// 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 stack

import (
	"fmt"
	"sync"
	"time"

	"gvisor.dev/gvisor/pkg/sleep"
	"gvisor.dev/gvisor/pkg/tcpip"
)

const linkAddrCacheSize = 512 // max cache entries

// linkAddrCache is a fixed-sized cache mapping IP addresses to link addresses.
//
// The entries are stored in a ring buffer, oldest entry replaced first.
//
// This struct is safe for concurrent use.
type linkAddrCache struct {
	// ageLimit is how long a cache entry is valid for.
	ageLimit time.Duration

	// resolutionTimeout is the amount of time to wait for a link request to
	// resolve an address.
	resolutionTimeout time.Duration

	// resolutionAttempts is the number of times an address is attempted to be
	// resolved before failing.
	resolutionAttempts int

	cache struct {
		sync.Mutex
		table map[tcpip.FullAddress]*linkAddrEntry
		lru   linkAddrEntryList
	}
}

// entryState controls the state of a single entry in the cache.
type entryState int

const (
	// incomplete means that there is an outstanding request to resolve the
	// address. This is the initial state.
	incomplete entryState = iota
	// ready means that the address has been resolved and can be used.
	ready
	// failed means that address resolution timed out and the address
	// could not be resolved.
	failed
)

// String implements Stringer.
func (s entryState) String() string {
	switch s {
	case incomplete:
		return "incomplete"
	case ready:
		return "ready"
	case failed:
		return "failed"
	default:
		return fmt.Sprintf("unknown(%d)", s)
	}
}

// A linkAddrEntry is an entry in the linkAddrCache.
// This struct is thread-compatible.
type linkAddrEntry struct {
	linkAddrEntryEntry

	addr       tcpip.FullAddress
	linkAddr   tcpip.LinkAddress
	expiration time.Time
	s          entryState

	// wakers is a set of waiters for address resolution result. Anytime
	// state transitions out of incomplete these waiters are notified.
	wakers map[*sleep.Waker]struct{}

	// done is used to allow callers to wait on address resolution. It is nil iff
	// s is incomplete and resolution is not yet in progress.
	done chan struct{}
}

// changeState sets the entry's state to ns, notifying any waiters.
//
// The entry's expiration is bumped up to the greater of itself and the passed
// expiration; the zero value indicates immediate expiration, and is set
// unconditionally - this is an implementation detail that allows for entries
// to be reused.
func (e *linkAddrEntry) changeState(ns entryState, expiration time.Time) {
	// Notify whoever is waiting on address resolution when transitioning
	// out of incomplete.
	if e.s == incomplete && ns != incomplete {
		for w := range e.wakers {
			w.Assert()
		}
		e.wakers = nil
		if ch := e.done; ch != nil {
			close(ch)
		}
		e.done = nil
	}

	if expiration.IsZero() || expiration.After(e.expiration) {
		e.expiration = expiration
	}
	e.s = ns
}

func (e *linkAddrEntry) removeWaker(w *sleep.Waker) {
	delete(e.wakers, w)
}

// add adds a k -> v mapping to the cache.
func (c *linkAddrCache) add(k tcpip.FullAddress, v tcpip.LinkAddress) {
	// Calculate expiration time before acquiring the lock, since expiration is
	// relative to the time when information was learned, rather than when it
	// happened to be inserted into the cache.
	expiration := time.Now().Add(c.ageLimit)

	c.cache.Lock()
	entry := c.getOrCreateEntryLocked(k)
	entry.linkAddr = v

	entry.changeState(ready, expiration)
	c.cache.Unlock()
}

// getOrCreateEntryLocked retrieves a cache entry associated with k. The
// returned entry is always refreshed in the cache (it is reachable via the
// map, and its place is bumped in LRU).
//
// If a matching entry exists in the cache, it is returned. If no matching
// entry exists and the cache is full, an existing entry is evicted via LRU,
// reset to state incomplete, and returned. If no matching entry exists and the
// cache is not full, a new entry with state incomplete is allocated and
// returned.
func (c *linkAddrCache) getOrCreateEntryLocked(k tcpip.FullAddress) *linkAddrEntry {
	if entry, ok := c.cache.table[k]; ok {
		c.cache.lru.Remove(entry)
		c.cache.lru.PushFront(entry)
		return entry
	}
	var entry *linkAddrEntry
	if len(c.cache.table) == linkAddrCacheSize {
		entry = c.cache.lru.Back()

		delete(c.cache.table, entry.addr)
		c.cache.lru.Remove(entry)

		// Wake waiters and mark the soon-to-be-reused entry as expired. Note
		// that the state passed doesn't matter when the zero time is passed.
		entry.changeState(failed, time.Time{})
	} else {
		entry = new(linkAddrEntry)
	}

	*entry = linkAddrEntry{
		addr: k,
		s:    incomplete,
	}
	c.cache.table[k] = entry
	c.cache.lru.PushFront(entry)
	return entry
}

// get reports any known link address for k.
func (c *linkAddrCache) get(k tcpip.FullAddress, linkRes LinkAddressResolver, localAddr tcpip.Address, linkEP LinkEndpoint, waker *sleep.Waker) (tcpip.LinkAddress, <-chan struct{}, *tcpip.Error) {
	if linkRes != nil {
		if addr, ok := linkRes.ResolveStaticAddress(k.Addr); ok {
			return addr, nil, nil
		}
	}

	c.cache.Lock()
	defer c.cache.Unlock()
	entry := c.getOrCreateEntryLocked(k)
	switch s := entry.s; s {
	case ready, failed:
		if !time.Now().After(entry.expiration) {
			// Not expired.
			switch s {
			case ready:
				return entry.linkAddr, nil, nil
			case failed:
				return entry.linkAddr, nil, tcpip.ErrNoLinkAddress
			default:
				panic(fmt.Sprintf("invalid cache entry state: %s", s))
			}
		}

		entry.changeState(incomplete, time.Time{})
		fallthrough
	case incomplete:
		if waker != nil {
			if entry.wakers == nil {
				entry.wakers = make(map[*sleep.Waker]struct{})
			}
			entry.wakers[waker] = struct{}{}
		}

		if entry.done == nil {
			// Address resolution needs to be initiated.
			if linkRes == nil {
				return entry.linkAddr, nil, tcpip.ErrNoLinkAddress
			}

			entry.done = make(chan struct{})
			go c.startAddressResolution(k, linkRes, localAddr, linkEP, entry.done) // S/R-SAFE: link non-savable; wakers dropped synchronously.
		}

		return entry.linkAddr, entry.done, tcpip.ErrWouldBlock
	default:
		panic(fmt.Sprintf("invalid cache entry state: %s", s))
	}
}

// removeWaker removes a waker previously added through get().
func (c *linkAddrCache) removeWaker(k tcpip.FullAddress, waker *sleep.Waker) {
	c.cache.Lock()
	defer c.cache.Unlock()

	if entry, ok := c.cache.table[k]; ok {
		entry.removeWaker(waker)
	}
}

func (c *linkAddrCache) startAddressResolution(k tcpip.FullAddress, linkRes LinkAddressResolver, localAddr tcpip.Address, linkEP LinkEndpoint, done <-chan struct{}) {
	for i := 0; ; i++ {
		// Send link request, then wait for the timeout limit and check
		// whether the request succeeded.
		linkRes.LinkAddressRequest(k.Addr, localAddr, linkEP)

		select {
		case now := <-time.After(c.resolutionTimeout):
			if stop := c.checkLinkRequest(now, k, i); stop {
				return
			}
		case <-done:
			return
		}
	}
}

// checkLinkRequest checks whether previous attempt to resolve address has succeeded
// and mark the entry accordingly, e.g. ready, failed, etc. Return true if request
// can stop, false if another request should be sent.
func (c *linkAddrCache) checkLinkRequest(now time.Time, k tcpip.FullAddress, attempt int) bool {
	c.cache.Lock()
	defer c.cache.Unlock()
	entry, ok := c.cache.table[k]
	if !ok {
		// Entry was evicted from the cache.
		return true
	}
	switch s := entry.s; s {
	case ready, failed:
		// Entry was made ready by resolver or failed. Either way we're done.
	case incomplete:
		if attempt+1 < c.resolutionAttempts {
			// No response yet, need to send another ARP request.
			return false
		}
		// Max number of retries reached, mark entry as failed.
		entry.changeState(failed, now.Add(c.ageLimit))
	default:
		panic(fmt.Sprintf("invalid cache entry state: %s", s))
	}
	return true
}

func newLinkAddrCache(ageLimit, resolutionTimeout time.Duration, resolutionAttempts int) *linkAddrCache {
	c := &linkAddrCache{
		ageLimit:           ageLimit,
		resolutionTimeout:  resolutionTimeout,
		resolutionAttempts: resolutionAttempts,
	}
	c.cache.table = make(map[tcpip.FullAddress]*linkAddrEntry, linkAddrCacheSize)
	return c
}