Make base::FastHash non-deterministic in DCHECK_IS_ON() builds.

This is a prerequisite for migrating more things over to base::FastHash,
to ensure they don't inadverdently have a dependency on a stable mapping
of inputs to hash values.

Also change the FastHash(const std::string&) overload to
FastHash(StringPiece) to avoid potentially constructing unnecessary
temporaries.

Bug: 1025358
Change-Id: I2e3d67ab74f869437ae2bdf12c9560f1be19688d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919804Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#715940}

base/hash/hash.cc

@@ -4,6 +4,7 @@
 
 #include "base/hash/hash.h"
 
+#include "base/rand_util.h"
 #include "base/third_party/cityhash/city.h"
 #include "build/build_config.h"
 
@@ -17,7 +18,11 @@ namespace base {
 size_t FastHash(base::span<const uint8_t> data) {
   // We use the updated CityHash within our namespace (not the deprecated
   // version from third_party/smhasher).
-#if defined(ARCH_CPU_64_BITS)
+#if DCHECK_IS_ON()
+  static const uint64_t seed = base::RandUint64();
+  return base::internal::cityhash_v111::CityHash64WithSeed(
+      reinterpret_cast<const char*>(data.data()), data.size(), seed);
+#elif defined(ARCH_CPU_64_BITS)
   return base::internal::cityhash_v111::CityHash64(
       reinterpret_cast<const char*>(data.data()), data.size());
 #else
@@ -26,14 +31,6 @@ size_t FastHash(base::span<const uint8_t> data) {
 #endif
 }
 
-size_t FastHash(const std::string& str) {
-#if defined(ARCH_CPU_64_BITS)
-  return base::internal::cityhash_v111::CityHash64(str.data(), str.size());
-#else
-  return base::internal::cityhash_v111::CityHash32(str.data(), str.size());
-#endif
-}
-
 uint32_t Hash(const void* data, size_t length) {
   // Currently our in-memory hash is the same as the persistent hash. The
   // split between in-memory and persistent hash functions is maintained to

base/hash/hash.h

@@ -16,6 +16,7 @@
 #include "base/containers/span.h"
 #include "base/logging.h"
 #include "base/strings/string16.h"
+#include "base/strings/string_piece.h"
 
 namespace base {
 
@@ -25,7 +26,7 @@ namespace base {
 // Deprecated: Computes a hash of a memory buffer, use FastHash() instead.
 // If you need to persist a change on disk or between computers, use
 // PersistentHash().
-// TODO(cavalcantii): Migrate client code to new hash function.
+// TODO(https://crbug.com/1025358): Migrate client code to new hash function.
 BASE_EXPORT uint32_t Hash(const void* data, size_t length);
 BASE_EXPORT uint32_t Hash(const std::string& str);
 BASE_EXPORT uint32_t Hash(const string16& str);
@@ -37,7 +38,9 @@ BASE_EXPORT uint32_t Hash(const string16& str);
 // publicly available.
 // May changed without warning, do not expect stability of outputs.
 BASE_EXPORT size_t FastHash(base::span<const uint8_t> data);
-BASE_EXPORT size_t FastHash(const std::string& str);
+inline size_t FastHash(StringPiece str) {
+  return FastHash(as_bytes(make_span(str)));
+}
 
 // Computes a hash of a memory buffer. This hash function must not change so
 // that code can use the hashed values for persistent storage purposes or

base/hash/hash_unittest.cc

@@ -79,4 +79,10 @@ TEST(HashTest, CString) {
   EXPECT_EQ(2794219650u, Hash(str, strlen("hello world")));
 }
 
+TEST(HashTest, FastHash) {
+  std::string s;
+  constexpr char kEmptyString[] = "";
+  EXPECT_EQ(FastHash(s), FastHash(kEmptyString));
+}
+
 }  // namespace base