Add a Reader-Writer Lock

(This is a merge of http://go/wvgerrit/70666)

We need a reader-writer lock implementation in order to make use of the
new threading guarantees in OEMCrypto v15. However, we do not have
access to an STL reader-writer lock due to only being on C++11. This
patch adds a home-grown reader-writer lock, as well as tests to verify
that its behavior is sound.

Bug: 70889998
Bug: 118584039
Test: CE CDM Unit Tests
Test: Android Unit Tests
Change-Id: Iaddcefb50e72452fbd27d04879eacf775484e675

libwvdrmengine/cdm/util/src/rw_lock.cpp

@@ -0,0 +1,60 @@
+// Copyright 2019 Google LLC. All Rights Reserved. This file and proprietary
+// source code may only be used and distributed under the Widevine Master
+// License Agreement.
+
+#include "rw_lock.h"
+
+#include "log.h"
+
+namespace wvcdm {
+
+shared_mutex::~shared_mutex() {
+  if (reader_count_ > 0) {
+    LOGE("shared_mutex destroyed with active readers!");
+  }
+  if (has_writer_) {
+    LOGE("shared_mutex destroyed with an active writer!");
+  }
+}
+
+void shared_mutex::lock_shared() {
+  std::unique_lock<std::mutex> lock(mutex_);
+
+  while (has_writer_) {
+    condition_variable_.wait(lock);
+  }
+
+  ++reader_count_;
+}
+
+void shared_mutex::unlock_shared() {
+  std::unique_lock<std::mutex> lock(mutex_);
+
+  --reader_count_;
+
+  if (reader_count_ == 0) {
+    condition_variable_.notify_all();
+  }
+}
+
+bool shared_mutex::lock_implementation(bool abort_if_unavailable) {
+  std::unique_lock<std::mutex> lock(mutex_);
+
+  while (reader_count_ > 0 || has_writer_) {
+    if (abort_if_unavailable) return false;
+    condition_variable_.wait(lock);
+  }
+
+  has_writer_ = true;
+  return true;
+}
+
+void shared_mutex::unlock() {
+  std::unique_lock<std::mutex> lock(mutex_);
+
+  has_writer_ = false;
+
+  condition_variable_.notify_all();
+}
+
+}  // namespace wvcdm