dist_lock::DistLockedTask Class Reference

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Table of Contents

• Detailed Description
• Example with retrying
• Example without retrying
• Public Types
• Public Member Functions
• Static Public Member Functions
• Member Typedef Documentation
• WorkerFunc
• Member Enumeration Documentation
• Importance
• State
• WaitMode
• Constructor & Destructor Documentation
• DistLockedTask() [1/2]
• DistLockedTask() [2/2]
• Member Function Documentation
• BlockingWait()
• GetLockedDuration()
• IsValid()
• Wait()
• WaitFor()
• WaitNothrow()
• WaitNothrowUntil()
• WaitUntil() [1/2]
• WaitUntil() [2/2]

#include <userver/dist_lock/dist_locked_task.hpp>

Detailed Description

A task that tries to acquire a distributed lock and runs user callback once while the lock is held.

When dist lock starts, the lock worker tries to take a lock in the loop. If succeeded, a task is launched that executes the user code. In the background, dist lock tries to extend the lock. In case of loss of the lock, the user task is canceled.

Example with retrying

    auto strategy = GetSomeDistLockStrategyForTheSample();
    std::atomic<std::size_t> counter = 0;
    dist_lock::DistLockedTask locked_task(
        "example",
        [&]() {
            counter++;
            if (counter < 5) {
                throw std::runtime_error("");
            }
        },
        strategy
    );
 
    UEXPECT_NO_THROW(locked_task.Get());
    EXPECT_EQ(counter, 5);

Example without retrying

    auto strategy = GetSomeDistLockStrategyForTheSample();
    std::atomic<std::size_t> counter = 0;
    dist_lock::DistLockedTask locked_task(
        "example",
        [&]() {
            counter++;
            throw std::runtime_error("123");
        },
        strategy,
        /*default settings*/ {},
        dist_lock::DistLockWaitingMode::kWait,
        dist_lock::DistLockRetryMode::kSingleAttempt
    );
 
    try {
        locked_task.Get();
        FAIL() << "Should have thrown";
    } catch (const std::runtime_error& exception) {
        EXPECT_EQ(exception.what(), std::string("123"));
    }
    EXPECT_EQ(counter, 1);

See also

Periodics and DistLocks

AlwaysBusyDistLockStrategy

Definition at line 48 of file dist_locked_task.hpp.

Inheritance diagram for dist_lock::DistLockedTask:

Public Types
using	WorkerFunc = std::function<void()>
enum class	Importance {   kNormal ,   kCritical }
	Task importance. More...
enum class	State {   kInvalid ,   kNew ,   kQueued ,   kRunning ,   kSuspended ,   kCancelled ,   kCompleted }
	Task state. More...
enum class	WaitMode {   kSingleWaiter ,   kMultipleWaiters }
	Task wait mode. More...

Public Member Functions
	DistLockedTask ()=default
	DistLockedTask (DistLockedTask &&)=delete
DistLockedTask &	operator= (DistLockedTask &&)=delete
	DistLockedTask (const DistLockedTask &)=delete
DistLockedTask &	operator= (const DistLockedTask &&)=delete
	DistLockedTask (std::string name, WorkerFunc worker_func, std::shared_ptr< DistLockStrategyBase > strategy, const DistLockSettings &settings={}, DistLockWaitingMode mode=DistLockWaitingMode::kWait, DistLockRetryMode retry_mode=DistLockRetryMode::kRetry)
	DistLockedTask (engine::TaskProcessor &task_processor, std::string name, WorkerFunc worker_func, std::shared_ptr< DistLockStrategyBase > strategy, const DistLockSettings &settings={}, DistLockWaitingMode mode=DistLockWaitingMode::kWait, DistLockRetryMode retry_mode=DistLockRetryMode::kRetry)
	Creates a DistLockedTask to be run in a specific engine::TaskProcessor.
std::optional< std::chrono::steady_clock::duration >	GetLockedDuration () const
void	Get () noexcept(false)
bool	IsValid () const
	Checks whether this object owns an actual task (not State::kInvalid)
State	GetState () const
	Gets the task State.
bool	IsFinished () const
	Returns whether the task finished execution.
void	Wait () const noexcept(false)
	Suspends execution until the task finishes or caller is cancelled. Can be called from coroutine context only. For non-coroutine context use BlockingWait().
template<typename Rep, typename Period>
void	WaitFor (const std::chrono::duration< Rep, Period > &) const noexcept(false)
	Suspends execution until the task finishes or after the specified timeout or until caller is cancelled.
template<typename Clock, typename Duration>
void	WaitUntil (const std::chrono::time_point< Clock, Duration > &) const noexcept(false)
	Suspends execution until the task finishes or until the specified time point is reached or until caller is cancelled.
void	WaitUntil (Deadline) const
	Suspends execution until the task finishes or until the specified deadline is reached or until caller is cancelled.
bool	WaitNothrow () const noexcept
	Suspends execution until the task finishes or caller is cancelled. Can be called from coroutine context only. For non-coroutine context use BlockingWait().
FutureStatus	WaitNothrowUntil (Deadline) const noexcept
	Suspends execution until the task finishes or until the specified deadline is reached or until caller is cancelled.
void	RequestCancel ()
	Queues task cancellation request.
void	SyncCancel () noexcept
	Cancels the task and suspends execution until it is finished. Can be called from coroutine context only. For non-coroutine context use RequestCancel() + BlockingWait().
TaskCancellationReason	CancellationReason () const
	Gets task cancellation reason.
void	BlockingWait () const

Static Public Member Functions
static std::string_view	GetStateName (State state)

Member Typedef Documentation

WorkerFunc

using dist_lock::DistLockedTask::WorkerFunc = std::function<void()>

Definition at line 50 of file dist_locked_task.hpp.

Member Enumeration Documentation

Importance

enum class engine::TaskBase::Importance

stronginherited

Task importance.

Enumerator
kNormal	Normal task.
kCritical	Critical task. The task will be started regardless of cancellations, e.g. due to user request, deadline or TaskProcessor overload. After the task starts, it may be cancelled. In particular, if it received any cancellation requests before starting, then it will start as cancelled.

Definition at line 36 of file task_base.hpp.

State

enum class engine::TaskBase::State

stronginherited

Task state.

Enumerator
kInvalid	Unusable.
kNew	just created, not registered with task processor
kQueued	awaits execution
kRunning	executing user code
kSuspended	suspended, e.g. waiting for blocking call to complete
kCancelled	exited user code because of external request
kCompleted	exited user code with return or throw

Definition at line 48 of file task_base.hpp.

WaitMode

enum class engine::TaskBase::WaitMode

stronginherited

Task wait mode.

Enumerator
kSingleWaiter	Can be awaited by at most one task at a time.
kMultipleWaiters	Can be awaited by multiple tasks simultaneously.

Definition at line 59 of file task_base.hpp.

Constructor & Destructor Documentation

DistLockedTask() [1/2]

dist_lock::DistLockedTask::DistLockedTask ( )

default

Default constructor. Creates an invalid task.

DistLockedTask() [2/2]

dist_lock::DistLockedTask::DistLockedTask	(	std::string	name,
		WorkerFunc	worker_func,
		std::shared_ptr< DistLockStrategyBase >	strategy,
		const DistLockSettings &	settings = {},
		DistLockWaitingMode	mode = DistLockWaitingMode::kWait,
		DistLockRetryMode	retry_mode = DistLockRetryMode::kRetry )

Creates a DistLockedTask.

Parameters

name	name of the task
worker_func	a callback that is started once we've acquired the lock and is cancelled when the lock is lost.
settings	distributed lock settings
strategy	distributed locking strategy
mode	distributed lock waiting mode
retry_mode	run task continuously or once (needed mainly for tests)

Note

worker_func must honour task cancellation and stop ASAP when it is cancelled, otherwise brain split is possible (IOW, two different users do work assuming both of them hold the lock, which is not true).

Member Function Documentation

BlockingWait()

void engine::TaskBase::BlockingWait ( ) const

inherited

Waits for the task in non-coroutine context (e.g. non-TaskProcessor's std::thread).

GetLockedDuration()

std::optional< std::chrono::steady_clock::duration > dist_lock::DistLockedTask::GetLockedDuration

(

)

const

Returns for how long the lock is held (if held at all). Returned value may be less than the real duration.

IsValid()

bool engine::TaskBase::IsValid ( ) const

inherited

Checks whether this object owns an actual task (not State::kInvalid)

An invalid task cannot be used. The task becomes invalid after each of the following calls:

1. the default constructor
2. Detach()
3. Get() (see engine::TaskWithResult)

Wait()

void engine::TaskBase::Wait ( ) const

inherited

Suspends execution until the task finishes or caller is cancelled. Can be called from coroutine context only. For non-coroutine context use BlockingWait().

Exceptions

WaitInterruptedException

when current_task::IsCancelRequested() and no TaskCancellationBlockers are present.

WaitFor()

template<typename Rep, typename Period>

void engine::TaskBase::WaitFor ( const std::chrono::duration< Rep, Period > & duration ) const

inherited

Suspends execution until the task finishes or after the specified timeout or until caller is cancelled.

Exceptions

WaitInterruptedException

when current_task::IsCancelRequested() and no TaskCancellationBlockers are present.

Definition at line 180 of file task_base.hpp.

WaitNothrow()

bool engine::TaskBase::WaitNothrow ( ) const

nodiscardnoexceptinherited

Suspends execution until the task finishes or caller is cancelled. Can be called from coroutine context only. For non-coroutine context use BlockingWait().

Returns

false when current_task::IsCancelRequested() and no TaskCancellationBlockers are present.

WaitNothrowUntil()

FutureStatus engine::TaskBase::WaitNothrowUntil ( Deadline ) const

nodiscardnoexceptinherited

Suspends execution until the task finishes or until the specified deadline is reached or until caller is cancelled.

Returns

FutureStatus::kCancelled when current_task::IsCancelRequested() and no TaskCancellationBlockers are present.

WaitUntil() [1/2]

template<typename Clock, typename Duration>

void engine::TaskBase::WaitUntil ( const std::chrono::time_point< Clock, Duration > & until ) const

inherited

Suspends execution until the task finishes or until the specified time point is reached or until caller is cancelled.

Exceptions

WaitInterruptedException

when current_task::IsCancelRequested() and no TaskCancellationBlockers are present.

Definition at line 185 of file task_base.hpp.

WaitUntil() [2/2]

void engine::TaskBase::WaitUntil ( Deadline ) const

inherited

Suspends execution until the task finishes or until the specified deadline is reached or until caller is cancelled.

Exceptions

WaitInterruptedException

when current_task::IsCancelRequested() and no TaskCancellationBlockers are present.

---

The documentation for this class was generated from the following file:

• userver/dist_lock/dist_locked_task.hpp