codeberg.org/gruf/go-mutexes@v1.5.0/map.go

package mutexes

import (
	"sync"
	"sync/atomic"
	"unsafe"

	"codeberg.org/gruf/go-mempool"
	"github.com/dolthub/swiss"
)

const (
	// possible lock types.
	lockTypeRead  = uint8(1) << 0
	lockTypeWrite = uint8(1) << 1
)

// MutexMap is a structure that allows read / write locking
// per key, performing as you'd expect a map[string]*RWMutex
// to perform, without you needing to worry about deadlocks
// between competing read / write locks and the map's own mutex.
// It uses memory pooling for the internal "mutex" (ish) types
// and performs self-eviction of keys.
//
// Under the hood this is achieved using a single mutex for the
// map, state tracking for individual keys, and some sync.Cond{}
// like structures for sleeping / awaking awaiting goroutines.
type MutexMap struct {
	mapmu  sync.Mutex
	mumap  *swiss.Map[string, *rwmutex]
	mupool mempool.UnsafePool
}

// checkInit ensures MutexMap is initialized (UNSAFE).
func (mm *MutexMap) checkInit() {
	if mm.mumap == nil {
		mm.mumap = swiss.NewMap[string, *rwmutex](0)
		mm.mupool.DirtyFactor = 256
	}
}

// Lock acquires a write lock on key in map, returning unlock function.
func (mm *MutexMap) Lock(key string) func() {
	return mm.lock(key, lockTypeWrite)
}

// RLock acquires a read lock on key in map, returning runlock function.
func (mm *MutexMap) RLock(key string) func() {
	return mm.lock(key, lockTypeRead)
}

func (mm *MutexMap) lock(key string, lt uint8) func() {
	// Perform first map lock
	// and check initialization
	// OUTSIDE the main loop.
	mm.mapmu.Lock()
	mm.checkInit()

	for {
		// Check map for mutex.
		mu, _ := mm.mumap.Get(key)

		if mu == nil {
			// Allocate mutex.
			mu = mm.acquire()
			mm.mumap.Put(key, mu)
		}

		if !mu.Lock(lt) {
			// Wait on mutex unlock, after
			// immediately relocking map mu.
			mu.WaitRelock(&mm.mapmu)
			continue
		}

		// Done with map.
		mm.mapmu.Unlock()

		// Return mutex unlock function.
		return func() { mm.unlock(key, mu) }
	}
}

func (mm *MutexMap) unlock(key string, mu *rwmutex) {
	// Get map lock.
	mm.mapmu.Lock()

	// Unlock mutex.
	if !mu.Unlock() {

		// Fast path. Mutex still
		// used so no map change.
		mm.mapmu.Unlock()
		return
	}

	// Mutex fully unlocked
	// with zero waiters. Self
	// evict and release it.
	mm.mumap.Delete(key)
	mm.release(mu)

	// Maximum load factor before
	// 'swiss' allocates new hmap:
	// maxLoad = 7 / 8
	//
	// So we apply the inverse/2, once
	// $maxLoad/2 % of hmap is empty we
	// compact the map to drop buckets.
	len := mm.mumap.Count()
	cap := mm.mumap.Capacity()
	if cap-len > (cap*7)/(8*2) {

		// Create a new map only as big as required.
		mumap := swiss.NewMap[string, *rwmutex](uint32(len))
		mm.mumap.Iter(func(k string, v *rwmutex) (stop bool) {
			mumap.Put(k, v)
			return false
		})

		// Set new map.
		mm.mumap = mumap
	}

	// Done with map.
	mm.mapmu.Unlock()
}

// acquire will acquire mutex from memory pool, or alloc new.
func (mm *MutexMap) acquire() *rwmutex {
	if ptr := mm.mupool.Get(); ptr != nil {
		return (*rwmutex)(ptr)
	}
	return new(rwmutex)
}

// release will release given mutex to memory pool.
func (mm *MutexMap) release(mu *rwmutex) {
	ptr := unsafe.Pointer(mu)
	mm.mupool.Put(ptr)
}

// rwmutex represents a RW mutex when used correctly within
// a MapMutex. It should ONLY be access when protected by
// the outer map lock, except for the 'notifyList' which is
// a runtime internal structure borrowed from the sync.Cond{}.
//
// this functions very similarly to a sync.Cond{}, but with
// lock state tracking, and returning on 'Broadcast()' whether
// any goroutines were actually awoken. it also has a less
// confusing API than sync.Cond{} with the outer locking
// mechanism we use, otherwise all Cond{}.L would reference
// the same outer map mutex.
type rwmutex struct {
	n notifyList // 'trigger' mechanism
	l int32      // no. locks
	t uint8      // lock type
}

// Lock will lock the mutex for given lock type, in the
// sense that it will update the internal state tracker
// accordingly. Return value is true on successful lock.
func (mu *rwmutex) Lock(lt uint8) bool {
	switch mu.t {
	case lockTypeRead:
		// already read locked,
		// only permit more reads.
		if lt != lockTypeRead {
			return false
		}

	case lockTypeWrite:
		// already write locked,
		// no other locks allowed.
		return false

	default:
		// Fully unlocked,
		// set incoming type.
		mu.t = lt
	}

	// Update
	// count.
	mu.l++

	return true
}

// Unlock will unlock the mutex, in the sense that it
// will update the internal state tracker accordingly.
// On totally unlocked state, it will awaken all
// sleeping goroutines waiting on this mutex.
func (mu *rwmutex) Unlock() bool {
	switch mu.l--; {
	case mu.l > 0 && mu.t == lockTypeWrite:
		panic("BUG: multiple writer locks")
	case mu.l < 0:
		panic("BUG: negative lock count")

	case mu.l == 0:
		// Fully unlocked.
		mu.t = 0

		// Awake all blocked goroutines and check
		// for change in the last notified ticket.
		before := atomic.LoadUint32(&mu.n.notify)
		runtime_notifyListNotifyAll(&mu.n)
		after := atomic.LoadUint32(&mu.n.notify)

		// If ticket changed, this indicates
		// AT LEAST one goroutine was awoken.
		//
		// (before != after) => (waiters > 0)
		// (before == after) => (waiters = 0)
		return (before == after)

	default:
		// i.e. mutex still
		// locked by others.
		return false
	}
}

// WaitRelock expects a mutex to be passed in, already in the
// locked state. It incr the notifyList waiter count before
// unlocking the outer mutex and blocking on notifyList wait.
// On awake it will decr wait count and relock outer mutex.
func (mu *rwmutex) WaitRelock(outer *sync.Mutex) {

	// add ourselves to list while still
	// under protection of outer map lock.
	t := runtime_notifyListAdd(&mu.n)

	// Finished with
	// outer map lock.
	outer.Unlock()

	// Block until awoken by another
	// goroutine within mu.Unlock().
	runtime_notifyListWait(&mu.n, t)

	// Relock!
	outer.Lock()
}