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 000000000000..34629b304aa1
--- /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