Export of internal Abseil changes.

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636137f6f0de910691a3950387fefacfa4909fb8 by Abseil Team <absl-team@google.com>:

Add move semantics to absl::container_internal::CompressedTuple

PiperOrigin-RevId: 225394165

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43da91e4f95a196b2e6b76f1c2f4158817b0ebb0 by Greg Falcon <gfalcon@google.com>:

Add a constructor to allow for global absl::Mutex instances.

This adds a new constexpr constructor to absl::Mutex, invoked with the absl::kConstInit tag value, which is intended to be used to construct Mutex instances with static storage duration.

What's tricky about is absl::Mutex (like std::mutex) is not a trivially destructible class, so by the letter of the law, accessing a global Mutex instance after it is destroyed results in undefined behavior.  Despite this, we take care in the destructor to not invalidate the memory layout of the Mutex.  Using a kConstInit-constructed global Mutex after it is destroyed happens to work on the toolchains we use.  Google relies heavily on this behavior internally.

Code sanitizers that detect undefined behavior are able to notice use-after-free of globals, and might complain about this pattern.

PiperOrigin-RevId: 225389447

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7b553a54bc6460cc7008b028552e66799475ca64 by Abseil Team <absl-team@google.com>:

Internal change.

PiperOrigin-RevId: 225373389

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fd0c722d217b3b509102274765ccb1a0b596cf46 by Abseil Team <absl-team@google.com>:

Update absl/time/CMakeLists.txt to use new functions
i.e. absl_cc_(library|test)

PiperOrigin-RevId: 225246853

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9f8f3ba3b67a6d1ac4ecdc529c8b8eb0f02576d9 by Abseil Team <absl-team@google.com>:

Update absl/synchronisation/CMakeLists.txt to use new functions
i.e. absl_cc_(library|test)

PiperOrigin-RevId: 225237980

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a3fdd67dad2e596f804f5e100c8d3a74d8064faa by Abseil Team <absl-team@google.com>:

Internal cleanup

PiperOrigin-RevId: 225226813

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48fab23fb8cdca45e95da14fce0de56614d09c25 by Jon Cohen <cohenjon@google.com>:

Use a shim #define for wchar_t in msvc in int128.

On ancient versions of msvc and with some compatibility flags on wchar_t is a typedef for unsigned short, whereas on standards-conforming versions wchar_t is a typedef for __wchar_t.  The first situation causes int128 to not compile as you can't define both `operator wchar_t()` and `operator unsigned short()` because they are the same type.

This CL introduces a wrapper #define in order to abstract over the different typedefs for wchar_t.  We do a define instead of a typedef so that we can #undef at the end and not leak the symbol, since we need it in a header.

https://docs.microsoft.com/en-us/previous-versions/dh8che7s(v=vs.140) has more detail about the underlying problem.

PiperOrigin-RevId: 225223756
GitOrigin-RevId: 636137f6f0de910691a3950387fefacfa4909fb8
Change-Id: Iad94e52e9484c5acec115a2f09ef2d5ec22c2074

4 files changed, 78 insertions, 1 deletions

diff --git a/absl/base/BUILD.bazel b/absl/base/BUILD.bazel
index 1c31211..2717df0 100644
--- a/absl/base/BUILD.bazel
+++ b/absl/base/BUILD.bazel
@@ -67,6 +67,7 @@ cc_library(
     name = "core_headers",
     hdrs = [
         "attributes.h",
+        "const_init.h",
         "macros.h",
         "optimization.h",
         "port.h",
diff --git a/absl/base/CMakeLists.txt b/absl/base/CMakeLists.txt
index 212dd08..5178d2b 100644
--- a/absl/base/CMakeLists.txt
+++ b/absl/base/CMakeLists.txt
@@ -60,6 +60,7 @@ absl_cc_library(
     core_headers
   HDRS
     "attributes.h"
+    "const_init.h"
     "macros.h"
     "optimization.h"
     "port.h"
diff --git a/absl/base/call_once_test.cc b/absl/base/call_once_test.cc
index aa7eb95..183b92e 100644
--- a/absl/base/call_once_test.cc
+++ b/absl/base/call_once_test.cc
@@ -19,6 +19,7 @@
 
 #include "gtest/gtest.h"
 #include "absl/base/attributes.h"
+#include "absl/base/const_init.h"
 #include "absl/base/thread_annotations.h"
 #include "absl/synchronization/mutex.h"
 
@@ -26,7 +27,8 @@ namespace absl {
 namespace {
 
 absl::once_flag once;
-Mutex counters_mu;
+
+ABSL_CONST_INIT Mutex counters_mu(absl::kConstInit);
 
 int running_thread_count GUARDED_BY(counters_mu) = 0;
 int call_once_invoke_count GUARDED_BY(counters_mu) = 0;
diff --git a/absl/base/const_init.h b/absl/base/const_init.h
new file mode 100644
index 0000000..fc88b26
--- /dev/null
+++ b/absl/base/const_init.h
@@ -0,0 +1,73 @@
+// Copyright 2017 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+//
+// -----------------------------------------------------------------------------
+// kConstInit
+// -----------------------------------------------------------------------------
+//
+// A constructor tag used to mark an object as safe for use as a global
+// variable, avoiding the usual lifetime issues that can affect globals.
+
+#ifndef ABSL_BASE_CONST_INIT_H_
+#define ABSL_BASE_CONST_INIT_H_
+
+// In general, objects with static storage duration (such as global variables)
+// can trigger tricky object lifetime situations.  Attempting to access them
+// from the constructors or destructors of other global objects can result in
+// undefined behavior, unless their constructors and destructors are designed
+// with this issue in mind.
+//
+// The normal way to deal with this issue in C++11 is to use constant
+// initialization and trivial destructors.
+//
+// Constant initialization is guaranteed to occur before any other code
+// executes.  Constructors that are declared 'constexpr' are eligible for
+// constant initialization.  You can annotate a variable declaration with the
+// ABSL_CONST_INIT macro to express this intent.  For compilers that support
+// it, this annotation will cause a compilation error for declarations that
+// aren't subject to constant initialization (perhaps because a runtime value
+// was passed as a constructor argument).
+//
+// On program shutdown, lifetime issues can be avoided on global objects by
+// ensuring that they contain  trivial destructors.  A class has a trivial
+// destructor unless it has a user-defined destructor, a virtual method or base
+// class, or a data member or base class with a non-trivial destructor of its
+// own.  Objects with static storage duration and a trivial destructor are not
+// cleaned up on program shutdown, and are thus safe to access from other code
+// running during shutdown.
+//
+// For a few core Abseil classes, we make a best effort to allow for safe global
+// instances, even though these classes have non-trivial destructors.  These
+// objects can be created with the absl::kConstInit tag.  For example:
+//   ABSL_CONST_INIT absl::Mutex global_mutex(absl::kConstInit);
+//
+// The line above declares a global variable of type absl::Mutex which can be
+// accessed at any point during startup or shutdown.  global_mutex's destructor
+// will still run, but will not invalidate the object.  Note that C++ specifies
+// that accessing an object after its destructor has run results in undefined
+// behavior, but this pattern works on the toolchains we support.
+//
+// The absl::kConstInit tag should only be used to define objects with static
+// or thread_local storage duration.
+//
+
+namespace absl {
+
+enum ConstInitType {
+  kConstInit,
+};
+
+}  // namespace absl
+
+#endif  // ABSL_BASE_CONST_INIT_H_