[GWP-ASan] Allow late initialisation if single-threaded.

[hctim]

Summary

This patch allows for late initialisation of the GWP-ASan allocator. Previously, if late initialisation occurred, the sample counter was never updated, meaning we would end up having to wait for 2^32 allocations before getting a sampled allocation.

Now, we initialise the sampling mechanism in init() as well. We require init() to be called single-threaded, so this isn't a problem.

[merge_guards_bot]

Unit tests: fail. 62352 tests passed, 1 failed and 839 were skipped.

failed: libc++.std/containers/sequences/array/array_creation/to_array.fail.cpp

clang-tidy: fail. clang-tidy found 2 errors and 1 warnings. 0 of them are added as review comments below (why?).

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

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[eugenis]

LGTM

compiler-rt/lib/gwp_asan/guarded_pool_allocator.cpp

void GuardedPoolAllocator::init(const options::Options &Opts) {
 
   // Multiply the sample rate by 2 to give a good, fast approximation for (1 /
   // SampleRate) chance of sampling.
   if (Opts.SampleRate != 1)
     AdjustedSampleRatePlusOne = static_cast<uint32_t>(Opts.SampleRate) * 2 + 1;
   else
     AdjustedSampleRatePlusOne = 2;
 
+  ThreadLocals.NextSampleCounter =
+      (getRandomUnsigned32() % (AdjustedSampleRatePlusOne - 1)) + 1;
+
   GuardedPagePool = reinterpret_cast<uintptr_t>(GuardedPoolMemory);
   GuardedPagePoolEnd =
       reinterpret_cast<uintptr_t>(GuardedPoolMemory) + PoolBytesRequired;
 
   // Ensure that signal handlers are installed as late as possible, as the class
   // is not thread-safe until init() is finished, and thus a SIGSEGV may cause a
   // race to members if received during init().
   if (Opts.InstallSignalHandlers)

compiler-rt/lib/gwp_asan/tests/CMakeLists.txt

set(GWP_ASAN_UNITTESTS
   basic.cpp
   compression.cpp
   iterate.cpp
   driver.cpp
   mutex_test.cpp
   slot_reuse.cpp
   thread_contention.cpp
   harness.cpp
-  enable_disable.cpp)
+  enable_disable.cpp
+  late_init.cpp)
 
 set(GWP_ASAN_UNIT_TEST_HEADERS
   ${GWP_ASAN_HEADERS}
   harness.h)
 
 add_custom_target(GwpAsanUnitTests)
 set_target_properties(GwpAsanUnitTests PROPERTIES FOLDER "Compiler-RT Tests")
 

compiler-rt/lib/gwp_asan/tests/late_init.cpp

+//===-- late_init.cpp -------------------------------------------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include "gwp_asan/guarded_pool_allocator.h"
+#include "gwp_asan/options.h"
+#include "gtest/gtest.h"
+
+TEST(LateInit, CheckLateInitIsOK) {
+  gwp_asan::GuardedPoolAllocator GPA;
+
+  for (size_t i = 0; i < 0x100; ++i)
+    EXPECT_FALSE(GPA.shouldSample());
+
+  gwp_asan::options::Options Opts;
+  Opts.Enabled = true;
+  Opts.SampleRate = 1;
+
+  GPA.init(Opts);
+  EXPECT_TRUE(GPA.shouldSample());
+}