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// 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/strings/internal/cordz_handle.h"
#include <atomic>
#include "absl/base/internal/raw_logging.h" // For ABSL_RAW_CHECK
#include "absl/base/no_destructor.h"
#include "absl/synchronization/mutex.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace cord_internal {
namespace {
struct Queue {
Queue() = default;
absl::Mutex mutex;
std::atomic<CordzHandle*> dq_tail ABSL_GUARDED_BY(mutex){nullptr};
// Returns true if this delete queue is empty. This method does not acquire
// the lock, but does a 'load acquire' observation on the delete queue tail.
// It is used inside Delete() to check for the presence of a delete queue
// without holding the lock. The assumption is that the caller is in the
// state of 'being deleted', and can not be newly discovered by a concurrent
// 'being constructed' snapshot instance. Practically, this means that any
// such discovery (`find`, 'first' or 'next', etc) must have proper 'happens
// before / after' semantics and atomic fences.
bool IsEmpty() const ABSL_NO_THREAD_SAFETY_ANALYSIS {
return dq_tail.load(std::memory_order_acquire) == nullptr;
}
};
static Queue& GlobalQueue() {
static absl::NoDestructor<Queue> global_queue;
return *global_queue;
}
} // namespace
CordzHandle::CordzHandle(bool is_snapshot) : is_snapshot_(is_snapshot) {
Queue& global_queue = GlobalQueue();
if (is_snapshot) {
MutexLock lock(&global_queue.mutex);
CordzHandle* dq_tail = global_queue.dq_tail.load(std::memory_order_acquire);
if (dq_tail != nullptr) {
dq_prev_ = dq_tail;
dq_tail->dq_next_ = this;
}
global_queue.dq_tail.store(this, std::memory_order_release);
}
}
CordzHandle::~CordzHandle() {
Queue& global_queue = GlobalQueue();
if (is_snapshot_) {
std::vector<CordzHandle*> to_delete;
{
MutexLock lock(&global_queue.mutex);
CordzHandle* next = dq_next_;
if (dq_prev_ == nullptr) {
// We were head of the queue, delete every CordzHandle until we reach
// either the end of the list, or a snapshot handle.
while (next && !next->is_snapshot_) {
to_delete.push_back(next);
next = next->dq_next_;
}
} else {
// Another CordzHandle existed before this one, don't delete anything.
dq_prev_->dq_next_ = next;
}
if (next) {
next->dq_prev_ = dq_prev_;
} else {
global_queue.dq_tail.store(dq_prev_, std::memory_order_release);
}
}
for (CordzHandle* handle : to_delete) {
delete handle;
}
}
}
bool CordzHandle::SafeToDelete() const {
return is_snapshot_ || GlobalQueue().IsEmpty();
}
void CordzHandle::Delete(CordzHandle* handle) {
assert(handle);
if (handle) {
Queue& queue = GlobalQueue();
if (!handle->SafeToDelete()) {
MutexLock lock(&queue.mutex);
CordzHandle* dq_tail = queue.dq_tail.load(std::memory_order_acquire);
if (dq_tail != nullptr) {
handle->dq_prev_ = dq_tail;
dq_tail->dq_next_ = handle;
queue.dq_tail.store(handle, std::memory_order_release);
return;
}
}
delete handle;
}
}
std::vector<const CordzHandle*> CordzHandle::DiagnosticsGetDeleteQueue() {
std::vector<const CordzHandle*> handles;
Queue& global_queue = GlobalQueue();
MutexLock lock(&global_queue.mutex);
CordzHandle* dq_tail = global_queue.dq_tail.load(std::memory_order_acquire);
for (const CordzHandle* p = dq_tail; p; p = p->dq_prev_) {
handles.push_back(p);
}
return handles;
}
bool CordzHandle::DiagnosticsHandleIsSafeToInspect(
const CordzHandle* handle) const {
if (!is_snapshot_) return false;
if (handle == nullptr) return true;
if (handle->is_snapshot_) return false;
bool snapshot_found = false;
Queue& global_queue = GlobalQueue();
MutexLock lock(&global_queue.mutex);
for (const CordzHandle* p = global_queue.dq_tail; p; p = p->dq_prev_) {
if (p == handle) return !snapshot_found;
if (p == this) snapshot_found = true;
}
ABSL_ASSERT(snapshot_found); // Assert that 'this' is in delete queue.
return true;
}
std::vector<const CordzHandle*>
CordzHandle::DiagnosticsGetSafeToInspectDeletedHandles() {
std::vector<const CordzHandle*> handles;
if (!is_snapshot()) {
return handles;
}
Queue& global_queue = GlobalQueue();
MutexLock lock(&global_queue.mutex);
for (const CordzHandle* p = dq_next_; p != nullptr; p = p->dq_next_) {
if (!p->is_snapshot()) {
handles.push_back(p);
}
}
return handles;
}
} // namespace cord_internal
ABSL_NAMESPACE_END
} // namespace absl
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