Export of internal Abseil changes

--
074a799119ac881b8b8ce59ef7a3166d1aa025ac by Tom Manshreck <shreck@google.com>:

nit: Add return info for StrCat

PiperOrigin-RevId: 278647298

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d58a2a39ab6f50266cc695506ba2e86bdb45d795 by Mark Barolak <mbar@google.com>:

Stop suppressing no-nested-anon-types warnings because there aren't actually any warnings to suppress.

PiperOrigin-RevId: 278440548

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445051bd280b9a6f608a8c80b3d7cafcc1377a03 by Abseil Team <absl-team@google.com>:

ResetThreadIdentity does not need to clear identity->waiter_state.

ResetThreadIdentity is only called by NewThreadIdentity. NewThreadIdentity is
only called by CreateThreadIdentity. CreateThreadIdentity calls
PerThreadSem::Init, which initializes identity->waiter_state, immediately after
calling NewThreadIdentity. Therefore ResetThreadIdentity does not need to clear
identity->waiter_state.
PiperOrigin-RevId: 278429844

--
c2079b664d92be40d5e365abcca4e9b3505a75a6 by Abseil Team <absl-team@google.com>:

Delete the f->header.magic check in LowLevelAlloc::Free().

The f->header.magic check in LowLevelAlloc::Free() is redundant, because
AddToFreeList() will immediately perform the same check.

Also fix a typo in the comment that documents the lock requirements for
Next(). The comment should say "L >= arena->mu", which is equivalent to
EXCLUSIVE_LOCKS_REQUIRED(arena->mu).

NOTE: LowLevelAlloc::Free() performs the f->header.magic check without
holding the arena lock. This may have caused the TSAN data race warning
reported in bug 143697235.
PiperOrigin-RevId: 278414140

--
5534f35ce677165700117d868f51607ed1f0d73b by Greg Falcon <gfalcon@google.com>:

Add an internal (unsupported) PiecewiseCombiner class to allow hashing buffers piecewise.

PiperOrigin-RevId: 278388902
GitOrigin-RevId: 074a799119ac881b8b8ce59ef7a3166d1aa025ac
Change-Id: I61734850cbbb01c7585e8c736a5bb56e416512a8

absl/hash/hash_test.cc

@@ -274,8 +274,8 @@ TEST(HashValueTest, Strings) {
 
   const std::string small = "foo";
   const std::string dup = "foofoo";
-  const std::string large = "large";
-  const std::string huge = std::string(5000, 'a');
+  const std::string large = std::string(2048, 'x');  // multiple of chunk size
+  const std::string huge = std::string(5000, 'a');   // not a multiple
 
   EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly(std::make_tuple(
       std::string(), absl::string_view(),
@@ -378,6 +378,116 @@ struct Private {
   }
 };
 
+// Test helper for combine_piecewise_buffer.  It holds a string_view to the
+// buffer-to-be-hashed.  Its AbslHashValue specialization will split up its
+// contents at the character offsets requested.
+class PiecewiseHashTester {
+ public:
+  // Create a hash view of a buffer to be hashed contiguously.
+  explicit PiecewiseHashTester(absl::string_view buf)
+      : buf_(buf), piecewise_(false), split_locations_() {}
+
+  // Create a hash view of a buffer to be hashed piecewise, with breaks at the
+  // given locations.
+  PiecewiseHashTester(absl::string_view buf, std::set<size_t> split_locations)
+      : buf_(buf),
+        piecewise_(true),
+        split_locations_(std::move(split_locations)) {}
+
+  template <typename H>
+  friend H AbslHashValue(H h, const PiecewiseHashTester& p) {
+    if (!p.piecewise_) {
+      return H::combine_contiguous(std::move(h), p.buf_.data(), p.buf_.size());
+    }
+    absl::hash_internal::PiecewiseCombiner combiner;
+    if (p.split_locations_.empty()) {
+      h = combiner.add_buffer(std::move(h), p.buf_.data(), p.buf_.size());
+      return combiner.finalize(std::move(h));
+    }
+    size_t begin = 0;
+    for (size_t next : p.split_locations_) {
+      absl::string_view chunk = p.buf_.substr(begin, next - begin);
+      h = combiner.add_buffer(std::move(h), chunk.data(), chunk.size());
+      begin = next;
+    }
+    absl::string_view last_chunk = p.buf_.substr(begin);
+    if (!last_chunk.empty()) {
+      h = combiner.add_buffer(std::move(h), last_chunk.data(),
+                              last_chunk.size());
+    }
+    return combiner.finalize(std::move(h));
+  }
+
+ private:
+  absl::string_view buf_;
+  bool piecewise_;
+  std::set<size_t> split_locations_;
+};
+
+// Dummy object that hashes as two distinct contiguous buffers, "foo" followed
+// by "bar"
+struct DummyFooBar {
+  template <typename H>
+  friend H AbslHashValue(H h, const DummyFooBar&) {
+    const char* foo = "foo";
+    const char* bar = "bar";
+    h = H::combine_contiguous(std::move(h), foo, 3);
+    h = H::combine_contiguous(std::move(h), bar, 3);
+    return std::move(h);
+  }
+};
+
+TEST(HashValueTest, CombinePiecewiseBuffer) {
+  absl::Hash<PiecewiseHashTester> hash;
+
+  // Check that hashing an empty buffer through the piecewise API works.
+  EXPECT_EQ(hash(PiecewiseHashTester("")), hash(PiecewiseHashTester("", {})));
+
+  // Similarly, small buffers should give consistent results
+  EXPECT_EQ(hash(PiecewiseHashTester("foobar")),
+            hash(PiecewiseHashTester("foobar", {})));
+  EXPECT_EQ(hash(PiecewiseHashTester("foobar")),
+            hash(PiecewiseHashTester("foobar", {3})));
+
+  // But hashing "foobar" in pieces gives a different answer than hashing "foo"
+  // contiguously, then "bar" contiguously.
+  EXPECT_NE(hash(PiecewiseHashTester("foobar", {3})),
+            absl::Hash<DummyFooBar>()(DummyFooBar{}));
+
+  // Test hashing a large buffer incrementally, broken up in several different
+  // ways.  Arrange for breaks on and near the stride boundaries to look for
+  // off-by-one errors in the implementation.
+  //
+  // This test is run on a buffer that is a multiple of the stride size, and one
+  // that isn't.
+  for (size_t big_buffer_size : {1024 * 2 + 512, 1024 * 3}) {
+    SCOPED_TRACE(big_buffer_size);
+    std::string big_buffer;
+    for (int i = 0; i < big_buffer_size; ++i) {
+      // Arbitrary std::string
+      big_buffer.push_back(32 + (i * (i / 3)) % 64);
+    }
+    auto big_buffer_hash = hash(PiecewiseHashTester(big_buffer));
+
+    const int possible_breaks = 9;
+    size_t breaks[possible_breaks] = {1,    512,  1023, 1024, 1025,
+                                      1536, 2047, 2048, 2049};
+    for (unsigned test_mask = 0; test_mask < (1u << possible_breaks);
+         ++test_mask) {
+      SCOPED_TRACE(test_mask);
+      std::set<size_t> break_locations;
+      for (int j = 0; j < possible_breaks; ++j) {
+        if (test_mask & (1u << j)) {
+          break_locations.insert(breaks[j]);
+        }
+      }
+      EXPECT_EQ(
+          hash(PiecewiseHashTester(big_buffer, std::move(break_locations))),
+          big_buffer_hash);
+    }
+  }
+}
+
 TEST(HashValueTest, PrivateSanity) {
   // Sanity check that Private is working as the tests below expect it to work.
   EXPECT_TRUE(is_hashable<Private>::value);