pool.go

package puddle

import (
	"context"
	"errors"
	"sync"
	"time"
)

const (
	resourceStatusConstructing = 0
	resourceStatusIdle         = iota
	resourceStatusAcquired     = iota
	resourceStatusHijacked     = iota
)

// ErrClosedPool occurs on an attempt to acquire a connection from a closed pool
// or a pool that is closed while the acquire is waiting.
var ErrClosedPool = errors.New("closed pool")

// ErrNotAvailable occurs on an attempt to acquire a resource from a pool
// that is at maximum capacity and has no available resources.
var ErrNotAvailable = errors.New("resource not available")

// Constructor is a function called by the pool to construct a resource.
type Constructor func(ctx context.Context) (res interface{}, err error)

// Destructor is a function called by the pool to destroy a resource.
type Destructor func(res interface{})

// Resource is the resource handle returned by acquiring from the pool.
type Resource struct {
	value        interface{}
	pool         *Pool
	creationTime time.Time
	lastUsedNano int64
	status       byte
}

// Value returns the resource value.
func (res *Resource) Value() interface{} {
	if !(res.status == resourceStatusAcquired || res.status == resourceStatusHijacked) {
		panic("tried to access resource that is not acquired or hijacked")
	}
	return res.value
}

// Release returns the resource to the pool. res must not be subsequently used.
func (res *Resource) Release() {
	if res.status != resourceStatusAcquired {
		panic("tried to release resource that is not acquired")
	}
	res.pool.releaseAcquiredResource(res, nanotime())
}

// ReleaseUnused returns the resource to the pool without updating when it was last used used. i.e. LastUsedNanotime
// will not change. res must not be subsequently used.
func (res *Resource) ReleaseUnused() {
	if res.status != resourceStatusAcquired {
		panic("tried to release resource that is not acquired")
	}
	res.pool.releaseAcquiredResource(res, res.lastUsedNano)
}

// Destroy returns the resource to the pool for destruction. res must not be
// subsequently used.
func (res *Resource) Destroy() {
	if res.status != resourceStatusAcquired {
		panic("tried to destroy resource that is not acquired")
	}
	go res.pool.destroyAcquiredResource(res)
}

// Hijack assumes ownership of the resource from the pool. Caller is responsible
// for cleanup of resource value.
func (res *Resource) Hijack() {
	if res.status != resourceStatusAcquired {
		panic("tried to hijack resource that is not acquired")
	}
	res.pool.hijackAcquiredResource(res)
}

// CreationTime returns when the resource was created by the pool.
func (res *Resource) CreationTime() time.Time {
	if !(res.status == resourceStatusAcquired || res.status == resourceStatusHijacked) {
		panic("tried to access resource that is not acquired or hijacked")
	}
	return res.creationTime
}

// LastUsedNanotime returns when Release was last called on the resource measured in nanoseconds from an arbitrary time
// (a monotonic time). Returns creation time if Release has never been called. This is only useful to compare with
// other calls to LastUsedNanotime. In almost all cases, IdleDuration should be used instead.
func (res *Resource) LastUsedNanotime() int64 {
	if !(res.status == resourceStatusAcquired || res.status == resourceStatusHijacked) {
		panic("tried to access resource that is not acquired or hijacked")
	}

	return res.lastUsedNano
}

// IdleDuration returns the duration since Release was last called on the resource. This is equivalent to subtracting
// LastUsedNanotime to the current nanotime.
func (res *Resource) IdleDuration() time.Duration {
	if !(res.status == resourceStatusAcquired || res.status == resourceStatusHijacked) {
		panic("tried to access resource that is not acquired or hijacked")
	}

	return time.Duration(nanotime() - res.lastUsedNano)
}

// Pool is a concurrency-safe resource pool.
type Pool struct {
	cond       *sync.Cond
	destructWG *sync.WaitGroup

	allResources  []*Resource
	idleResources []*Resource

	constructor Constructor
	destructor  Destructor
	maxSize     int32

	acquireCount         int64
	acquireDuration      time.Duration
	emptyAcquireCount    int64
	canceledAcquireCount int64

	closed bool
}

// NewPool creates a new pool. Panics if maxSize is less than 1.
func NewPool(constructor Constructor, destructor Destructor, maxSize int32) *Pool {
	if maxSize < 1 {
		panic("maxSize is less than 1")
	}

	return &Pool{
		cond:        sync.NewCond(new(sync.Mutex)),
		destructWG:  &sync.WaitGroup{},
		maxSize:     maxSize,
		constructor: constructor,
		destructor:  destructor,
	}
}

// Close destroys all resources in the pool and rejects future Acquire calls.
// Blocks until all resources are returned to pool and destroyed.
func (p *Pool) Close() {
	p.cond.L.Lock()
	if p.closed {
		p.cond.L.Unlock()
		return
	}
	p.closed = true

	for _, res := range p.idleResources {
		p.allResources = removeResource(p.allResources, res)
		go p.destructResourceValue(res.value)
	}
	p.idleResources = nil
	p.cond.L.Unlock()

	// Wake up all go routines waiting for a resource to be returned so they can terminate.
	p.cond.Broadcast()

	p.destructWG.Wait()
}

// Stat is a snapshot of Pool statistics.
type Stat struct {
	constructingResources int32
	acquiredResources     int32
	idleResources         int32
	maxResources          int32
	acquireCount          int64
	acquireDuration       time.Duration
	emptyAcquireCount     int64
	canceledAcquireCount  int64
}

// TotalResource returns the total number of resources currently in the pool.
// The value is the sum of ConstructingResources, AcquiredResources, and
// IdleResources.
func (s *Stat) TotalResources() int32 {
	return s.constructingResources + s.acquiredResources + s.idleResources
}

// ConstructingResources returns the number of resources with construction in progress in
// the pool.
func (s *Stat) ConstructingResources() int32 {
	return s.constructingResources
}

// AcquiredResources returns the number of currently acquired resources in the pool.
func (s *Stat) AcquiredResources() int32 {
	return s.acquiredResources
}

// IdleResources returns the number of currently idle resources in the pool.
func (s *Stat) IdleResources() int32 {
	return s.idleResources
}

// MaxResources returns the maximum size of the pool.
func (s *Stat) MaxResources() int32 {
	return s.maxResources
}

// AcquireCount returns the cumulative count of successful acquires from the pool.
func (s *Stat) AcquireCount() int64 {
	return s.acquireCount
}

// AcquireDuration returns the total duration of all successful acquires from
// the pool.
func (s *Stat) AcquireDuration() time.Duration {
	return s.acquireDuration
}

// EmptyAcquireCount returns the cumulative count of successful acquires from the pool
// that waited for a resource to be released or constructed because the pool was
// empty.
func (s *Stat) EmptyAcquireCount() int64 {
	return s.emptyAcquireCount
}

// CanceledAcquireCount returns the cumulative count of acquires from the pool
// that were canceled by a context.
func (s *Stat) CanceledAcquireCount() int64 {
	return s.canceledAcquireCount
}

// Stat returns the current pool statistics.
func (p *Pool) Stat() *Stat {
	p.cond.L.Lock()
	s := &Stat{
		maxResources:         p.maxSize,
		acquireCount:         p.acquireCount,
		emptyAcquireCount:    p.emptyAcquireCount,
		canceledAcquireCount: p.canceledAcquireCount,
		acquireDuration:      p.acquireDuration,
	}

	for _, res := range p.allResources {
		switch res.status {
		case resourceStatusConstructing:
			s.constructingResources += 1
		case resourceStatusIdle:
			s.idleResources += 1
		case resourceStatusAcquired:
			s.acquiredResources += 1
		}
	}

	p.cond.L.Unlock()
	return s
}

// Acquire gets a resource from the pool. If no resources are available and the pool
// is not at maximum capacity it will create a new resource. If the pool is at
// maximum capacity it will block until a resource is available. ctx can be used
// to cancel the Acquire.
func (p *Pool) Acquire(ctx context.Context) (*Resource, error) {
	startNano := nanotime()
	if doneChan := ctx.Done(); doneChan != nil {
		select {
		case <-ctx.Done():
			p.cond.L.Lock()
			p.canceledAcquireCount += 1
			p.cond.L.Unlock()
			return nil, ctx.Err()
		default:
		}
	}

	p.cond.L.Lock()

	emptyAcquire := false

	for {
		if p.closed {
			p.cond.L.Unlock()
			return nil, ErrClosedPool
		}

		// If a resource is available now
		if len(p.idleResources) > 0 {
			res := p.idleResources[len(p.idleResources)-1]
			p.idleResources[len(p.idleResources)-1] = nil // Avoid memory leak
			p.idleResources = p.idleResources[:len(p.idleResources)-1]
			res.status = resourceStatusAcquired
			if emptyAcquire {
				p.emptyAcquireCount += 1
			}
			p.acquireCount += 1
			p.acquireDuration += time.Duration(nanotime() - startNano)
			p.cond.L.Unlock()
			return res, nil
		}

		emptyAcquire = true

		// If there is room to create a resource do so
		if len(p.allResources) < int(p.maxSize) {
			res := &Resource{pool: p, creationTime: time.Now(), lastUsedNano: nanotime(), status: resourceStatusConstructing}
			p.allResources = append(p.allResources, res)
			p.destructWG.Add(1)
			p.cond.L.Unlock()

			// we create the resource in the background because the constructor might
			// outlive the context and we want to continue constructing it as long as
			// necessary but the acquire should be cancelled when the context is cancelled
			// see: https://github.com/jackc/pgx/issues/1287 and https://github.com/jackc/pgx/issues/1259
			constructErrCh := make(chan error)
			go func() {
				value, err := p.constructResourceValue(ctx)
				p.cond.L.Lock()
				if err != nil {
					p.allResources = removeResource(p.allResources, res)
					p.destructWG.Done()

					// we can't use default here in case we get here before the caller is
					// in the select
					select {
					case constructErrCh <- err:
					case <-ctx.Done():
						p.canceledAcquireCount += 1
					}
					p.cond.L.Unlock()
					p.cond.Signal()
					return
				}
				res.value = value

				// assume that we will acquire it
				res.status = resourceStatusAcquired
				// we can't use default here in case we get here before the caller is
				// in the select
				select {
				case constructErrCh <- nil:
					p.emptyAcquireCount += 1
					p.acquireCount += 1
					p.acquireDuration += time.Duration(nanotime() - startNano)
					p.cond.L.Unlock()
					// we don't call Signal here we didn't change any of the resource pools
				case <-ctx.Done():
					p.canceledAcquireCount += 1
					p.cond.L.Unlock()
					// we don't call Signal here we didn't change any of the resopurce pools
					// since we couldn't send the constructed resource to the acquire
					// function that means the caller has stopped waiting and we should
					// just put this resource back in the pool
					p.releaseAcquiredResource(res, res.lastUsedNano)
				}
			}()

			select {
			case <-ctx.Done():
				return nil, ctx.Err()
			case err := <-constructErrCh:
				if err != nil {
					return nil, err
				}
				// we don't call signal here because we didn't change the resource pools
				// at all so waking anything else up won't help
				return res, nil
			}
		}

		if ctx.Done() == nil {
			p.cond.Wait()
		} else {
			// Convert p.cond.Wait into a channel
			waitChan := make(chan struct{}, 1)
			go func() {
				p.cond.Wait()
				waitChan <- struct{}{}
			}()

			select {
			case <-ctx.Done():
				// Allow goroutine waiting for signal to exit. Re-signal since we couldn't
				// do anything with it. Another goroutine might be waiting.
				go func() {
					<-waitChan
					p.cond.L.Unlock()
					p.cond.Signal()
				}()

				p.cond.L.Lock()
				p.canceledAcquireCount += 1
				p.cond.L.Unlock()
				return nil, ctx.Err()
			case <-waitChan:
			}
		}
	}
}

// TryAcquire gets a resource from the pool if one is immediately available. If not, it returns ErrNotAvailable. If no
// resources are available but the pool has room to grow, a resource will be created in the background. ctx is only
// used to cancel the background creation.
func (p *Pool) TryAcquire(ctx context.Context) (*Resource, error) {
	p.cond.L.Lock()
	defer p.cond.L.Unlock()

	if p.closed {
		return nil, ErrClosedPool
	}

	// If a resource is available now
	if len(p.idleResources) > 0 {
		res := p.idleResources[len(p.idleResources)-1]
		p.idleResources[len(p.idleResources)-1] = nil // Avoid memory leak
		p.idleResources = p.idleResources[:len(p.idleResources)-1]
		p.acquireCount += 1
		res.status = resourceStatusAcquired
		return res, nil
	}

	if len(p.allResources) < int(p.maxSize) {
		res := &Resource{pool: p, creationTime: time.Now(), lastUsedNano: nanotime(), status: resourceStatusConstructing}
		p.allResources = append(p.allResources, res)
		p.destructWG.Add(1)

		go func() {
			value, err := p.constructResourceValue(ctx)
			defer p.cond.Signal()
			p.cond.L.Lock()
			defer p.cond.L.Unlock()

			if err != nil {
				p.allResources = removeResource(p.allResources, res)
				p.destructWG.Done()
				return
			}

			res.value = value
			res.status = resourceStatusIdle
			p.idleResources = append(p.idleResources, res)
		}()
	}

	return nil, ErrNotAvailable
}

// AcquireAllIdle atomically acquires all currently idle resources. Its intended
// use is for health check and keep-alive functionality. It does not update pool
// statistics.
func (p *Pool) AcquireAllIdle() []*Resource {
	p.cond.L.Lock()
	if p.closed {
		p.cond.L.Unlock()
		return nil
	}

	for _, res := range p.idleResources {
		res.status = resourceStatusAcquired
	}
	resources := p.idleResources // Swap out current slice
	p.idleResources = nil

	p.cond.L.Unlock()
	return resources
}

// CreateResource constructs a new resource without acquiring it.
// It goes straight in the IdlePool. It does not check against maxSize.
// It can be useful to maintain warm resources under little load.
func (p *Pool) CreateResource(ctx context.Context) error {
	p.cond.L.Lock()
	if p.closed {
		p.cond.L.Unlock()
		return ErrClosedPool
	}
	p.cond.L.Unlock()

	value, err := p.constructResourceValue(ctx)
	if err != nil {
		return err
	}

	res := &Resource{
		pool:         p,
		creationTime: time.Now(),
		status:       resourceStatusIdle,
		value:        value,
		lastUsedNano: nanotime(),
	}
	p.destructWG.Add(1)

	p.cond.L.Lock()
	// If closed while constructing resource then destroy it and return an error
	if p.closed {
		go p.destructResourceValue(res.value)
		p.cond.L.Unlock()
		return ErrClosedPool
	}
	p.allResources = append(p.allResources, res)
	p.idleResources = append(p.idleResources, res)
	p.cond.L.Unlock()

	return nil
}

// releaseAcquiredResource returns res to the the pool.
func (p *Pool) releaseAcquiredResource(res *Resource, lastUsedNano int64) {
	p.cond.L.Lock()

	if !p.closed {
		res.lastUsedNano = lastUsedNano
		res.status = resourceStatusIdle
		p.idleResources = append(p.idleResources, res)
	} else {
		p.allResources = removeResource(p.allResources, res)
		go p.destructResourceValue(res.value)
	}

	p.cond.L.Unlock()
	p.cond.Signal()
}

// Remove removes res from the pool and closes it. If res is not part of the
// pool Remove will panic.
func (p *Pool) destroyAcquiredResource(res *Resource) {
	p.destructResourceValue(res.value)
	p.cond.L.Lock()
	p.allResources = removeResource(p.allResources, res)
	p.cond.L.Unlock()
	p.cond.Signal()
}

func (p *Pool) hijackAcquiredResource(res *Resource) {
	p.cond.L.Lock()

	p.allResources = removeResource(p.allResources, res)
	res.status = resourceStatusHijacked
	p.destructWG.Done() // not responsible for destructing hijacked resources

	p.cond.L.Unlock()
	p.cond.Signal()
}

func removeResource(slice []*Resource, res *Resource) []*Resource {
	for i := range slice {
		if slice[i] == res {
			slice[i] = slice[len(slice)-1]
			slice[len(slice)-1] = nil // Avoid memory leak
			return slice[:len(slice)-1]
		}
	}

	panic("BUG: removeResource could not find res in slice")
}

func (p *Pool) constructResourceValue(ctx context.Context) (interface{}, error) {
	return p.constructor(ctx)
}

func (p *Pool) destructResourceValue(value interface{}) {
	p.destructor(value)
	p.destructWG.Done()
}