionCallback& callback,
+                                 int error_code) {
+    if (error_code == net::ERR_FAILED) {
+      OpenNextEntry(entry, callback);
+      return;
+    }
+    callback.Run(error_code);
+  }
+
+ private:
+  base::WeakPtr<SimpleBackendImpl> backend_;
+  scoped_ptr<std::vector<uint64> > hashes_to_enumerate_;
+  base::WeakPtrFactory<SimpleIterator> weak_factory_;
+};
+
+scoped_ptr<Backend::Iterator> SimpleBackendImpl::CreateIterator() {
+  return scoped_ptr<Iterator>(new SimpleIterator(AsWeakPtr()));
 }
 
 void SimpleBackendImpl::GetStats(
@@ -499,27 +559,6 @@ void SimpleBackendImpl::OnExternalCacheHit(const std::string& key) {
   index_->UseIfExists(simple_util::GetEntryHashKey(key));
 }
 
-// static
-SimpleBackendImpl::ActiveEnumerationMap::KeyType
-    SimpleBackendImpl::IteratorToEnumerationId(void** iter) {
-  COMPILE_ASSERT(sizeof(ptrdiff_t) >= sizeof(*iter),
-                 integer_type_must_fit_ptr_type_for_cast_to_be_reversible);
-  const ptrdiff_t ptrdiff_enumeration_id = reinterpret_cast<ptrdiff_t>(*iter);
-  const ActiveEnumerationMap::KeyType enumeration_id = ptrdiff_enumeration_id;
-  DCHECK_EQ(enumeration_id, ptrdiff_enumeration_id);
-  return enumeration_id;
-}
-
-// static
-void* SimpleBackendImpl::EnumerationIdToIterator(
-    ActiveEnumerationMap::KeyType enumeration_id) {
-  const ptrdiff_t ptrdiff_enumeration_id = enumeration_id;
-  DCHECK_EQ(enumeration_id, ptrdiff_enumeration_id);
-  COMPILE_ASSERT(sizeof(ptrdiff_t) >= sizeof(void*),
-                 integer_type_must_fit_ptr_type_for_cast_to_be_reversible);
-  return reinterpret_cast<void*>(ptrdiff_enumeration_id);
-}
-
 void SimpleBackendImpl::InitializeIndex(const CompletionCallback& callback,
                                         const DiskStatResult& result) {
   if (result.net_error == net::OK) {
@@ -633,49 +672,6 @@ int SimpleBackendImpl::DoomEntryFromHash(uint64 entry_hash,
   return net::ERR_IO_PENDING;
 }
 
-void SimpleBackendImpl::GetNextEntryInIterator(
-    void** iter,
-    Entry** next_entry,
-    const CompletionCallback& callback,
-    int error_code) {
-  if (error_code != net::OK) {
-    callback.Run(error_code);
-    return;
-  }
-  std::vector<uint64>* entry_list = NULL;
-  if (*iter == NULL) {
-    const ActiveEnumerationMap::KeyType new_enumeration_id =
-        active_enumerations_.Add(
-            entry_list = index()->GetAllHashes().release());
-    *iter = EnumerationIdToIterator(new_enumeration_id);
-  } else {
-    entry_list = active_enumerations_.Lookup(IteratorToEnumerationId(iter));
-  }
-  while (entry_list->size() > 0) {
-    uint64 entry_hash = entry_list->back();
-    entry_list->pop_back();
-    if (index()->Has(entry_hash)) {
-      *next_entry = NULL;
-      CompletionCallback continue_iteration = base::Bind(
-          &SimpleBackendImpl::CheckIterationReturnValue,
-          AsWeakPtr(),
-          iter,
-          next_entry,
-          callback);
-      int error_code_open = OpenEntryFromHash(entry_hash,
-                                              next_entry,
-                                              continue_iteration);
-      if (error_code_open == net::ERR_IO_PENDING)
-        return;
-      if (error_code_open != net::ERR_FAILED) {
-        callback.Run(error_code_open);
-        return;
-      }
-    }
-  }
-  callback.Run(net::ERR_FAILED);
-}
-
 void SimpleBackendImpl::OnEntryOpenedFromHash(
     uint64 hash,
     Entry** entry,
@@ -729,18 +725,6 @@ void SimpleBackendImpl::OnEntryOpenedFromKey(
   callback.Run(final_code);
 }
 
-void SimpleBackendImpl::CheckIterationReturnValue(
-    void** iter,
-    Entry** entry,
-    const CompletionCallback& callback,
-    int error_code) {
-  if (error_code == net::ERR_FAILED) {
-    OpenNextEntry(iter, entry, callback);
-    return;
-  }
-  callback.Run(error_code);
-}
-
 void SimpleBackendImpl::DoomEntriesComplete(
     scoped_ptr<std::vector<uint64> > entry_hashes,
     const net::CompletionCallback& callback,
diff --git a/net/disk_cache/simple/simple_backend_impl.h b/net/disk_cache/simple/simple_backend_impl.h
index 907ee5a..48c422f 100644
--- a/net/disk_cache/simple/simple_backend_impl.h
+++ b/net/disk_cache/simple/simple_backend_impl.h
@@ -13,7 +13,6 @@
 #include "base/compiler_specific.h"
 #include "base/containers/hash_tables.h"
 #include "base/files/file_path.h"
-#include "base/id_map.h"
 #include "base/memory/ref_counted.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/weak_ptr.h"
@@ -35,6 +34,11 @@ namespace disk_cache {
 // files.
 // See http://www.chromium.org/developers/design-documents/network-stack/disk-cache/very-simple-backend
 //
+// The SimpleBackendImpl provides safe iteration; mutating entries during
+// iteration cannot cause a crash. It is undefined whether entries created or
+// destroyed during the iteration will be included in any pre-existing
+// iterations.
+//
 // The non-static functions below must be called on the IO thread unless
 // otherwise stated.
 
@@ -98,17 +102,16 @@ class NET_EXPORT_PRIVATE SimpleBackendImpl : public Backend,
                                  const CompletionCallback& callback) OVERRIDE;
   virtual int DoomEntriesSince(base::Time initial_time,
                                const CompletionCallback& callback) OVERRIDE;
-  virtual int OpenNextEntry(void** iter, Entry** next_entry,
-                            const CompletionCallback& callback) OVERRIDE;
-  virtual void EndEnumeration(void** iter) OVERRIDE;
+  virtual scoped_ptr<Iterator> CreateIterator() OVERRIDE;
   virtual void GetStats(
       std::vector<std::pair<std::string, std::string> >* stats) OVERRIDE;
   virtual void OnExternalCacheHit(const std::string& key) OVERRIDE;
 
  private:
-  typedef base::hash_map<uint64, SimpleEntryImpl*> EntryMap;
+  class SimpleIterator;
+  friend class SimpleIterator;
 
-  typedef IDMap<std::vector<uint64>, IDMapOwnPointer> ActiveEnumerationMap;
+  typedef base::hash_map<uint64, SimpleEntryImpl*> EntryMap;
 
   typedef base::Callback<void(base::Time mtime, uint64 max_size, int result)>
       InitializeIndexCallback;
@@ -124,17 +127,6 @@ class NET_EXPORT_PRIVATE SimpleBackendImpl : public Backend,
     int net_error;
   };
 
-  // Convert an iterator from OpenNextEntry() to the key type for
-  // ActiveEnumerationMap. Note it takes a void** argument; this is for safety;
-  // if it took a void*, that would be type compatible with a void** permitting
-  // easy calls missing the dereference.
-  static ActiveEnumerationMap::KeyType IteratorToEnumerationId(void** iter);
-
-  // Convert a key from ActiveEnumerationMap back to a void*, suitable for
-  // storing in the iterator argument to OpenNextEntry().
-  static void* EnumerationIdToIterator(
-      ActiveEnumerationMap::KeyType enumeration_id);
-
   void InitializeIndex(const CompletionCallback& callback,
                        const DiskStatResult& result);
 
@@ -169,14 +161,6 @@ class NET_EXPORT_PRIVATE SimpleBackendImpl : public Backend,
   // which is very important to prevent races in DoomEntries() above.
   int DoomEntryFromHash(uint64 entry_hash, const CompletionCallback & callback);
 
-  // Called when the index is initilized to find the next entry in the iterator
-  // |iter|. If there are no more hashes in the iterator list, net::ERR_FAILED
-  // is returned. Otherwise, calls OpenEntryFromHash.
-  void GetNextEntryInIterator(void** iter,
-                              Entry** next_entry,
-                              const CompletionCallback& callback,
-                              int error_code);
-
   // Called when we tried to open an entry with hash alone. When a blank entry
   // has been created and filled in with information from the disk - based on a
   // hash alone - this checks that a duplicate active entry was not created
@@ -195,14 +179,6 @@ class NET_EXPORT_PRIVATE SimpleBackendImpl : public Backend,
                             const CompletionCallback& callback,
                             int error_code);
 
-  // Called at the end of the asynchronous operation triggered by
-  // OpenEntryFromHash. Makes sure to continue iterating if the open entry was
-  // not a success.
-  void CheckIterationReturnValue(void** iter,
-                                 Entry** entry,
-                                 const CompletionCallback& callback,
-                                 int error_code);
-
   // A callback thunk used by DoomEntries to clear the |entries_pending_doom_|
   // after a mass doom.
   void DoomEntriesComplete(scoped_ptr<std::vector<uint64> > entry_hashes,
@@ -220,9 +196,6 @@ class NET_EXPORT_PRIVATE SimpleBackendImpl : public Backend,
 
   EntryMap active_entries_;
 
-  // One entry for every enumeration in progress.
-  ActiveEnumerationMap active_enumerations_;
-
   // The set of all entries which are currently being doomed. To avoid races,
   // these entries cannot have Doom/Create/Open operations run until the doom
   // is complete. The base::Closure map target is used to store deferred