Gio.Cancellable¶

class — extends GObject.Object

GCancellable allows operations to be cancelled.

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

Constructors¶

new¶

@classmethod
def new(cls) -> 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.

Methods¶

cancel¶

def cancel(self) -> None

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 None, 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 GAsyncReadyCallback will not be invoked until the application returns to the main loop.

It is safe (although useless, since it will be a no-op) to call this function from a Cancellable.cancelled signal handler.

connect¶

def connect(self, callback: GObject.Callback) -> int

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 exactly once each time cancellable is cancelled, either directly at the time of the connect if cancellable is already cancelled, or when cancellable is cancelled in some thread. In case the cancellable is reset via Cancellable.reset then the callback can be called again if the cancellable is cancelled and if it had not been previously cancelled at the time Cancellable.connect was called (e.g. if the connection actually took place, returning a non-zero value).

data_destroy_func 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. Cancellable.cancel. Note that since 2.82 GLib still holds a lock during the callback but it’s designed in a way that most of the Cancellable methods can be called, including Cancellable.cancel or GObject.Object.unref.

There are still some methods that will deadlock (by design) when called from the Cancellable.cancelled callbacks: - Cancellable.connect - Cancellable.disconnect - Cancellable.reset - Cancellable.make_pollfd - Cancellable.release_fd

Parameters:

callback — The GCallback to connect.

disconnect¶

def disconnect(self, handler_id: int) -> None

Disconnects a handler from a cancellable instance similar to GObject.signal_handler_disconnect. 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 None or handler_id is 0 this function does nothing.

Parameters:

handler_id — Handler id of the handler to be disconnected, or 0.

get_fd¶

def get_fd(self) -> int

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 Cancellable.reset.

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

is_cancelled¶

def is_cancelled(self) -> bool

Checks if a cancellable job has been cancelled.

make_pollfd¶

def make_pollfd(self, pollfd: GLib.PollFD) -> bool

Creates a GLib.PollFD corresponding to cancellable; this can be passed to GLib.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 Cancellable.release_fd to free up resources allocated for the pollfd. After a False return, do not call Cancellable.release_fd.

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 Cancellable.reset.

Note that in the event that a Cancellable.cancelled signal handler is currently running, this call will block until the handler has finished. Calling this function from a signal handler will therefore result in a deadlock.

Parameters:

pollfd — a pointer to a GLib.PollFD

pop_current¶

def pop_current(self) -> None

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

push_current¶

def push_current(self) -> None

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

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.

release_fd¶

def release_fd(self) -> None

Releases a resources previously allocated by Cancellable.get_fd or Cancellable.make_pollfd.

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.

Note that in the event that a Cancellable.cancelled signal handler is currently running, this call will block until the handler has finished. Calling this function from a signal handler will therefore result in a deadlock.

reset¶

def reset(self) -> None

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.

In the event that a Cancellable.cancelled signal handler is currently running, this call will block until the handler has finished. Calling this function from a signal handler will therefore result in a deadlock.

set_error_if_cancelled¶

def set_error_if_cancelled(self) -> bool

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

source_new¶

def source_new(self) -> GLib.Source

Creates a source that triggers if cancellable is cancelled and calls its callback of type GCancellableSourceFunc. This is primarily useful for attaching to another (non-cancellable) source with GLib.Source.add_child_source to add cancellability to it.

For convenience, you can call this with a None Cancellable, in which case the source will never trigger.

The new GLib.Source will hold a reference to the Cancellable.

Static functions¶

get_current¶

@staticmethod
def get_current() -> Cancellable | None

Gets the top cancellable from the stack.

Virtual methods¶

do_cancelled¶

def do_cancelled(self) -> None

Signals¶

cancelled¶

def on_cancelled(self) -> None: ...

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 GObject.signal_handler_disconnect 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: Cancellable.connect and Cancellable.disconnect which protect against problems like this.

An example of how to us this:

// 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.