GI.Gio.Objects.Cancellable

GCancellable is a thread-safe operation cancellation stack used throughout GIO to allow for cancellation of synchronous and asynchronous operations.

Exported types

newtype Cancellable

Memory-managed wrapper type.

Constructors

Cancellable (ManagedPtr Cancellable)

Instances

GObject Cancellable

Defined in GI.Gio.Objects.Cancellable

Methods

gobjectType :: IO GType

HasParentTypes Cancellable

Defined in GI.Gio.Objects.Cancellable

type ParentTypes Cancellable

Defined in GI.Gio.Objects.Cancellable

type ParentTypes Cancellable = Object ': ([] :: [Type])

class (GObject o, IsDescendantOf Cancellable o) => IsCancellable o

Type class for types which can be safely cast to Cancellable, for instance with toCancellable.

Instances

(GObject o, IsDescendantOf Cancellable o) => IsCancellable o

Defined in GI.Gio.Objects.Cancellable

toCancellable :: (MonadIO m, IsCancellable o) => o -> m Cancellable

Cast to Cancellable, for types for which this is known to be safe. For general casts, use castTo.

noCancellable :: Maybe Cancellable

A convenience alias for Nothing :: Maybe Cancellable.

Methods

cancel

cancellableCancel

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: a Cancellable object.

-> m ()

Will set cancellable to cancelled, and will emit the Cancellable::cancelled signal. (However, see the warning about race conditions in the documentation for that signal if you are planning to connect to it.)

This function is thread-safe. In other words, you can safely call it from a thread other than the one running the operation that was passed the cancellable.

If cancellable is Nothing, this function returns immediately for convenience.

The convention within GIO is that cancelling an asynchronous operation causes it to complete asynchronously. That is, if you cancel the operation from the same thread in which it is running, then the operation's AsyncReadyCallback will not be invoked until the application returns to the main loop.

connect

cancellableConnect

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: A Cancellable.

-> Callback

callback: The Callback to connect.

-> m CULong

Returns: The id of the signal handler or 0 if cancellable has already been cancelled.

Convenience function to connect to the Cancellable::cancelled signal. Also handles the race condition that may happen if the cancellable is cancelled right before connecting.

callback is called at most once, either directly at the time of the connect if cancellable is already cancelled, or when cancellable is cancelled in some thread.

dataDestroyFunc will be called when the handler is disconnected, or immediately if the cancellable is already cancelled.

See Cancellable::cancelled for details on how to use this.

Since GLib 2.40, the lock protecting cancellable is not held when callback is invoked. This lifts a restriction in place for earlier GLib versions which now makes it easier to write cleanup code that unconditionally invokes e.g. cancellableCancel.

Since: 2.22

disconnect

cancellableDisconnect

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: A Cancellable or Nothing.

-> CULong

handlerId: Handler id of the handler to be disconnected, or 0.

-> m ()

Disconnects a handler from a cancellable instance similar to signalHandlerDisconnect. Additionally, in the event that a signal handler is currently running, this call will block until the handler has finished. Calling this function from a Cancellable::cancelled signal handler will therefore result in a deadlock.

This avoids a race condition where a thread cancels at the same time as the cancellable operation is finished and the signal handler is removed. See Cancellable::cancelled for details on how to use this.

If cancellable is Nothing or handlerId is 0 this function does nothing.

Since: 2.22

getCurrent

cancellableGetCurrent

Arguments

:: (HasCallStack, MonadIO m)

=> m (Maybe Cancellable)

Returns: a Cancellable from the top of the stack, or Nothing if the stack is empty.

Gets the top cancellable from the stack.

getFd

cancellableGetFd

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: a Cancellable.

-> m Int32

Returns: A valid file descriptor. -1 if the file descriptor is not supported, or on errors.

Gets the file descriptor for a cancellable job. This can be used to implement cancellable operations on Unix systems. The returned fd will turn readable when cancellable is cancelled.

You are not supposed to read from the fd yourself, just check for readable status. Reading to unset the readable status is done with cancellableReset.

After a successful return from this function, you should use cancellableReleaseFd to free up resources allocated for the returned file descriptor.

See also cancellableMakePollfd.

isCancelled

cancellableIsCancelled

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: a Cancellable or Nothing

-> m Bool

Returns: True if cancellable is cancelled, FALSE if called with Nothing or if item is not cancelled.

Checks if a cancellable job has been cancelled.

makePollfd

cancellableMakePollfd

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: a Cancellable or Nothing

-> PollFD

pollfd: a pointer to a PollFD

-> m Bool

Returns: True if pollfd was successfully initialized, False on failure to prepare the cancellable.

Creates a PollFD corresponding to cancellable; this can be passed to poll and used to poll for cancellation. This is useful both for unix systems without a native poll and for portability to windows.

When this function returns True, you should use cancellableReleaseFd to free up resources allocated for the pollfd. After a False return, do not call cancellableReleaseFd.

If this function returns False, either no cancellable was given or resource limits prevent this function from allocating the necessary structures for polling. (On Linux, you will likely have reached the maximum number of file descriptors.) The suggested way to handle these cases is to ignore the cancellable.

You are not supposed to read from the fd yourself, just check for readable status. Reading to unset the readable status is done with cancellableReset.

Since: 2.22

new

cancellableNew

Arguments

:: (HasCallStack, MonadIO m)

=> m Cancellable

Returns: a Cancellable.

Creates a new Cancellable object.

Applications that want to start one or more operations that should be cancellable should create a Cancellable and pass it to the operations.

One Cancellable can be used in multiple consecutive operations or in multiple concurrent operations.

popCurrent

cancellablePopCurrent

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: a Cancellable object

-> m ()

Pops cancellable off the cancellable stack (verifying that cancellable is on the top of the stack).

pushCurrent

cancellablePushCurrent

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: a Cancellable object

-> m ()

Pushes cancellable onto the cancellable stack. The current cancellable can then be received using cancellableGetCurrent.

This is useful when implementing cancellable operations in code that does not allow you to pass down the cancellable object.

This is typically called automatically by e.g. File operations, so you rarely have to call this yourself.

releaseFd

cancellableReleaseFd

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: a Cancellable

-> m ()

Releases a resources previously allocated by cancellableGetFd or cancellableMakePollfd.

For compatibility reasons with older releases, calling this function is not strictly required, the resources will be automatically freed when the cancellable is finalized. However, the cancellable will block scarce file descriptors until it is finalized if this function is not called. This can cause the application to run out of file descriptors when many GCancellables are used at the same time.

Since: 2.22

reset

cancellableReset

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: a Cancellable object.

-> m ()

Resets cancellable to its uncancelled state.

If cancellable is currently in use by any cancellable operation then the behavior of this function is undefined.

Note that it is generally not a good idea to reuse an existing cancellable for more operations after it has been cancelled once, as this function might tempt you to do. The recommended practice is to drop the reference to a cancellable after cancelling it, and let it die with the outstanding async operations. You should create a fresh cancellable for further async operations.

setErrorIfCancelled

cancellableSetErrorIfCancelled

Arguments

:: (HasCallStack, MonadIO m, IsCancellable a)

=> a

cancellable: a Cancellable or Nothing

-> m ()

(Can throw GError)

If the cancellable is cancelled, sets the error to notify that the operation was cancelled.

Signals

cancelled

type C_CancellableCancelledCallback = Ptr () -> Ptr () -> IO ()

Type for the callback on the (unwrapped) C side.

type CancellableCancelledCallback = IO ()

Emitted when the operation has been cancelled.

Can be used by implementations of cancellable operations. If the operation is cancelled from another thread, the signal will be emitted in the thread that cancelled the operation, not the thread that is running the operation.

Note that disconnecting from this signal (or any signal) in a multi-threaded program is prone to race conditions. For instance it is possible that a signal handler may be invoked even after a call to signalHandlerDisconnect for that handler has already returned.

There is also a problem when cancellation happens right before connecting to the signal. If this happens the signal will unexpectedly not be emitted, and checking before connecting to the signal leaves a race condition where this is still happening.

In order to make it safe and easy to connect handlers there are two helper functions: cancellableConnect and cancellableDisconnect which protect against problems like this.

An example of how to us this:

C code

   // Make sure we don't do unnecessary work if already cancelled
   if (g_cancellable_set_error_if_cancelled (cancellable, error))
     return;

   // Set up all the data needed to be able to handle cancellation
   // of the operation
   my_data = my_data_new (...);

   id = 0;
   if (cancellable)
     id = g_cancellable_connect (cancellable,
   			      G_CALLBACK (cancelled_handler)
   			      data, NULL);

   // cancellable operation here...

   g_cancellable_disconnect (cancellable, id);

   // cancelled_handler is never called after this, it is now safe
   // to free the data
   my_data_free (my_data);

Note that the cancelled signal is emitted in the thread that the user cancelled from, which may be the main thread. So, the cancellable signal should not do something that can block.

afterCancellableCancelled :: (IsCancellable a, MonadIO m) => a -> CancellableCancelledCallback -> m SignalHandlerId

Connect a signal handler for the “cancelled” signal, to be run after the default handler. When overloading is enabled, this is equivalent to

after cancellable #cancelled callback

genClosure_CancellableCancelled :: MonadIO m => CancellableCancelledCallback -> m (GClosure C_CancellableCancelledCallback)

Wrap the callback into a GClosure.

mk_CancellableCancelledCallback :: C_CancellableCancelledCallback -> IO (FunPtr C_CancellableCancelledCallback)

Generate a function pointer callable from C code, from a C_CancellableCancelledCallback.

noCancellableCancelledCallback :: Maybe CancellableCancelledCallback

A convenience synonym for Nothing :: Maybe CancellableCancelledCallback.

onCancellableCancelled :: (IsCancellable a, MonadIO m) => a -> CancellableCancelledCallback -> m SignalHandlerId

Connect a signal handler for the “cancelled” signal, to be run before the default handler. When overloading is enabled, this is equivalent to

on cancellable #cancelled callback

wrap_CancellableCancelledCallback :: CancellableCancelledCallback -> C_CancellableCancelledCallback

Wrap a CancellableCancelledCallback into a C_CancellableCancelledCallback.