Reduce the false positive rate for SafeBrowsing gethash requests

chrome/browser/safe_browsing/safe_browsing_database_unittest.cc

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 //
 // Unit tests for the SafeBrowsing storage system.
 
 #include "base/file_path.h"
 #include "base/file_util.h"
 #include "base/logging.h"
 #include "base/path_service.h"
@@ skipping 887 matching lines @@
 
   // Prefixes with no full results are misses.
   EXPECT_EQ(database->prefix_miss_cache()->size(), 2U);
 
   // Update the database.
   PopulateDatabaseForCacheTest(database);
 
   // Prefix miss cache should be cleared.
   EXPECT_EQ(database->prefix_miss_cache()->size(), 0U);
 
+  // Cache a GetHash miss for a particular prefix, and even though the prefix is
+  // in the database, it is flagged as a miss so looking up the associated URL
+  // will not succeed.
+  prefixes.clear();
+  full_hashes.clear();
+  prefix_misses.clear();
+  empty_full_hash.clear();
+  prefix_misses.push_back(Sha256Prefix("www.evil.com/phishing.html"));
+  database->CacheHashResults(prefix_misses, empty_full_hash);
+  EXPECT_FALSE(database->ContainsUrl(GURL("http://www.evil.com/phishing.html"),
+                                     &listname, &prefixes,
+                                     &full_hashes, Time::Now()));
+
   lists.clear();
   prefixes.clear();
   full_hashes.clear();
 
   // Test receiving a full add chunk.
   host.host = Sha256Prefix("www.fullevil.com/");
   host.entry = SBEntry::Create(SBEntry::ADD_FULL_HASH, 2);
   host.entry->set_chunk_id(20);
   SBFullHash full_add1;
   base::SHA256HashString("www.fullevil.com/bad1.html",
@@ skipping 310 matching lines @@
 TEST(SafeBrowsingDatabase, DISABLED_DatabaseOldLotsofDeletesIO) {
   std::vector<ChunksInfo> chunks;
   std::vector<SBChunkDelete>* deletes = new std::vector<SBChunkDelete>;
   SBChunkDelete del;
   del.is_sub_del = false;
   del.list_name = safe_browsing_util::kMalwareList;
   del.chunk_del.push_back(ChunkRange(3539, 3579));
   deletes->push_back(del);
   PeformUpdate(GetOldSafeBrowsingPath(), chunks, deletes);
 }