Export of internal Abseil changes

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d6748c733a70cd74ad9b76a0c9cd6b3fe2cecacf by Xiaoyi Zhang <zhangxy@google.com>:

Remove empty block, to address alerts reported in
https://github.com/abseil/abseil-cpp/issues/368.

PiperOrigin-RevId: 265099887

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232e2036b5668d6d1296b881f9347756d84541ee by Derek Mauro <dmauro@google.com>:

Make the Linux Bazel CI scripts test with the exception mode explicitly set.

PiperOrigin-RevId: 265092105

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942a40696c2c9b833be03e92d22a6ede7bccb6d4 by Xiaoyi Zhang <zhangxy@google.com>:

Import https://github.com/abseil/abseil-cpp/pull/372.
Suppress the unused variable warning on GCC, i.e. "-Wunused-variable".

PiperOrigin-RevId: 265063925

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7ef90796b52cbdc260afc77cf47206f9356471d0 by Xiaoyi Zhang <zhangxy@google.com>:

Add quotes to `ABSL_COMMON_INCLUDE_DIRS` since it's a list and may contain a
`;`. This addresses https://github.com/abseil/abseil-cpp/issues/373.

PiperOrigin-RevId: 265059077

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43f3ae742e00b83672ad6c5bc5b17fdb8f9fe6fe by Gennadiy Rozental <rogeeff@google.com>:

Internal re-organization

PiperOrigin-RevId: 264913945

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6a2adf9c08ee1d98cc6b2855a676345c6495294a by Andy Soffer <asoffer@google.com>:

Publicly expose type names for uniform interval tags as in, for example, absl::IntervalClosedClosedTag, and add equality comparison operators.

PiperOrigin-RevId: 264861162

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3c90c6e05fd61d56b419cd2d39dab8f17b8711b8 by Abseil Team <absl-team@google.com>:

Add validity check on returned frame pointer.

PiperOrigin-RevId: 264858823

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2db87e0cfa0c6bea7ba81684b834cb8a73b7d748 by Gennadiy Rozental <rogeeff@google.com>:

Add MUST_USE_RESULT attribute to absl::GetFlag to prevent accidental misuse.

PiperOrigin-RevId: 264782762
GitOrigin-RevId: d6748c733a70cd74ad9b76a0c9cd6b3fe2cecacf
Change-Id: I169e9c5358e4f63000c1255e806d26b8afecf5ff

1 files changed, 51 insertions, 0 deletions

diff --git a/absl/flags/internal/flag.cc b/absl/flags/internal/flag.cc
new file mode 100644
index 00000000..34629b30
--- /dev/null
+++ b/absl/flags/internal/flag.cc
@@ -0,0 +1,51 @@
+//
+// Copyright 2019 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
+//
+//      https://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.
+
+#include "absl/flags/internal/flag.h"
+
+#include "absl/synchronization/mutex.h"
+
+namespace absl {
+namespace flags_internal {
+
+// If the flag has a mutation callback this function invokes it. While the
+// callback is being invoked the primary flag's mutex is unlocked and it is
+// re-locked back after call to callback is completed. Callback invocation is
+// guarded by flag's secondary mutex instead which prevents concurrent
+// callback invocation. Note that it is possible for other thread to grab the
+// primary lock and update flag's value at any time during the callback
+// invocation. This is by design. Callback can get a value of the flag if
+// necessary, but it might be different from the value initiated the callback
+// and it also can be different by the time the callback invocation is
+// completed. Requires that *primary_lock be held in exclusive mode; it may be
+// released and reacquired by the implementation.
+void InvokeCallback(absl::Mutex* primary_mu, absl::Mutex* callback_mu,
+                    FlagCallback cb) EXCLUSIVE_LOCKS_REQUIRED(primary_mu) {
+  if (!cb) return;
+
+  // When executing the callback we need the primary flag's mutex to be
+  // unlocked so that callback can retrieve the flag's value.
+  primary_mu->Unlock();
+
+  {
+    absl::MutexLock lock(callback_mu);
+    cb();
+  }
+
+  primary_mu->Lock();
+}
+
+}  // namespace flags_internal
+}  // namespace absl