Documentation
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Overview ¶
Package waiter provides the implementation of a wait queue, where waiters can be enqueued to be notified when an event of interest happens.
Becoming readable and/or writable are examples of events. Waiters are expected to use a pattern similar to this to make a blocking function out of a non-blocking one:
func (o *object) blockingRead(...) error {
err := o.nonBlockingRead(...)
if err != ErrAgain {
// Completed with no need to wait!
return err
}
e := createOrGetWaiterEntry(...)
o.EventRegister(&e, waiter.EventIn)
defer o.EventUnregister(&e)
// We need to try to read again after registration because the
// object may have become readable between the last attempt to
// read and read registration.
err = o.nonBlockingRead(...)
for err == ErrAgain {
wait()
err = o.nonBlockingRead(...)
}
return err
}
Another goroutine needs to notify waiters when events happen. For example:
func (o *object) Write(...) ... {
// Do write work.
[...]
if oldDataAvailableSize == 0 && dataAvailableSize > 0 {
// If no data was available and now some data is
// available, the object became readable, so notify
// potential waiters about this.
o.Notify(waiter.EventIn)
}
}
Index ¶
Constants ¶
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Variables ¶
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Functions ¶
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Types ¶
type AlwaysReady ¶
type AlwaysReady struct {
}
AlwaysReady implements the Waitable interface but is always ready. Embedding this struct into another struct makes it implement the boilerplate empty functions automatically.
func (*AlwaysReady) EventRegister ¶
func (*AlwaysReady) EventRegister(*Entry, EventMask)
EventRegister doesn't do anything because this object doesn't need to issue notifications because its readiness never changes.
func (*AlwaysReady) EventUnregister ¶
func (*AlwaysReady) EventUnregister(e *Entry)
EventUnregister doesn't do anything because this object doesn't need to issue notifications because its readiness never changes.
func (*AlwaysReady) Readiness ¶
func (*AlwaysReady) Readiness(mask EventMask) EventMask
Readiness always returns the input mask because this object is always ready.
type Entry ¶
type Entry struct {
// Context stores any state the waiter may wish to store in the entry
// itself, which may be used at wake up time.
//
// Note that use of this field is optional and state may alternatively be
// stored in the callback itself.
Context interface{}
Callback EntryCallback
// contains filtered or unexported fields
}
Entry represents a waiter that can be add to the a wait queue. It can only be in one queue at a time, and is added "intrusively" to the queue with no extra memory allocations.
+stateify savable
func NewChannelEntry ¶
func NewChannelEntry(c chan struct{}) (Entry, chan struct{})
NewChannelEntry initializes a new Entry that does a non-blocking write to a struct{} channel when the callback is called. It returns the new Entry instance and the channel being used.
If a channel isn't specified (i.e., if "c" is nil), then NewChannelEntry allocates a new channel.
func (*Entry) Next ¶
func (e *Entry) Next() *Entry
Next returns the entry that follows e in the list.
func (*Entry) Prev ¶
func (e *Entry) Prev() *Entry
Prev returns the entry that precedes e in the list.
type EntryCallback ¶
type EntryCallback interface {
// Callback is the function to be called when the waiter entry is
// notified. It is responsible for doing whatever is needed to wake up
// the waiter.
//
// The callback is supposed to perform minimal work, and cannot call
// any method on the queue itself because it will be locked while the
// callback is running.
Callback(e *Entry)
}
EntryCallback provides a notify callback.
type EventMask ¶
type EventMask uint16
EventMask represents io events as used in the poll() syscall.
const ( EventIn EventMask = 0x01 // POLLIN EventPri EventMask = 0x02 // POLLPRI EventOut EventMask = 0x04 // POLLOUT EventErr EventMask = 0x08 // POLLERR EventHUp EventMask = 0x10 // POLLHUP )
Events that waiters can wait on. The meaning is the same as those in the poll() syscall.
func EventMaskFromLinux ¶
EventMaskFromLinux returns an EventMask representing the supported events from the Linux events e, which is in the format used by poll(2).
type Queue ¶
type Queue struct {
// contains filtered or unexported fields
}
Queue represents the wait queue where waiters can be added and notifiers can notify them when events happen.
The zero value for waiter.Queue is an empty queue ready for use.
+stateify savable
func (*Queue) EventRegister ¶
EventRegister adds a waiter to the wait queue; the waiter will be notified when at least one of the events specified in mask happens.
func (*Queue) EventUnregister ¶
EventUnregister removes the given waiter entry from the wait queue.
func (*Queue) Events ¶
Events returns the set of events being waited on. It is the union of the masks of all registered entries.
type Waitable ¶
type Waitable interface {
// Readiness returns what the object is currently ready for. If it's
// not ready for a desired purpose, the caller may use EventRegister and
// EventUnregister to get notifications once the object becomes ready.
//
// Implementations should allow for events like EventHUp and EventErr
// to be returned regardless of whether they are in the input EventMask.
Readiness(mask EventMask) EventMask
// EventRegister registers the given waiter entry to receive
// notifications when an event occurs that makes the object ready for
// at least one of the events in mask.
EventRegister(e *Entry, mask EventMask)
// EventUnregister unregisters a waiter entry previously registered with
// EventRegister().
EventUnregister(e *Entry)
}
Waitable contains the methods that need to be implemented by waitable objects.
Source Files