puddle/pool.go

package puddle

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

const (
	resourceStatusCreating  = 0
	resourceStatusAvailable = iota
	resourceStatusBorrowed  = iota
)

const maxUint = ^uint(0)
const maxInt = int(maxUint >> 1)

// ErrClosedPool occurs on an attempt to get a connection from a closed pool.
var ErrClosedPool = errors.New("cannot get from closed pool")

type CreateFunc func() (res interface{}, err error)
type CloseFunc func(res interface{}) (err error)

// BackgroundErrorHandler is the type of function that handles background
// errors. It may be called while the pool is locked. Therefore it must not call
// any pool methods and should not perform any lengthy operations.
type BackgroundErrorHandler func(err error)

type resourceWrapper struct {
	resource interface{}
	status   byte
}

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

	allResources       map[interface{}]*resourceWrapper
	availableResources []*resourceWrapper
	maxSize            int
	closed             bool

	create                 CreateFunc
	closeRes               CloseFunc
	backgroundErrorHandler BackgroundErrorHandler
}

func NewPool(create CreateFunc, closeRes CloseFunc) *Pool {
	return &Pool{
		cond:                   sync.NewCond(new(sync.Mutex)),
		allResources:           make(map[interface{}]*resourceWrapper),
		maxSize:                maxInt,
		create:                 create,
		closeRes:               closeRes,
		backgroundErrorHandler: func(error) {},
	}
}

// Close closes all resources in the pool and rejects future Get calls.
// Unavailable resources will be closes when they are returned to the pool.
func (p *Pool) Close() {
	p.cond.L.Lock()
	p.closed = true

	for _, rw := range p.availableResources {
		err := p.closeRes(rw.resource)
		if err != nil {
			p.backgroundErrorHandler(err)
		}
		delete(p.allResources, rw.resource)
	}
	p.availableResources = nil
	p.cond.L.Unlock()

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

// Size returns the current size of the pool.
func (p *Pool) Size() int {
	p.cond.L.Lock()
	n := len(p.allResources)
	p.cond.L.Unlock()
	return n
}

// MaxSize returns the current maximum size of the pool.
func (p *Pool) MaxSize() int {
	p.cond.L.Lock()
	n := p.maxSize
	p.cond.L.Unlock()
	return n
}

// SetMaxSize sets the maximum size of the pool. It panics if n < 1.
func (p *Pool) SetMaxSize(n int) {
	if n < 1 {
		panic("pool MaxSize cannot be < 1")
	}
	p.cond.L.Lock()
	p.maxSize = n
	p.cond.L.Unlock()
}

// SetBackgroundErrorHandler assigns a handler for errors that have no other
// place to be reported. For example, Get is called when no resources are
// available. Get begins creating a new resource (in a goroutine). Before the
// new resource is completed, the context passed to Get is canceled. Then the
// new resource creation fails. f will be called with that error.
func (p *Pool) SetBackgroundErrorHandler(f BackgroundErrorHandler) {
	p.cond.L.Lock()
	p.backgroundErrorHandler = f
	p.cond.L.Unlock()
}

// Get 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 Get.
func (p *Pool) Get(ctx context.Context) (interface{}, error) {
	if doneChan := ctx.Done(); doneChan != nil {
		select {
		case <-ctx.Done():
			return nil, ctx.Err()
		default:
		}
	}

	p.cond.L.Lock()

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

	// If a resource is available now
	if len(p.availableResources) > 0 {
		res := p.lockedAvailableGet()
		p.cond.L.Unlock()
		return res, nil
	}

	// If there is room to create a resource start the process asynchronously
	var createResChan chan interface{}
	var createErrChan chan error
	if len(p.allResources) < p.maxSize {
		createResChan, createErrChan = p.startCreate()
	}
	p.cond.L.Unlock()

	// Wait for a resource to be returned to the pool.
	waitResChan := make(chan interface{})
	abortWaitResChan := make(chan struct{})
	go func() {
		p.cond.L.Lock()
		for len(p.availableResources) == 0 {
			p.cond.Wait()
		}
		if p.closed {
			p.cond.L.Unlock()
			return
		}
		res := p.lockedAvailableGet()
		p.cond.L.Unlock()

		select {
		case <-abortWaitResChan:
			p.Return(res)
		case waitResChan <- res:
		}
	}()

	select {
	case <-ctx.Done():
		close(abortWaitResChan)
		p.backgroundFinishCreate(createResChan, createErrChan)
		return nil, ctx.Err()
	case err := <-createErrChan:
		close(abortWaitResChan)
		return nil, err
	case res := <-createResChan:
		close(abortWaitResChan)
		return res, nil
	case res := <-waitResChan:
		p.backgroundFinishCreate(createResChan, createErrChan)
		return res, nil
	}
}

// func (p *Pool) backgroundReportError(errChan chan error) {
// 	go func() {
// 		err := <-errChan
// 		if err != nil {
// 			p.cond.L.Lock()
// 			p.backgroundErrorHandler(err)
// 			p.cond.L.Unlock()
// 		}
// 	}()
// }

// lockedAvailableGet gets the top resource from p.availableResources. p.cond.L
// must already be locked. len(p.availableResources) must be > 0.
func (p *Pool) lockedAvailableGet() interface{} {
	rw := p.availableResources[len(p.availableResources)-1]
	p.availableResources = p.availableResources[:len(p.availableResources)-1]
	if rw.status != resourceStatusAvailable {
		panic("BUG: unavailable resource gotten from availableResources")
	}
	rw.status = resourceStatusBorrowed
	return rw.resource
}

// startCreate starts creating a new resource. p.cond.L must already be
// locked. The newly created resource will be sent on resChan (already checked
// out) or an error will be sent on errChan.
func (p *Pool) startCreate() (resChan chan interface{}, errChan chan error) {
	resChan = make(chan interface{})
	errChan = make(chan error)

	var localVal int
	placeholder := &localVal
	p.allResources[placeholder] = &resourceWrapper{resource: placeholder, status: resourceStatusCreating}

	go func() {
		res, err := p.create()
		p.cond.L.Lock()
		delete(p.allResources, placeholder)
		if err != nil {
			p.cond.L.Unlock()
			errChan <- err
			return
		}

		rw := &resourceWrapper{resource: res, status: resourceStatusBorrowed}
		p.allResources[res] = rw
		p.cond.L.Unlock()
		resChan <- res
	}()

	return resChan, errChan
}

func (p *Pool) backgroundFinishCreate(resChan chan interface{}, errChan chan error) {
	go func() {
		select {
		case res := <-resChan:
			p.Return(res)
		case err := <-errChan:
			p.cond.L.Lock()
			p.backgroundErrorHandler(err)
			p.cond.L.Unlock()
		}
	}()
}

// Return returns res to the the pool. If res is not part of the pool Return
// will panic.
func (p *Pool) Return(res interface{}) {
	p.cond.L.Lock()

	rw, present := p.allResources[res]
	if !present {
		p.cond.L.Unlock()
		panic("Return called on resource that does not belong to pool")
	}

	if p.closed {
		err := p.closeRes(rw.resource)
		if err != nil {
			p.backgroundErrorHandler(err)
		}
		delete(p.allResources, rw.resource)
		p.cond.L.Unlock()
		return
	}

	rw.status = resourceStatusAvailable
	p.availableResources = append(p.availableResources, rw)

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