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// Copyright 2016 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
#include "base/task/single_thread_task_runner.h"
#include <memory>
#include <utility>
#include "base/check.h"
#include "base/check_op.h"
#include "base/compiler_specific.h"
#include "base/dcheck_is_on.h"
#include "base/functional/bind.h"
#include "base/lazy_instance.h"
#include "base/run_loop.h"
#include "third_party/abseil-cpp/absl/base/attributes.h"
namespace base {
namespace {
ABSL_CONST_INIT thread_local SingleThreadTaskRunner::CurrentDefaultHandle*
current_default_handle = nullptr;
// This function can be removed, and the calls below replaced with direct
// variable accesses, once the MSAN workaround is not necessary.
SingleThreadTaskRunner::CurrentDefaultHandle* GetCurrentDefaultHandle() {
// Workaround false-positive MSAN use-of-uninitialized-value on
// thread_local storage for loaded libraries:
// https://github.com/google/sanitizers/issues/1265
MSAN_UNPOISON(¤t_default_handle,
sizeof(SingleThreadTaskRunner::CurrentDefaultHandle*));
return current_default_handle;
}
} // namespace
// static
const scoped_refptr<SingleThreadTaskRunner>&
SingleThreadTaskRunner::GetCurrentDefault() {
const auto* const handle = GetCurrentDefaultHandle();
CHECK(handle)
<< "Error: This caller requires a single-threaded context (i.e. the "
"current task needs to run from a SingleThreadTaskRunner). If you're "
"in a test refer to //docs/threading_and_tasks_testing.md."
<< (SequencedTaskRunner::HasCurrentDefault()
? " Note: base::SequencedTaskRunner::GetCurrentDefault() "
"is set; "
"consider using it if the current task can run from a "
"SequencedTaskRunner."
: "");
return handle->task_runner_;
}
// static
bool SingleThreadTaskRunner::HasCurrentDefault() {
return !!GetCurrentDefaultHandle();
}
SingleThreadTaskRunner::CurrentDefaultHandle::CurrentDefaultHandle(
scoped_refptr<SingleThreadTaskRunner> task_runner)
: resetter_(¤t_default_handle, this, nullptr),
task_runner_(std::move(task_runner)),
sequenced_task_runner_current_default_(task_runner_) {
DCHECK(task_runner_->BelongsToCurrentThread());
}
SingleThreadTaskRunner::CurrentDefaultHandle::~CurrentDefaultHandle() {
DCHECK(task_runner_->BelongsToCurrentThread());
DCHECK_EQ(GetCurrentDefaultHandle(), this);
}
SingleThreadTaskRunner::CurrentHandleOverride::CurrentHandleOverride(
scoped_refptr<SingleThreadTaskRunner> overriding_task_runner,
bool allow_nested_runloop) {
DCHECK(!SequencedTaskRunner::HasCurrentDefault() ||
SingleThreadTaskRunner::HasCurrentDefault())
<< "SingleThreadTaskRunner::CurrentHandleOverride is not compatible with "
"a SequencedTaskRunner::CurrentDefaultHandle already "
"being set on this thread (except when it's "
"set by the current "
"SingleThreadTaskRunner::CurrentDefaultHandle).";
if (!SingleThreadTaskRunner::HasCurrentDefault()) {
top_level_thread_task_runner_current_default_.emplace(
std::move(overriding_task_runner));
return;
}
#if DCHECK_IS_ON()
expected_task_runner_before_restore_ = overriding_task_runner.get();
#endif
auto* const handle = GetCurrentDefaultHandle();
SequencedTaskRunner::SetCurrentDefaultHandleTaskRunner(
handle->sequenced_task_runner_current_default_, overriding_task_runner);
handle->task_runner_.swap(overriding_task_runner);
// Due to the swap, now `handle->task_runner_` points to the overriding task
// runner and `overriding_task_runner_` points to the previous task runner.
task_runner_to_restore_ = std::move(overriding_task_runner);
if (!allow_nested_runloop) {
no_running_during_override_ =
std::make_unique<ScopedDisallowRunningRunLoop>();
}
}
SingleThreadTaskRunner::CurrentHandleOverride::~CurrentHandleOverride() {
if (task_runner_to_restore_) {
auto* const handle = GetCurrentDefaultHandle();
#if DCHECK_IS_ON()
DCHECK_EQ(expected_task_runner_before_restore_, handle->task_runner_.get())
<< "Nested overrides must expire their "
"SingleThreadTaskRunner::CurrentHandleOverride "
"in LIFO order.";
#endif
SequencedTaskRunner::SetCurrentDefaultHandleTaskRunner(
handle->sequenced_task_runner_current_default_,
task_runner_to_restore_);
handle->task_runner_.swap(task_runner_to_restore_);
}
}
} // namespace base
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