diff -r 000000000000 -r 6474c204b198 toolkit/components/url-classifier/Classifier.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/components/url-classifier/Classifier.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,761 @@
+//* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "Classifier.h"
+#include "nsIPrefBranch.h"
+#include "nsIPrefService.h"
+#include "nsISimpleEnumerator.h"
+#include "nsIRandomGenerator.h"
+#include "nsIInputStream.h"
+#include "nsISeekableStream.h"
+#include "nsIFile.h"
+#include "nsAutoPtr.h"
+#include "mozilla/Telemetry.h"
+#include "prlog.h"
+
+// NSPR_LOG_MODULES=UrlClassifierDbService:5
+extern PRLogModuleInfo *gUrlClassifierDbServiceLog;
+#if defined(PR_LOGGING)
+#define LOG(args) PR_LOG(gUrlClassifierDbServiceLog, PR_LOG_DEBUG, args)
+#define LOG_ENABLED() PR_LOG_TEST(gUrlClassifierDbServiceLog, 4)
+#else
+#define LOG(args)
+#define LOG_ENABLED() (false)
+#endif
+
+#define STORE_DIRECTORY      NS_LITERAL_CSTRING("safebrowsing")
+#define TO_DELETE_DIR_SUFFIX NS_LITERAL_CSTRING("-to_delete")
+#define BACKUP_DIR_SUFFIX    NS_LITERAL_CSTRING("-backup")
+
+namespace mozilla {
+namespace safebrowsing {
+
+void
+Classifier::SplitTables(const nsACString& str, nsTArray<nsCString>& tables)
+{
+  tables.Clear();
+
+  nsACString::const_iterator begin, iter, end;
+  str.BeginReading(begin);
+  str.EndReading(end);
+  while (begin != end) {
+    iter = begin;
+    FindCharInReadable(',', iter, end);
+    nsDependentCSubstring table = Substring(begin,iter);
+    if (!table.IsEmpty()) {
+      tables.AppendElement(Substring(begin, iter));
+    }
+    begin = iter;
+    if (begin != end) {
+      begin++;
+    }
+  }
+}
+
+Classifier::Classifier()
+  : mFreshTime(45 * 60)
+{
+}
+
+Classifier::~Classifier()
+{
+  Close();
+}
+
+nsresult
+Classifier::SetupPathNames()
+{
+  // Get the root directory where to store all the databases.
+  nsresult rv = mCacheDirectory->Clone(getter_AddRefs(mStoreDirectory));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = mStoreDirectory->AppendNative(STORE_DIRECTORY);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Make sure LookupCaches (which are persistent and survive updates)
+  // are reading/writing in the right place. We will be moving their
+  // files "underneath" them during backup/restore.
+  for (uint32_t i = 0; i < mLookupCaches.Length(); i++) {
+    mLookupCaches[i]->UpdateDirHandle(mStoreDirectory);
+  }
+
+  // Directory where to move a backup before an update.
+  rv = mCacheDirectory->Clone(getter_AddRefs(mBackupDirectory));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = mBackupDirectory->AppendNative(STORE_DIRECTORY + BACKUP_DIR_SUFFIX);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Directory where to move the backup so we can atomically
+  // delete (really move) it.
+  rv = mCacheDirectory->Clone(getter_AddRefs(mToDeleteDirectory));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = mToDeleteDirectory->AppendNative(STORE_DIRECTORY + TO_DELETE_DIR_SUFFIX);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return NS_OK;
+}
+
+nsresult
+Classifier::CreateStoreDirectory()
+{
+  // Ensure the safebrowsing directory exists.
+  bool storeExists;
+  nsresult rv = mStoreDirectory->Exists(&storeExists);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (!storeExists) {
+    rv = mStoreDirectory->Create(nsIFile::DIRECTORY_TYPE, 0755);
+    NS_ENSURE_SUCCESS(rv, rv);
+  } else {
+    bool storeIsDir;
+    rv = mStoreDirectory->IsDirectory(&storeIsDir);
+    NS_ENSURE_SUCCESS(rv, rv);
+    if (!storeIsDir)
+      return NS_ERROR_FILE_DESTINATION_NOT_DIR;
+  }
+
+  return NS_OK;
+}
+
+nsresult
+Classifier::Open(nsIFile& aCacheDirectory)
+{
+  // Remember the Local profile directory.
+  nsresult rv = aCacheDirectory.Clone(getter_AddRefs(mCacheDirectory));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Create the handles to the update and backup directories.
+  rv = SetupPathNames();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Clean up any to-delete directories that haven't been deleted yet.
+  rv = CleanToDelete();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Check whether we have an incomplete update and recover from the
+  // backup if so.
+  rv = RecoverBackups();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Make sure the main store directory exists.
+  rv = CreateStoreDirectory();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  mCryptoHash = do_CreateInstance(NS_CRYPTO_HASH_CONTRACTID, &rv);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Build the list of know urlclassifier lists
+  // XXX: Disk IO potentially on the main thread during startup
+  RegenActiveTables();
+
+  return NS_OK;
+}
+
+void
+Classifier::Close()
+{
+  DropStores();
+}
+
+void
+Classifier::Reset()
+{
+  DropStores();
+
+  mStoreDirectory->Remove(true);
+  mBackupDirectory->Remove(true);
+  mToDeleteDirectory->Remove(true);
+
+  CreateStoreDirectory();
+
+  mTableFreshness.Clear();
+  RegenActiveTables();
+}
+
+void
+Classifier::TableRequest(nsACString& aResult)
+{
+  nsTArray<nsCString> tables;
+  ActiveTables(tables);
+  for (uint32_t i = 0; i < tables.Length(); i++) {
+    nsAutoPtr<HashStore> store(new HashStore(tables[i], mStoreDirectory));
+    if (!store)
+      continue;
+
+    nsresult rv = store->Open();
+    if (NS_FAILED(rv))
+      continue;
+
+    aResult.Append(store->TableName());
+    aResult.Append(";");
+
+    ChunkSet &adds = store->AddChunks();
+    ChunkSet &subs = store->SubChunks();
+
+    if (adds.Length() > 0) {
+      aResult.Append("a:");
+      nsAutoCString addList;
+      adds.Serialize(addList);
+      aResult.Append(addList);
+    }
+
+    if (subs.Length() > 0) {
+      if (adds.Length() > 0)
+        aResult.Append(':');
+      aResult.Append("s:");
+      nsAutoCString subList;
+      subs.Serialize(subList);
+      aResult.Append(subList);
+    }
+
+    aResult.Append('\n');
+  }
+}
+
+nsresult
+Classifier::Check(const nsACString& aSpec,
+                  const nsACString& aTables,
+                  LookupResultArray& aResults)
+{
+  Telemetry::AutoTimer<Telemetry::URLCLASSIFIER_CL_CHECK_TIME> timer;
+
+  // Get the set of fragments based on the url. This is necessary because we
+  // only look up at most 5 URLs per aSpec, even if aSpec has more than 5
+  // components.
+  nsTArray<nsCString> fragments;
+  nsresult rv = LookupCache::GetLookupFragments(aSpec, &fragments);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsTArray<nsCString> activeTables;
+  SplitTables(aTables, activeTables);
+
+  nsTArray<LookupCache*> cacheArray;
+  for (uint32_t i = 0; i < activeTables.Length(); i++) {
+    LOG(("Checking table %s", activeTables[i].get()));
+    LookupCache *cache = GetLookupCache(activeTables[i]);
+    if (cache) {
+      cacheArray.AppendElement(cache);
+    } else {
+      return NS_ERROR_FAILURE;
+    }
+  }
+
+  // Now check each lookup fragment against the entries in the DB.
+  for (uint32_t i = 0; i < fragments.Length(); i++) {
+    Completion lookupHash;
+    lookupHash.FromPlaintext(fragments[i], mCryptoHash);
+
+    // Get list of host keys to look up
+    Completion hostKey;
+    rv = LookupCache::GetKey(fragments[i], &hostKey, mCryptoHash);
+    if (NS_FAILED(rv)) {
+      // Local host on the network.
+      continue;
+    }
+
+#if DEBUG && defined(PR_LOGGING)
+    if (LOG_ENABLED()) {
+      nsAutoCString checking;
+      lookupHash.ToHexString(checking);
+      LOG(("Checking fragment %s, hash %s (%X)", fragments[i].get(),
+           checking.get(), lookupHash.ToUint32()));
+    }
+#endif
+    for (uint32_t i = 0; i < cacheArray.Length(); i++) {
+      LookupCache *cache = cacheArray[i];
+      bool has, complete;
+      rv = cache->Has(lookupHash, &has, &complete);
+      NS_ENSURE_SUCCESS(rv, rv);
+      if (has) {
+        LookupResult *result = aResults.AppendElement();
+        if (!result)
+          return NS_ERROR_OUT_OF_MEMORY;
+
+        int64_t age;
+        bool found = mTableFreshness.Get(cache->TableName(), &age);
+        if (!found) {
+          age = 24 * 60 * 60; // just a large number
+        } else {
+          int64_t now = (PR_Now() / PR_USEC_PER_SEC);
+          age = now - age;
+        }
+
+        LOG(("Found a result in %s: %s (Age: %Lds)",
+             cache->TableName().get(),
+             complete ? "complete." : "Not complete.",
+             age));
+
+        result->hash.complete = lookupHash;
+        result->mComplete = complete;
+        result->mFresh = (age < mFreshTime);
+        result->mTableName.Assign(cache->TableName());
+      }
+    }
+
+  }
+
+  return NS_OK;
+}
+
+nsresult
+Classifier::ApplyUpdates(nsTArray<TableUpdate*>* aUpdates)
+{
+  Telemetry::AutoTimer<Telemetry::URLCLASSIFIER_CL_UPDATE_TIME> timer;
+
+#if defined(PR_LOGGING)
+  PRIntervalTime clockStart = 0;
+  if (LOG_ENABLED() || true) {
+    clockStart = PR_IntervalNow();
+  }
+#endif
+
+  LOG(("Backup before update."));
+
+  nsresult rv = BackupTables();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  LOG(("Applying table updates."));
+
+  for (uint32_t i = 0; i < aUpdates->Length(); i++) {
+    // Previous ApplyTableUpdates() may have consumed this update..
+    if ((*aUpdates)[i]) {
+      // Run all updates for one table
+      nsCString updateTable(aUpdates->ElementAt(i)->TableName());
+      rv = ApplyTableUpdates(aUpdates, updateTable);
+      if (NS_FAILED(rv)) {
+        if (rv != NS_ERROR_OUT_OF_MEMORY) {
+          Reset();
+        }
+        return rv;
+      }
+    }
+  }
+  aUpdates->Clear();
+
+  rv = RegenActiveTables();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  LOG(("Cleaning up backups."));
+
+  // Move the backup directory away (signaling the transaction finished
+  // successfully). This is atomic.
+  rv = RemoveBackupTables();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Do the actual deletion of the backup files.
+  rv = CleanToDelete();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  LOG(("Done applying updates."));
+
+#if defined(PR_LOGGING)
+  if (LOG_ENABLED() || true) {
+    PRIntervalTime clockEnd = PR_IntervalNow();
+    LOG(("update took %dms\n",
+         PR_IntervalToMilliseconds(clockEnd - clockStart)));
+  }
+#endif
+
+  return NS_OK;
+}
+
+nsresult
+Classifier::MarkSpoiled(nsTArray<nsCString>& aTables)
+{
+  for (uint32_t i = 0; i < aTables.Length(); i++) {
+    LOG(("Spoiling table: %s", aTables[i].get()));
+    // Spoil this table by marking it as no known freshness
+    mTableFreshness.Remove(aTables[i]);
+    // Remove any cached Completes for this table
+    LookupCache *cache = GetLookupCache(aTables[i]);
+    if (cache) {
+      cache->ClearCompleteCache();
+    }
+  }
+  return NS_OK;
+}
+
+void
+Classifier::DropStores()
+{
+  for (uint32_t i = 0; i < mHashStores.Length(); i++) {
+    delete mHashStores[i];
+  }
+  mHashStores.Clear();
+  for (uint32_t i = 0; i < mLookupCaches.Length(); i++) {
+    delete mLookupCaches[i];
+  }
+  mLookupCaches.Clear();
+}
+
+nsresult
+Classifier::RegenActiveTables()
+{
+  mActiveTablesCache.Clear();
+
+  nsTArray<nsCString> foundTables;
+  ScanStoreDir(foundTables);
+
+  for (uint32_t i = 0; i < foundTables.Length(); i++) {
+    nsAutoPtr<HashStore> store(new HashStore(nsCString(foundTables[i]), mStoreDirectory));
+    if (!store)
+      return NS_ERROR_OUT_OF_MEMORY;
+
+    nsresult rv = store->Open();
+    if (NS_FAILED(rv))
+      continue;
+
+    LookupCache *lookupCache = GetLookupCache(store->TableName());
+    if (!lookupCache) {
+      continue;
+    }
+
+    if (!lookupCache->IsPrimed())
+      continue;
+
+    const ChunkSet &adds = store->AddChunks();
+    const ChunkSet &subs = store->SubChunks();
+
+    if (adds.Length() == 0 && subs.Length() == 0)
+      continue;
+
+    LOG(("Active table: %s", store->TableName().get()));
+    mActiveTablesCache.AppendElement(store->TableName());
+  }
+
+  return NS_OK;
+}
+
+nsresult
+Classifier::ScanStoreDir(nsTArray<nsCString>& aTables)
+{
+  nsCOMPtr<nsISimpleEnumerator> entries;
+  nsresult rv = mStoreDirectory->GetDirectoryEntries(getter_AddRefs(entries));
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  bool hasMore;
+  while (NS_SUCCEEDED(rv = entries->HasMoreElements(&hasMore)) && hasMore) {
+    nsCOMPtr<nsISupports> supports;
+    rv = entries->GetNext(getter_AddRefs(supports));
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    nsCOMPtr<nsIFile> file = do_QueryInterface(supports);
+
+    nsCString leafName;
+    rv = file->GetNativeLeafName(leafName);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    nsCString suffix(NS_LITERAL_CSTRING(".sbstore"));
+
+    int32_t dot = leafName.RFind(suffix, 0);
+    if (dot != -1) {
+      leafName.Cut(dot, suffix.Length());
+      aTables.AppendElement(leafName);
+    }
+  }
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return NS_OK;
+}
+
+nsresult
+Classifier::ActiveTables(nsTArray<nsCString>& aTables)
+{
+  aTables = mActiveTablesCache;
+  return NS_OK;
+}
+
+nsresult
+Classifier::CleanToDelete()
+{
+  bool exists;
+  nsresult rv = mToDeleteDirectory->Exists(&exists);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (exists) {
+    rv = mToDeleteDirectory->Remove(true);
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  return NS_OK;
+}
+
+nsresult
+Classifier::BackupTables()
+{
+  // We have to work in reverse here: first move the normal directory
+  // away to be the backup directory, then copy the files over
+  // to the normal directory. This ensures that if we crash the backup
+  // dir always has a valid, complete copy, instead of a partial one,
+  // because that's the one we will copy over the normal store dir.
+
+  nsCString backupDirName;
+  nsresult rv = mBackupDirectory->GetNativeLeafName(backupDirName);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  nsCString storeDirName;
+  rv = mStoreDirectory->GetNativeLeafName(storeDirName);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = mStoreDirectory->MoveToNative(nullptr, backupDirName);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = mStoreDirectory->CopyToNative(nullptr, storeDirName);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // We moved some things to new places, so move the handles around, too.
+  rv = SetupPathNames();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return NS_OK;
+}
+
+nsresult
+Classifier::RemoveBackupTables()
+{
+  nsCString toDeleteName;
+  nsresult rv = mToDeleteDirectory->GetNativeLeafName(toDeleteName);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  rv = mBackupDirectory->MoveToNative(nullptr, toDeleteName);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // mBackupDirectory now points to toDelete, fix that up.
+  rv = SetupPathNames();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  return NS_OK;
+}
+
+nsresult
+Classifier::RecoverBackups()
+{
+  bool backupExists;
+  nsresult rv = mBackupDirectory->Exists(&backupExists);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (backupExists) {
+    // Remove the safebrowsing dir if it exists
+    nsCString storeDirName;
+    rv = mStoreDirectory->GetNativeLeafName(storeDirName);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    bool storeExists;
+    rv = mStoreDirectory->Exists(&storeExists);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    if (storeExists) {
+      rv = mStoreDirectory->Remove(true);
+      NS_ENSURE_SUCCESS(rv, rv);
+    }
+
+    // Move the backup to the store location
+    rv = mBackupDirectory->MoveToNative(nullptr, storeDirName);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    // mBackupDirectory now points to storeDir, fix up.
+    rv = SetupPathNames();
+    NS_ENSURE_SUCCESS(rv, rv);
+  }
+
+  return NS_OK;
+}
+
+/*
+ * This will consume+delete updates from the passed nsTArray.
+*/
+nsresult
+Classifier::ApplyTableUpdates(nsTArray<TableUpdate*>* aUpdates,
+                              const nsACString& aTable)
+{
+  LOG(("Classifier::ApplyTableUpdates(%s)", PromiseFlatCString(aTable).get()));
+
+  nsAutoPtr<HashStore> store(new HashStore(aTable, mStoreDirectory));
+
+  if (!store)
+    return NS_ERROR_FAILURE;
+
+  // take the quick exit if there is no valid update for us
+  // (common case)
+  uint32_t validupdates = 0;
+
+  for (uint32_t i = 0; i < aUpdates->Length(); i++) {
+    TableUpdate *update = aUpdates->ElementAt(i);
+    if (!update || !update->TableName().Equals(store->TableName()))
+      continue;
+    if (update->Empty()) {
+      aUpdates->ElementAt(i) = nullptr;
+      delete update;
+      continue;
+    }
+    validupdates++;
+  }
+
+  if (!validupdates) {
+    // This can happen if the update was only valid for one table.
+    return NS_OK;
+  }
+
+  nsresult rv = store->Open();
+  NS_ENSURE_SUCCESS(rv, rv);
+  rv = store->BeginUpdate();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Read the part of the store that is (only) in the cache
+  LookupCache *prefixSet = GetLookupCache(store->TableName());
+  if (!prefixSet) {
+    return NS_ERROR_FAILURE;
+  }
+  nsTArray<uint32_t> AddPrefixHashes;
+  rv = prefixSet->GetPrefixes(&AddPrefixHashes);
+  NS_ENSURE_SUCCESS(rv, rv);
+  rv = store->AugmentAdds(AddPrefixHashes);
+  NS_ENSURE_SUCCESS(rv, rv);
+  AddPrefixHashes.Clear();
+
+  uint32_t applied = 0;
+  bool updateFreshness = false;
+  bool hasCompletes = false;
+
+  for (uint32_t i = 0; i < aUpdates->Length(); i++) {
+    TableUpdate *update = aUpdates->ElementAt(i);
+    if (!update || !update->TableName().Equals(store->TableName()))
+      continue;
+
+    rv = store->ApplyUpdate(*update);
+    NS_ENSURE_SUCCESS(rv, rv);
+
+    applied++;
+
+    LOG(("Applied update to table %s:", store->TableName().get()));
+    LOG(("  %d add chunks", update->AddChunks().Length()));
+    LOG(("  %d add prefixes", update->AddPrefixes().Length()));
+    LOG(("  %d add completions", update->AddCompletes().Length()));
+    LOG(("  %d sub chunks", update->SubChunks().Length()));
+    LOG(("  %d sub prefixes", update->SubPrefixes().Length()));
+    LOG(("  %d sub completions", update->SubCompletes().Length()));
+    LOG(("  %d add expirations", update->AddExpirations().Length()));
+    LOG(("  %d sub expirations", update->SubExpirations().Length()));
+
+    if (!update->IsLocalUpdate()) {
+      updateFreshness = true;
+      LOG(("Remote update, updating freshness"));
+    }
+
+    if (update->AddCompletes().Length() > 0
+        || update->SubCompletes().Length() > 0) {
+      hasCompletes = true;
+      LOG(("Contains Completes, keeping cache."));
+    }
+
+    aUpdates->ElementAt(i) = nullptr;
+    delete update;
+  }
+
+  LOG(("Applied %d update(s) to %s.", applied, store->TableName().get()));
+
+  rv = store->Rebuild();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // Not an update with Completes, clear all completes data.
+  if (!hasCompletes) {
+    store->ClearCompletes();
+  }
+
+  LOG(("Table %s now has:", store->TableName().get()));
+  LOG(("  %d add chunks", store->AddChunks().Length()));
+  LOG(("  %d add prefixes", store->AddPrefixes().Length()));
+  LOG(("  %d add completions", store->AddCompletes().Length()));
+  LOG(("  %d sub chunks", store->SubChunks().Length()));
+  LOG(("  %d sub prefixes", store->SubPrefixes().Length()));
+  LOG(("  %d sub completions", store->SubCompletes().Length()));
+
+  rv = store->WriteFile();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  // At this point the store is updated and written out to disk, but
+  // the data is still in memory.  Build our quick-lookup table here.
+  rv = prefixSet->Build(store->AddPrefixes(), store->AddCompletes());
+  NS_ENSURE_SUCCESS(rv, rv);
+
+#if defined(DEBUG) && defined(PR_LOGGING)
+  prefixSet->Dump();
+#endif
+  rv = prefixSet->WriteFile();
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (updateFreshness) {
+    int64_t now = (PR_Now() / PR_USEC_PER_SEC);
+    LOG(("Successfully updated %s", store->TableName().get()));
+    mTableFreshness.Put(store->TableName(), now);
+  }
+
+  return NS_OK;
+}
+
+LookupCache *
+Classifier::GetLookupCache(const nsACString& aTable)
+{
+  for (uint32_t i = 0; i < mLookupCaches.Length(); i++) {
+    if (mLookupCaches[i]->TableName().Equals(aTable)) {
+      return mLookupCaches[i];
+    }
+  }
+
+  LookupCache *cache = new LookupCache(aTable, mStoreDirectory);
+  nsresult rv = cache->Init();
+  if (NS_FAILED(rv)) {
+    return nullptr;
+  }
+  rv = cache->Open();
+  if (NS_FAILED(rv)) {
+    if (rv == NS_ERROR_FILE_CORRUPTED) {
+      Reset();
+    }
+    return nullptr;
+  }
+  mLookupCaches.AppendElement(cache);
+  return cache;
+}
+
+nsresult
+Classifier::ReadNoiseEntries(const Prefix& aPrefix,
+                             const nsACString& aTableName,
+                             uint32_t aCount,
+                             PrefixArray* aNoiseEntries)
+{
+  LookupCache *cache = GetLookupCache(aTableName);
+  if (!cache) {
+    return NS_ERROR_FAILURE;
+  }
+
+  nsTArray<uint32_t> prefixes;
+  nsresult rv = cache->GetPrefixes(&prefixes);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  uint32_t idx = prefixes.BinaryIndexOf(aPrefix.ToUint32());
+
+  if (idx == nsTArray<uint32_t>::NoIndex) {
+    NS_WARNING("Could not find prefix in PrefixSet during noise lookup");
+    return NS_ERROR_FAILURE;
+  }
+
+  idx -= idx % aCount;
+
+  for (uint32_t i = 0; (i < aCount) && ((idx+i) < prefixes.Length()); i++) {
+    Prefix newPref;
+    newPref.FromUint32(prefixes[idx+i]);
+    if (newPref != aPrefix) {
+      aNoiseEntries->AppendElement(newPref);
+    }
+  }
+
+  return NS_OK;
+}
+
+} // namespace safebrowsing
+} // namespace mozilla