Export of internal Abseil changes

--
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, 42 insertions, 7 deletions

diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h
index bc50de54..fad36877 100644
--- a/absl/flags/internal/flag.h
+++ b/absl/flags/internal/flag.h
@@ -22,20 +22,30 @@
 namespace absl {
 namespace flags_internal {
 
+// Signature for the mutation callback used by watched Flags
+// The callback is noexcept.
+// TODO(rogeeff): add noexcept after C++17 support is added.
+using FlagCallback = void (*)();
+
+void InvokeCallback(absl::Mutex* primary_mu, absl::Mutex* callback_mu,
+                    FlagCallback cb) EXCLUSIVE_LOCKS_REQUIRED(primary_mu);
+
 // This is "unspecified" implementation of absl::Flag<T> type.
 template <typename T>
-class Flag : public flags_internal::CommandLineFlag {
+class Flag final : public flags_internal::CommandLineFlag {
  public:
-  constexpr Flag(const char* name, const flags_internal::HelpGenFunc help_gen,
-                 const char* filename,
+  constexpr Flag(const char* name_arg,
+                 const flags_internal::HelpGenFunc help_gen,
+                 const char* filename_arg,
                  const flags_internal::FlagMarshallingOpFn marshalling_op_arg,
                  const flags_internal::InitialValGenFunc initial_value_gen)
       : flags_internal::CommandLineFlag(
-            name, flags_internal::HelpText::FromFunctionPointer(help_gen),
-            filename, &flags_internal::FlagOps<T>, marshalling_op_arg,
+            name_arg, flags_internal::HelpText::FromFunctionPointer(help_gen),
+            filename_arg, &flags_internal::FlagOps<T>, marshalling_op_arg,
             initial_value_gen,
-            /*retired_arg=*/false, /*def_arg=*/nullptr,
-            /*cur_arg=*/nullptr) {}
+            /*def_arg=*/nullptr,
+            /*cur_arg=*/nullptr),
+        callback_(nullptr) {}
 
   T Get() const {
     // Implementation notes:
@@ -76,6 +86,31 @@ class Flag : public flags_internal::CommandLineFlag {
   }
 
   void Set(const T& v) { this->Write(&v, &flags_internal::FlagOps<T>); }
+
+  void SetCallback(const flags_internal::FlagCallback mutation_callback) {
+    absl::MutexLock l(InitFlagIfNecessary());
+
+    callback_ = mutation_callback;
+
+    InvokeCallback();
+  }
+  void InvokeCallback() override
+      EXCLUSIVE_LOCKS_REQUIRED(this->locks->primary_mu) {
+    flags_internal::InvokeCallback(&this->locks->primary_mu,
+                                   &this->locks->callback_mu, callback_);
+  }
+
+ private:
+  void Destroy() const override {
+    // Values are heap allocated Abseil Flags.
+    if (this->cur) Delete(this->op, this->cur);
+    if (this->def) Delete(this->op, this->def);
+
+    delete this->locks;
+  }
+
+  // Flag's data
+  FlagCallback callback_;  // Mutation callback
 };
 
 // This class facilitates Flag object registration and tail expression-based