1 files changed, 250 insertions, 0 deletions
diff --git a/pkg/waiter/waiter.go b/pkg/waiter/waiter.go
new file mode 100644
index 000000000..707eb085b
--- /dev/null
+++ b/pkg/waiter/waiter.go
@@ -0,0 +1,250 @@
+// Copyright 2018 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// 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)
+//		}
+//	}
+package waiter
+
+import (
+	"gvisor.dev/gvisor/pkg/sync"
+)
+
+// EventMask represents io events as used in the poll() syscall.
+type EventMask uint64
+
+// Events that waiters can wait on. The meaning is the same as those 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
+
+	allEvents EventMask = 0x1f
+)
+
+// EventMaskFromLinux returns an EventMask representing the supported events
+// from the Linux events e, which is in the format used by poll(2).
+func EventMaskFromLinux(e uint32) EventMask {
+	// Our flag definitions are currently identical to Linux.
+	return EventMask(e) & allEvents
+}
+
+// ToLinux returns e in the format used by Linux poll(2).
+func (e EventMask) ToLinux() uint32 {
+	// Our flag definitions are currently identical to Linux.
+	return uint32(e)
+}
+
+// Waitable contains the methods that need to be implemented by waitable
+// objects.
+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)
+}
+
+// EntryCallback provides a notify callback.
+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)
+}
+
+// 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
+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
+
+	// The following fields are protected by the queue lock.
+	mask EventMask
+	waiterEntry
+}
+
+type channelCallback struct{}
+
+// Callback implements EntryCallback.Callback.
+func (*channelCallback) Callback(e *Entry) {
+	ch := e.Context.(chan struct{})
+	select {
+	case ch <- struct{}{}:
+	default:
+	}
+}
+
+// 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 NewChannelEntry(c chan struct{}) (Entry, chan struct{}) {
+	if c == nil {
+		c = make(chan struct{}, 1)
+	}
+
+	return Entry{Context: c, Callback: &channelCallback{}}, c
+}
+
+// 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
+type Queue struct {
+	list waiterList   `state:"zerovalue"`
+	mu   sync.RWMutex `state:"nosave"`
+}
+
+// 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 (q *Queue) EventRegister(e *Entry, mask EventMask) {
+	q.mu.Lock()
+	e.mask = mask
+	q.list.PushBack(e)
+	q.mu.Unlock()
+}
+
+// EventUnregister removes the given waiter entry from the wait queue.
+func (q *Queue) EventUnregister(e *Entry) {
+	q.mu.Lock()
+	q.list.Remove(e)
+	q.mu.Unlock()
+}
+
+// Notify notifies all waiters in the queue whose masks have at least one bit
+// in common with the notification mask.
+func (q *Queue) Notify(mask EventMask) {
+	q.mu.RLock()
+	for e := q.list.Front(); e != nil; e = e.Next() {
+		if mask&e.mask != 0 {
+			e.Callback.Callback(e)
+		}
+	}
+	q.mu.RUnlock()
+}
+
+// Events returns the set of events being waited on. It is the union of the
+// masks of all registered entries.
+func (q *Queue) Events() EventMask {
+	ret := EventMask(0)
+
+	q.mu.RLock()
+	for e := q.list.Front(); e != nil; e = e.Next() {
+		ret |= e.mask
+	}
+	q.mu.RUnlock()
+
+	return ret
+}
+
+// IsEmpty returns if the wait queue is empty or not.
+func (q *Queue) IsEmpty() bool {
+	q.mu.Lock()
+	defer q.mu.Unlock()
+
+	return q.list.Front() == nil
+}
+
+// AlwaysReady implements the Waitable interface but is always ready. Embedding
+// this struct into another struct makes it implement the boilerplate empty
+// functions automatically.
+type AlwaysReady struct {
+}
+
+// Readiness always returns the input mask because this object is always ready.
+func (*AlwaysReady) Readiness(mask EventMask) EventMask {
+	return mask
+}
+
+// EventRegister doesn't do anything because this object doesn't need to issue
+// notifications because its readiness never changes.
+func (*AlwaysReady) EventRegister(*Entry, EventMask) {
+}
+
+// EventUnregister doesn't do anything because this object doesn't need to issue
+// notifications because its readiness never changes.
+func (*AlwaysReady) EventUnregister(e *Entry) {
+}