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pool.go
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pool.go
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package puddle
import (
"context"
"errors"
"sync"
"sync/atomic"
"time"
"github.com/jackc/puddle/v2/internal/circ"
"golang.org/x/sync/semaphore"
)
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[T any] func(ctx context.Context) (res T, err error)
// Destructor is a function called by the pool to destroy a resource.
type Destructor[T any] func(res T)
// Resource is the resource handle returned by acquiring from the pool.
type Resource[T any] struct {
value T
pool *Pool[T]
creationTime time.Time
lastUsedNano int64
poolResetCount int
status byte
}
// Value returns the resource value.
func (res *Resource[T]) Value() T {
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[T]) 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[T]) 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[T]) 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[T]) 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[T]) 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[T]) 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[T]) 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[T any] struct {
mux sync.Mutex
acquireSem *semaphore.Weighted
destructWG sync.WaitGroup
allResources []*Resource[T]
idleResources *circ.Queue[*Resource[T]]
constructor Constructor[T]
destructor Destructor[T]
maxSize int32
acquireCount int64
acquireDuration time.Duration
emptyAcquireCount int64
canceledAcquireCount atomic.Int64
resetCount int
baseAcquireCtx context.Context
cancelBaseAcquireCtx context.CancelFunc
closed bool
}
type Config[T any] struct {
Constructor Constructor[T]
Destructor Destructor[T]
MaxSize int32
}
// NewPool creates a new pool. Panics if maxSize is less than 1.
func NewPool[T any](config *Config[T]) (*Pool[T], error) {
if config.MaxSize < 1 {
return nil, errors.New("MaxSize must be >= 1")
}
baseAcquireCtx, cancelBaseAcquireCtx := context.WithCancel(context.Background())
return &Pool[T]{
acquireSem: semaphore.NewWeighted(int64(config.MaxSize)),
idleResources: circ.NewQueue[*Resource[T]](int(config.MaxSize)),
maxSize: config.MaxSize,
constructor: config.Constructor,
destructor: config.Destructor,
baseAcquireCtx: baseAcquireCtx,
cancelBaseAcquireCtx: cancelBaseAcquireCtx,
}, nil
}
// 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[T]) Close() {
defer p.destructWG.Wait()
p.mux.Lock()
defer p.mux.Unlock()
if p.closed {
return
}
p.closed = true
p.cancelBaseAcquireCtx()
for p.idleResources.Len() > 0 {
res := p.idleResources.Dequeue()
p.allResources = removeResource(p.allResources, res)
go p.destructResourceValue(res.value)
}
p.idleResources = nil
}
// 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
}
// TotalResources 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[T]) Stat() *Stat {
p.mux.Lock()
defer p.mux.Unlock()
s := &Stat{
maxResources: p.maxSize,
acquireCount: p.acquireCount,
emptyAcquireCount: p.emptyAcquireCount,
canceledAcquireCount: p.canceledAcquireCount.Load(),
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
}
}
return s
}
// tryAcquireIdleResource checks if there is any idle resource. If there is
// some, this method removes it from idle list and returns it. If the idle pool
// is empty, this method returns nil and doesn't modify the idleResources slice.
//
// WARNING: Caller of this method must hold the pool mutex!
func (p *Pool[T]) tryAcquireIdleResource() *Resource[T] {
if p.idleResources.Len() == 0 {
return nil
}
return p.idleResources.Dequeue()
}
// createNewResource creates a new resource and inserts it into list of pool
// resources.
//
// WARNING: Caller of this method must hold the pool mutex!
func (p *Pool[T]) createNewResource() *Resource[T] {
res := &Resource[T]{
pool: p,
creationTime: time.Now(),
lastUsedNano: nanotime(),
poolResetCount: p.resetCount,
status: resourceStatusConstructing,
}
p.allResources = append(p.allResources, res)
p.destructWG.Add(1)
return res
}
// 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.
//
// If Acquire creates a new resource the resource constructor function will receive a context that delegates Value() to
// ctx. Canceling ctx will cause Acquire to return immediately but it will not cancel the resource creation. This avoids
// the problem of it being impossible to create resources when the time to create a resource is greater than any one
// caller of Acquire is willing to wait.
func (p *Pool[T]) Acquire(ctx context.Context) (_ *Resource[T], err error) {
select {
case <-ctx.Done():
p.canceledAcquireCount.Add(1)
return nil, ctx.Err()
default:
}
return p.acquire(ctx)
}
// acquire is a continuation of Acquire function that doesn't check context
// validity. This function exists separatly only for benchmarking purposes.
func (p *Pool[T]) acquire(ctx context.Context) (*Resource[T], error) {
startNano := nanotime()
var waitedForLock bool
if !p.acquireSem.TryAcquire(1) {
waitedForLock = true
err := p.acquireSem.Acquire(ctx, 1)
if err != nil {
p.canceledAcquireCount.Add(1)
return nil, err
}
}
p.mux.Lock()
if p.closed {
p.mux.Unlock()
p.acquireSem.Release(1)
return nil, ErrClosedPool
}
// If a resource is available in the pool.
if res := p.tryAcquireIdleResource(); res != nil {
res.status = resourceStatusAcquired
if waitedForLock {
p.emptyAcquireCount += 1
}
p.acquireCount += 1
p.acquireDuration += time.Duration(nanotime() - startNano)
p.mux.Unlock()
return res, nil
}
if len(p.allResources) >= int(p.maxSize) {
p.mux.Unlock()
p.acquireSem.Release(1)
panic("bug: semaphore allowed more acquires than pool allows")
}
// The resource is not available, but there is enough space to create
// one.
res := p.createNewResource()
p.mux.Unlock()
// Create the resource in a goroutine to immediately return from Acquire if ctx is canceled without also canceling
// the constructor. See: https://github.com/jackc/pgx/issues/1287 and https://github.com/jackc/pgx/issues/1259
constructErrCh := make(chan error)
go func() {
constructorCtx := newValueCancelCtx(ctx, p.baseAcquireCtx)
value, err := p.constructResourceValue(constructorCtx)
if err != nil {
p.mux.Lock()
p.allResources = removeResource(p.allResources, res)
p.destructWG.Done()
p.mux.Unlock()
// We won't take the resource so we allow someone else
// to run Acquire.
p.acquireSem.Release(1)
select {
case constructErrCh <- err:
case <-ctx.Done():
// The caller is cancelled, so no-one awaits the
// error. This branch avoid goroutine leak.
}
return
}
res.value = value
res.status = resourceStatusAcquired
select {
case constructErrCh <- nil:
p.mux.Lock()
p.emptyAcquireCount += 1
p.acquireCount += 1
p.acquireDuration += time.Duration(nanotime() - startNano)
p.mux.Unlock()
case <-ctx.Done():
p.releaseAcquiredResource(res, res.lastUsedNano)
}
}()
select {
case <-ctx.Done():
p.canceledAcquireCount.Add(1)
return nil, ctx.Err()
case err := <-constructErrCh:
if err != nil {
return nil, err
}
return res, nil
}
}
// 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[T]) TryAcquire(ctx context.Context) (*Resource[T], error) {
if !p.acquireSem.TryAcquire(1) {
return nil, ErrNotAvailable
}
p.mux.Lock()
defer p.mux.Unlock()
if p.closed {
p.acquireSem.Release(1)
return nil, ErrClosedPool
}
// If a resource is available now
if res := p.tryAcquireIdleResource(); res != nil {
p.acquireCount += 1
res.status = resourceStatusAcquired
return res, nil
}
if len(p.allResources) >= int(p.maxSize) {
panic("bug: semaphore allowed more acquires than pool allows")
}
res := p.createNewResource()
go func() {
value, err := p.constructResourceValue(ctx)
// We have to create the resource and only then release the
// semaphore - For the time being there is no resource that
// someone could acquire.
defer p.acquireSem.Release(1)
p.mux.Lock()
defer p.mux.Unlock()
if err != nil {
p.allResources = removeResource(p.allResources, res)
p.destructWG.Done()
return
}
res.value = value
res.status = resourceStatusIdle
p.idleResources.Enqueue(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[T]) AcquireAllIdle() []*Resource[T] {
var cnt int
for p.acquireSem.TryAcquire(1) {
cnt++
}
if cnt == 0 {
return nil
}
resources := make([]*Resource[T], 0, cnt)
p.mux.Lock()
defer p.mux.Unlock()
if p.closed {
p.acquireSem.Release(int64(cnt))
return nil
}
// Some resources from the maxSize limit do not have to exist (i.e. be
// idle).
if diff := cnt - p.idleResources.Len(); diff > 0 {
p.acquireSem.Release(int64(diff))
cnt = p.idleResources.Len()
}
// We are not guaranteed that idleResources are empty after this loop.
// But we are guaranteed that all resources remain in idleResources
// after this loop will (1) either be acquired soon (semaphore was
// already acquired for them) or (2) were released after start of this
// function.
for i := 0; i < cnt; i++ {
res := p.idleResources.Dequeue()
res.status = resourceStatusAcquired
resources = append(resources, res)
}
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[T]) CreateResource(ctx context.Context) error {
p.mux.Lock()
if p.closed {
p.mux.Unlock()
return ErrClosedPool
}
p.destructWG.Add(1)
p.mux.Unlock()
value, err := p.constructResourceValue(ctx)
if err != nil {
p.destructWG.Done()
return err
}
res := &Resource[T]{
pool: p,
creationTime: time.Now(),
status: resourceStatusIdle,
value: value,
lastUsedNano: nanotime(),
poolResetCount: p.resetCount,
}
p.mux.Lock()
defer p.mux.Unlock()
// If closed while constructing resource then destroy it and return an error
if p.closed {
go p.destructResourceValue(res.value)
return ErrClosedPool
}
p.allResources = append(p.allResources, res)
p.idleResources.Enqueue(res)
return nil
}
// Reset destroys all resources, but leaves the pool open. It is intended for use when an error is detected that would
// disrupt all resources (such as a network interruption or a server state change).
//
// It is safe to reset a pool while resources are checked out. Those resources will be destroyed when they are returned
// to the pool.
func (p *Pool[T]) Reset() {
p.mux.Lock()
defer p.mux.Unlock()
p.resetCount++
for p.idleResources.Len() > 0 {
res := p.idleResources.Dequeue()
p.allResources = removeResource(p.allResources, res)
go p.destructResourceValue(res.value)
}
}
// releaseAcquiredResource returns res to the the pool.
func (p *Pool[T]) releaseAcquiredResource(res *Resource[T], lastUsedNano int64) {
defer p.acquireSem.Release(1)
p.mux.Lock()
defer p.mux.Unlock()
if p.closed || res.poolResetCount != p.resetCount {
p.allResources = removeResource(p.allResources, res)
go p.destructResourceValue(res.value)
} else {
res.lastUsedNano = lastUsedNano
res.status = resourceStatusIdle
p.idleResources.Enqueue(res)
}
}
// Remove removes res from the pool and closes it. If res is not part of the
// pool Remove will panic.
func (p *Pool[T]) destroyAcquiredResource(res *Resource[T]) {
p.destructResourceValue(res.value)
defer p.acquireSem.Release(1)
p.mux.Lock()
defer p.mux.Unlock()
p.allResources = removeResource(p.allResources, res)
}
func (p *Pool[T]) hijackAcquiredResource(res *Resource[T]) {
defer p.acquireSem.Release(1)
p.mux.Lock()
defer p.mux.Unlock()
p.allResources = removeResource(p.allResources, res)
res.status = resourceStatusHijacked
p.destructWG.Done() // not responsible for destructing hijacked resources
}
func removeResource[T any](slice []*Resource[T], res *Resource[T]) []*Resource[T] {
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[T]) constructResourceValue(ctx context.Context) (T, error) {
return p.constructor(ctx)
}
func (p *Pool[T]) destructResourceValue(value T) {
p.destructor(value)
p.destructWG.Done()
}