FTL: Add wrapper for shared_mutex

std::shared_mutex on Android is missing capabilities necessary for
threading annotations. b/135688034 tracks adding them, but is blocked on
b/175635923. In the meantime, add a simple wrapper so that we can start
using annotations.

Only add the methods we currently need. Other methods can be added as
needed.

Bug: 185536303
Test: ftl_test
Change-Id: Ic7c2152bc7e46b31eecdba42fac1126b26aafd60

3 files changed, 108 insertions, 0 deletions

diff --git a/include/ftl/shared_mutex.h b/include/ftl/shared_mutex.h
new file mode 100644
index 0000000000..146f5ba4a9
--- /dev/null
+++ b/include/ftl/shared_mutex.h
@@ -0,0 +1,47 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#pragma once
+
+#include <shared_mutex>
+
+namespace android::ftl {
+
+// Wrapper around std::shared_mutex to provide capabilities for thread-safety
+// annotations.
+// TODO(b/257958323): This class is no longer needed once b/135688034 is fixed (currently blocked on
+// b/175635923).
+class [[clang::capability("shared_mutex")]] SharedMutex final {
+ public:
+  [[clang::acquire_capability()]] void lock() {
+    mutex_.lock();
+  }
+  [[clang::release_capability()]] void unlock() {
+    mutex_.unlock();
+  }
+
+  [[clang::acquire_shared_capability()]] void lock_shared() {
+    mutex_.lock_shared();
+  }
+  [[clang::release_shared_capability()]] void unlock_shared() {
+    mutex_.unlock_shared();
+  }
+
+ private:
+  std::shared_mutex mutex_;
+};
+
+}  // namespace android::ftl
diff --git a/libs/ftl/Android.bp b/libs/ftl/Android.bp
index 81113bc211..df0b271a9b 100644
--- a/libs/ftl/Android.bp
+++ b/libs/ftl/Android.bp
@@ -24,6 +24,7 @@ cc_test {
         "match_test.cpp",
         "non_null_test.cpp",
         "optional_test.cpp",
+        "shared_mutex_test.cpp",
         "small_map_test.cpp",
         "small_vector_test.cpp",
         "static_vector_test.cpp",
diff --git a/libs/ftl/shared_mutex_test.cpp b/libs/ftl/shared_mutex_test.cpp
new file mode 100644
index 0000000000..6da7061ae0
--- /dev/null
+++ b/libs/ftl/shared_mutex_test.cpp
@@ -0,0 +1,60 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * 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.
+ */
+
+#include <ftl/shared_mutex.h>
+#include <gtest/gtest.h>
+#include <ftl/fake_guard.h>
+
+namespace android::test {
+
+TEST(SharedMutex, SharedLock) {
+  ftl::SharedMutex mutex;
+  std::shared_lock shared_lock(mutex);
+
+  { std::shared_lock shared_lock2(mutex); }
+}
+
+TEST(SharedMutex, ExclusiveLock) {
+  ftl::SharedMutex mutex;
+  std::unique_lock unique_lock(mutex);
+}
+
+TEST(SharedMutex, Annotations) {
+  struct {
+    void foo() FTL_ATTRIBUTE(requires_shared_capability(mutex)) { num++; }
+    void bar() FTL_ATTRIBUTE(requires_capability(mutex)) { num++; }
+    void baz() {
+      std::shared_lock shared_lock(mutex);
+      num++;
+    }
+    ftl::SharedMutex mutex;
+    int num = 0;
+
+  } s;
+
+  {
+    // TODO(b/257958323): Use an RAII class instead of locking manually.
+    s.mutex.lock_shared();
+    s.foo();
+    s.baz();
+    s.mutex.unlock_shared();
+  }
+  s.mutex.lock();
+  s.bar();
+  s.mutex.unlock();
+}
+
+}  // namespace android::test