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diff --git a/xpcom/tests/gtest/TestPLDHash.cpp b/xpcom/tests/gtest/TestPLDHash.cpp
new file mode 100644
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+++ b/xpcom/tests/gtest/TestPLDHash.cpp
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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 "PLDHashTable.h"
+#include "nsCOMPtr.h"
+#include "nsICrashReporter.h"
+#include "nsServiceManagerUtils.h"
+#include "gtest/gtest.h"
+
+// This test mostly focuses on edge cases. But more coverage of normal
+// operations wouldn't be a bad thing.
+
+#ifdef XP_UNIX
+#  include <unistd.h>
+#  include <sys/types.h>
+#  include <sys/wait.h>
+
+// This global variable is defined in toolkit/xre/nsSigHandlers.cpp.
+extern unsigned int _gdb_sleep_duration;
+#endif
+
+// We can test that certain operations cause expected aborts by forking
+// and then checking that the child aborted in the expected way (i.e. via
+// MOZ_CRASH). We skip this for the following configurations.
+// - On Windows, because it doesn't have fork().
+// - On non-DEBUG builds, because the crashes cause the crash reporter to pop
+//   up when running this test locally, which is surprising and annoying.
+// - On ASAN builds, because ASAN alters the way a MOZ_CRASHing process
+//   terminates, which makes it harder to test if the right thing has occurred.
+static void TestCrashyOperation(const char* label, void (*aCrashyOperation)()) {
+#if defined(XP_UNIX) && defined(DEBUG) && !defined(MOZ_ASAN)
+  // We're about to trigger a crash. When it happens don't pause to allow GDB
+  // to be attached.
+  unsigned int old_gdb_sleep_duration = _gdb_sleep_duration;
+  _gdb_sleep_duration = 0;
+
+  int pid = fork();
+  ASSERT_NE(pid, -1);
+
+  if (pid == 0) {
+    // Disable the crashreporter -- writing a crash dump in the child will
+    // prevent the parent from writing a subsequent dump. Crashes here are
+    // expected, so we don't want their stacks to show up in the log anyway.
+    nsCOMPtr<nsICrashReporter> crashreporter =
+        do_GetService("@mozilla.org/toolkit/crash-reporter;1");
+    if (crashreporter) {
+      crashreporter->SetEnabled(false);
+    }
+
+    // Child: perform the crashy operation.
+    fprintf(stderr,
+            "TestCrashyOperation %s: The following crash is expected. Do not "
+            "panic.\n",
+            label);
+    aCrashyOperation();
+    fprintf(stderr, "TestCrashyOperation %s: didn't crash?!\n", label);
+    ASSERT_TRUE(false);  // shouldn't reach here
+  }
+
+  // Parent: check that child crashed as expected.
+  int status;
+  ASSERT_NE(waitpid(pid, &status, 0), -1);
+
+  // The path taken here depends on the platform and configuration.
+  ASSERT_TRUE(WIFEXITED(status) || WTERMSIG(status));
+  if (WIFEXITED(status)) {
+    // This occurs if the ah_crap_handler() is run, i.e. we caught the crash.
+    // It returns the number of the caught signal.
+    int signum = WEXITSTATUS(status);
+    if (signum != SIGSEGV && signum != SIGBUS) {
+      fprintf(stderr, "TestCrashyOperation %s: 'exited' failure: %d\n", label,
+              signum);
+      ASSERT_TRUE(false);
+    }
+  } else if (WIFSIGNALED(status)) {
+    // This one occurs if we didn't catch the crash. The exit code is the
+    // number of the terminating signal.
+    int signum = WTERMSIG(status);
+    if (signum != SIGSEGV && signum != SIGBUS) {
+      fprintf(stderr, "TestCrashyOperation %s: 'signaled' failure: %d\n", label,
+              signum);
+      ASSERT_TRUE(false);
+    }
+  }
+
+  _gdb_sleep_duration = old_gdb_sleep_duration;
+#endif
+}
+
+static void InitCapacityOk_InitialLengthTooBig() {
+  PLDHashTable t(PLDHashTable::StubOps(), sizeof(PLDHashEntryStub),
+                 PLDHashTable::kMaxInitialLength + 1);
+}
+
+static void InitCapacityOk_InitialEntryStoreTooBig() {
+  // Try the smallest disallowed power-of-two entry store size, which is 2^32
+  // bytes (which overflows to 0). (Note that the 2^23 *length* gets converted
+  // to a 2^24 *capacity*.)
+  PLDHashTable t(PLDHashTable::StubOps(), (uint32_t)1 << 8, (uint32_t)1 << 23);
+}
+
+static void InitCapacityOk_EntrySizeTooBig() {
+  // Try the smallest disallowed entry size, which is 256 bytes.
+  PLDHashTable t(PLDHashTable::StubOps(), 256);
+}
+
+TEST(PLDHashTableTest, InitCapacityOk)
+{
+  // Try the largest allowed capacity.  With kMaxCapacity==1<<26, this
+  // would allocate (if we added an element) 0.5GB of entry store on 32-bit
+  // platforms and 1GB on 64-bit platforms.
+  PLDHashTable t1(PLDHashTable::StubOps(), sizeof(PLDHashEntryStub),
+                  PLDHashTable::kMaxInitialLength);
+
+  // Try the largest allowed power-of-two entry store size, which is 2^31 bytes
+  // (Note that the 2^23 *length* gets converted to a 2^24 *capacity*.)
+  PLDHashTable t2(PLDHashTable::StubOps(), (uint32_t)1 << 7, (uint32_t)1 << 23);
+
+  // Try a too-large capacity (which aborts).
+  TestCrashyOperation("length too big", InitCapacityOk_InitialLengthTooBig);
+
+  // Try a large capacity combined with a large entry size that when multiplied
+  // overflow (causing abort).
+  TestCrashyOperation("entry store too big",
+                      InitCapacityOk_InitialEntryStoreTooBig);
+
+  // Try the largest allowed entry size.
+  PLDHashTable t3(PLDHashTable::StubOps(), 255);
+
+  // Try an overly large entry size.
+  TestCrashyOperation("entry size too big", InitCapacityOk_EntrySizeTooBig);
+
+  // Ideally we'd also try a large-but-ok capacity that almost but doesn't
+  // quite overflow, but that would result in allocating slightly less than 4
+  // GiB of entry storage. That would be very likely to fail on 32-bit
+  // platforms, so such a test wouldn't be reliable.
+}
+
+TEST(PLDHashTableTest, LazyStorage)
+{
+  PLDHashTable t(PLDHashTable::StubOps(), sizeof(PLDHashEntryStub));
+
+  // PLDHashTable allocates entry storage lazily. Check that all the non-add
+  // operations work appropriately when the table is empty and the storage
+  // hasn't yet been allocated.
+
+  ASSERT_EQ(t.Capacity(), 0u);
+  ASSERT_EQ(t.EntrySize(), sizeof(PLDHashEntryStub));
+  ASSERT_EQ(t.EntryCount(), 0u);
+  ASSERT_EQ(t.Generation(), 0u);
+
+  ASSERT_TRUE(!t.Search((const void*)1));
+
+  // No result to check here, but call it to make sure it doesn't crash.
+  t.Remove((const void*)2);
+
+  for (auto iter = t.Iter(); !iter.Done(); iter.Next()) {
+    ASSERT_TRUE(false);  // shouldn't hit this on an empty table
+  }
+
+  ASSERT_EQ(t.ShallowSizeOfExcludingThis(moz_malloc_size_of), 0u);
+}
+
+// A trivial hash function is good enough here. It's also super-fast for the
+// GrowToMaxCapacity test because we insert the integers 0.., which means it's
+// collision-free.
+static PLDHashNumber TrivialHash(const void* key) {
+  return (PLDHashNumber)(size_t)key;
+}
+
+static void TrivialInitEntry(PLDHashEntryHdr* aEntry, const void* aKey) {
+  auto entry = static_cast<PLDHashEntryStub*>(aEntry);
+  entry->key = aKey;
+}
+
+static const PLDHashTableOps trivialOps = {
+    TrivialHash, PLDHashTable::MatchEntryStub, PLDHashTable::MoveEntryStub,
+    PLDHashTable::ClearEntryStub, TrivialInitEntry};
+
+TEST(PLDHashTableTest, MoveSemantics)
+{
+  PLDHashTable t1(&trivialOps, sizeof(PLDHashEntryStub));
+  t1.Add((const void*)88);
+  PLDHashTable t2(&trivialOps, sizeof(PLDHashEntryStub));
+  t2.Add((const void*)99);
+
+#if defined(__clang__)
+#  pragma clang diagnostic push
+#  pragma clang diagnostic ignored "-Wself-move"
+#endif
+  t1 = std::move(t1);  // self-move
+#if defined(__clang__)
+#  pragma clang diagnostic pop
+#endif
+
+  t1 = std::move(t2);  // empty overwritten with empty
+
+  PLDHashTable t3(&trivialOps, sizeof(PLDHashEntryStub));
+  PLDHashTable t4(&trivialOps, sizeof(PLDHashEntryStub));
+  t3.Add((const void*)88);
+
+  t3 = std::move(t4);  // non-empty overwritten with empty
+
+  PLDHashTable t5(&trivialOps, sizeof(PLDHashEntryStub));
+  PLDHashTable t6(&trivialOps, sizeof(PLDHashEntryStub));
+  t6.Add((const void*)88);
+
+  t5 = std::move(t6);  // empty overwritten with non-empty
+
+  PLDHashTable t7(&trivialOps, sizeof(PLDHashEntryStub));
+  PLDHashTable t8(std::move(t7));  // new table constructed with uninited
+
+  PLDHashTable t9(&trivialOps, sizeof(PLDHashEntryStub));
+  t9.Add((const void*)88);
+  PLDHashTable t10(std::move(t9));  // new table constructed with inited
+}
+
+TEST(PLDHashTableTest, Clear)
+{
+  PLDHashTable t1(&trivialOps, sizeof(PLDHashEntryStub));
+
+  t1.Clear();
+  ASSERT_EQ(t1.EntryCount(), 0u);
+
+  t1.ClearAndPrepareForLength(100);
+  ASSERT_EQ(t1.EntryCount(), 0u);
+
+  t1.Add((const void*)77);
+  t1.Add((const void*)88);
+  t1.Add((const void*)99);
+  ASSERT_EQ(t1.EntryCount(), 3u);
+
+  t1.Clear();
+  ASSERT_EQ(t1.EntryCount(), 0u);
+
+  t1.Add((const void*)55);
+  t1.Add((const void*)66);
+  t1.Add((const void*)77);
+  t1.Add((const void*)88);
+  t1.Add((const void*)99);
+  ASSERT_EQ(t1.EntryCount(), 5u);
+
+  t1.ClearAndPrepareForLength(8192);
+  ASSERT_EQ(t1.EntryCount(), 0u);
+}
+
+TEST(PLDHashTableTest, Iterator)
+{
+  PLDHashTable t(&trivialOps, sizeof(PLDHashEntryStub));
+
+  // Explicitly test the move constructor. We do this because, due to copy
+  // elision, compilers might optimize away move constructor calls for normal
+  // iterator use.
+  {
+    PLDHashTable::Iterator iter1(&t);
+    PLDHashTable::Iterator iter2(std::move(iter1));
+  }
+
+  // Iterate through the empty table.
+  for (PLDHashTable::Iterator iter(&t); !iter.Done(); iter.Next()) {
+    (void)iter.Get();
+    ASSERT_TRUE(false);  // shouldn't hit this
+  }
+
+  // Add three entries.
+  t.Add((const void*)77);
+  t.Add((const void*)88);
+  t.Add((const void*)99);
+
+  // Check the iterator goes through each entry once.
+  bool saw77 = false, saw88 = false, saw99 = false;
+  int n = 0;
+  for (auto iter(t.Iter()); !iter.Done(); iter.Next()) {
+    auto entry = static_cast<PLDHashEntryStub*>(iter.Get());
+    if (entry->key == (const void*)77) {
+      saw77 = true;
+    }
+    if (entry->key == (const void*)88) {
+      saw88 = true;
+    }
+    if (entry->key == (const void*)99) {
+      saw99 = true;
+    }
+    n++;
+  }
+  ASSERT_TRUE(saw77 && saw88 && saw99 && n == 3);
+
+  t.Clear();
+
+  // First, we insert 64 items, which results in a capacity of 128, and a load
+  // factor of 50%.
+  for (intptr_t i = 0; i < 64; i++) {
+    t.Add((const void*)i);
+  }
+  ASSERT_EQ(t.EntryCount(), 64u);
+  ASSERT_EQ(t.Capacity(), 128u);
+
+  // The first removing iterator does no removing; capacity and entry count are
+  // unchanged.
+  for (PLDHashTable::Iterator iter(&t); !iter.Done(); iter.Next()) {
+    (void)iter.Get();
+  }
+  ASSERT_EQ(t.EntryCount(), 64u);
+  ASSERT_EQ(t.Capacity(), 128u);
+
+  // The second removing iterator removes 16 items. This reduces the load
+  // factor to 37.5% (48 / 128), which isn't low enough to shrink the table.
+  for (auto iter = t.Iter(); !iter.Done(); iter.Next()) {
+    auto entry = static_cast<PLDHashEntryStub*>(iter.Get());
+    if ((intptr_t)(entry->key) % 4 == 0) {
+      iter.Remove();
+    }
+  }
+  ASSERT_EQ(t.EntryCount(), 48u);
+  ASSERT_EQ(t.Capacity(), 128u);
+
+  // The third removing iterator removes another 16 items. This reduces
+  // the load factor to 25% (32 / 128), so the table is shrunk.
+  for (auto iter = t.Iter(); !iter.Done(); iter.Next()) {
+    auto entry = static_cast<PLDHashEntryStub*>(iter.Get());
+    if ((intptr_t)(entry->key) % 2 == 0) {
+      iter.Remove();
+    }
+  }
+  ASSERT_EQ(t.EntryCount(), 32u);
+  ASSERT_EQ(t.Capacity(), 64u);
+
+  // The fourth removing iterator removes all remaining items. This reduces
+  // the capacity to the minimum.
+  for (auto iter = t.Iter(); !iter.Done(); iter.Next()) {
+    iter.Remove();
+  }
+  ASSERT_EQ(t.EntryCount(), 0u);
+  ASSERT_EQ(t.Capacity(), unsigned(PLDHashTable::kMinCapacity));
+}
+
+// This test involves resizing a table repeatedly up to 512 MiB in size. On
+// 32-bit platforms (Win32, Android) it sometimes OOMs, causing the test to
+// fail. (See bug 931062 and bug 1267227.) Therefore, we only run it on 64-bit
+// platforms where OOM is much less likely.
+//
+// Also, it's slow, and so should always be last.
+#ifdef HAVE_64BIT_BUILD
+TEST(PLDHashTableTest, GrowToMaxCapacity)
+{
+  // This is infallible.
+  PLDHashTable* t =
+      new PLDHashTable(&trivialOps, sizeof(PLDHashEntryStub), 128);
+
+  // Keep inserting elements until failure occurs because the table is full.
+  size_t numInserted = 0;
+  while (true) {
+    if (!t->Add((const void*)numInserted, mozilla::fallible)) {
+      break;
+    }
+    numInserted++;
+  }
+
+  // We stop when the element count is 96.875% of PLDHashTable::kMaxCapacity
+  // (see MaxLoadOnGrowthFailure()).
+  if (numInserted !=
+      PLDHashTable::kMaxCapacity - (PLDHashTable::kMaxCapacity >> 5)) {
+    delete t;
+    ASSERT_TRUE(false);
+  }
+
+  delete t;
+}
+#endif