src/syscall/zx/internal/context/context.go

// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// This is a copy of context.Context with all dependencies on "time" and "fmt"
// removed in order to avoid a dependency cycle.
//
// TODO(fxb/43246) Break the dependency cycle when using context.Context and
// use that instead.

package context

import (
	"errors"
	"reflect"
	"sync"
)

type Context interface {
	Done() <-chan struct{}

	Err() error

	Value(key interface{}) interface{}
}

var Canceled = errors.New("context canceled")

type emptyCtx int

func (*emptyCtx) Done() <-chan struct{} {
	return nil
}

func (*emptyCtx) Err() error {
	return nil
}

func (*emptyCtx) Value(key interface{}) interface{} {
	return nil
}

var (
	background = new(emptyCtx)
	todo       = new(emptyCtx)
)

func Background() Context {
	return background
}

func TODO() Context {
	return todo
}

type CancelFunc func()

func WithCancel(parent Context) (ctx Context, cancel CancelFunc) {
	c := newCancelCtx(parent)
	propagateCancel(parent, &c)
	return &c, func() { c.cancel(true, Canceled) }
}

func newCancelCtx(parent Context) cancelCtx {
	return cancelCtx{Context: parent}
}

func propagateCancel(parent Context, child canceler) {
	if parent.Done() == nil {
		return // parent is never canceled
	}
	if p, ok := parentCancelCtx(parent); ok {
		p.mu.Lock()
		if p.err != nil {
			// parent has already been canceled
			child.cancel(false, p.err)
		} else {
			if p.children == nil {
				p.children = make(map[canceler]struct{})
			}
			p.children[child] = struct{}{}
		}
		p.mu.Unlock()
	} else {
		go func() {
			select {
			case <-parent.Done():
				child.cancel(false, parent.Err())
			case <-child.Done():
			}
		}()
	}
}

func parentCancelCtx(parent Context) (*cancelCtx, bool) {
	for {
		switch c := parent.(type) {
		case *cancelCtx:
			return c, true
		case *valueCtx:
			parent = c.Context
		default:
			return nil, false
		}
	}
}

func removeChild(parent Context, child canceler) {
	p, ok := parentCancelCtx(parent)
	if !ok {
		return
	}
	p.mu.Lock()
	if p.children != nil {
		delete(p.children, child)
	}
	p.mu.Unlock()
}

type canceler interface {
	cancel(removeFromParent bool, err error)
	Done() <-chan struct{}
}

var closedchan = make(chan struct{})

func init() {
	close(closedchan)
}

type cancelCtx struct {
	Context

	mu       sync.Mutex
	done     chan struct{}
	children map[canceler]struct{}
	err      error
}

func (c *cancelCtx) Done() <-chan struct{} {
	c.mu.Lock()
	if c.done == nil {
		c.done = make(chan struct{})
	}
	d := c.done
	c.mu.Unlock()
	return d
}

func (c *cancelCtx) Err() error {
	c.mu.Lock()
	err := c.err
	c.mu.Unlock()
	return err
}

func (c *cancelCtx) cancel(removeFromParent bool, err error) {
	if err == nil {
		panic("context: internal error: missing cancel error")
	}
	c.mu.Lock()
	if c.err != nil {
		c.mu.Unlock()
		return // already canceled
	}
	c.err = err
	if c.done == nil {
		c.done = closedchan
	} else {
		close(c.done)
	}
	for child := range c.children {
		// NOTE: acquiring the child's lock while holding parent's lock.
		child.cancel(false, err)
	}
	c.children = nil
	c.mu.Unlock()

	if removeFromParent {
		removeChild(c.Context, c)
	}
}

func WithValue(parent Context, key, val interface{}) Context {
	if key == nil {
		panic("nil key")
	}
	if !reflect.TypeOf(key).Comparable() {
		panic("key is not comparable")
	}
	return &valueCtx{parent, key, val}
}

type valueCtx struct {
	Context
	key, val interface{}
}

func (c *valueCtx) Value(key interface{}) interface{} {
	if c.key == key {
		return c.val
	}
	return c.Context.Value(key)
}