// 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 context defines an internal context type.
//
// The given Context conforms to the standard Go context, but mandates
// additional methods that are specific to the kernel internals. Note however,
// that the Context described by this package carries additional constraints
// regarding concurrent access and retaining beyond the scope of a call.
//
// See the Context type for complete details.
package context

import (
	"context"
	"time"

	"inet.af/netstack/log"
)

// A Context represents a thread of execution (hereafter "goroutine" to reflect
// Go idiosyncrasy). It carries state associated with the goroutine across API
// boundaries.
//
// While Context exists for essentially the same reasons as Go's standard
// context.Context, the standard type represents the state of an operation
// rather than that of a goroutine. This is a critical distinction:
//
// - Unlike context.Context, which "may be passed to functions running in
// different goroutines", it is *not safe* to use the same Context in multiple
// concurrent goroutines.
//
// - It is *not safe* to retain a Context passed to a function beyond the scope
// of that function call.
//
// In both cases, values extracted from the Context should be used instead.
type Context interface {
	log.Logger
	context.Context

	ChannelSleeper

	// UninterruptibleSleepStart indicates the beginning of an uninterruptible
	// sleep state (equivalent to Linux's TASK_UNINTERRUPTIBLE). If deactivate
	// is true and the Context represents a Task, the Task's AddressSpace is
	// deactivated.
	UninterruptibleSleepStart(deactivate bool)

	// UninterruptibleSleepFinish indicates the end of an uninterruptible sleep
	// state that was begun by a previous call to UninterruptibleSleepStart. If
	// activate is true and the Context represents a Task, the Task's
	// AddressSpace is activated. Normally activate is the same value as the
	// deactivate parameter passed to UninterruptibleSleepStart.
	UninterruptibleSleepFinish(activate bool)
}

// A ChannelSleeper represents a goroutine that may sleep interruptibly, where
// interruption is indicated by a channel becoming readable.
type ChannelSleeper interface {
	// SleepStart is called before going to sleep interruptibly. If SleepStart
	// returns a non-nil channel and that channel becomes ready for receiving
	// while the goroutine is sleeping, the goroutine should be woken, and
	// SleepFinish(false) should be called. Otherwise, SleepFinish(true) should
	// be called after the goroutine stops sleeping.
	SleepStart() <-chan struct{}

	// SleepFinish is called after an interruptibly-sleeping goroutine stops
	// sleeping, as documented by SleepStart.
	SleepFinish(success bool)

	// Interrupted returns true if the channel returned by SleepStart is
	// ready for receiving.
	Interrupted() bool
}

// NoopSleeper is a noop implementation of ChannelSleeper and
// Context.UninterruptibleSleep* methods for anonymous embedding in other types
// that do not implement special behavior around sleeps.
type NoopSleeper struct{}

// SleepStart implements ChannelSleeper.SleepStart.
func (NoopSleeper) SleepStart() <-chan struct{} {
	return nil
}

// SleepFinish implements ChannelSleeper.SleepFinish.
func (NoopSleeper) SleepFinish(success bool) {}

// Interrupted implements ChannelSleeper.Interrupted.
func (NoopSleeper) Interrupted() bool {
	return false
}

// UninterruptibleSleepStart implements Context.UninterruptibleSleepStart.
func (NoopSleeper) UninterruptibleSleepStart(deactivate bool) {}

// UninterruptibleSleepFinish implements Context.UninterruptibleSleepFinish.
func (NoopSleeper) UninterruptibleSleepFinish(activate bool) {}

// Deadline implements context.Context.Deadline.
func (NoopSleeper) Deadline() (time.Time, bool) {
	return time.Time{}, false
}

// Done implements context.Context.Done.
func (NoopSleeper) Done() <-chan struct{} {
	return nil
}

// Err returns context.Context.Err.
func (NoopSleeper) Err() error {
	return nil
}

// logContext implements basic logging.
type logContext struct {
	log.Logger
	NoopSleeper
}

// Value implements Context.Value.
func (logContext) Value(key interface{}) interface{} {
	return nil
}

// bgContext is the context returned by context.Background.
var bgContext = &logContext{Logger: log.Log()}

// Background returns an empty context using the default logger.
// Generally, one should use the Task as their context when available, or avoid
// having to use a context in places where a Task is unavailable.
//
// Using a Background context for tests is fine, as long as no values are
// needed from the context in the tested code paths.
func Background() Context {
	return bgContext
}

// WithValue returns a copy of parent in which the value associated with key is
// val.
func WithValue(parent Context, key, val interface{}) Context {
	return &withValue{
		Context: parent,
		key:     key,
		val:     val,
	}
}

type withValue struct {
	Context
	key interface{}
	val interface{}
}

// Value implements Context.Value.
func (ctx *withValue) Value(key interface{}) interface{} {
	if key == ctx.key {
		return ctx.val
	}
	return ctx.Context.Value(key)
}