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// Copyright (c) Tailscale Inc & contributors
// SPDX-License-Identifier: BSD-3-Clause
// Package pool contains a generic type for managing a pool of resources; for
// example, connections to a database, or to a remote service.
//
// Unlike sync.Pool from the Go standard library, this pool does not remove
// items from the pool when garbage collection happens, nor is it safe for
// concurrent use like sync.Pool.
package pool
import (
"fmt"
"math/rand/v2"
)
// consistencyCheck enables additional runtime checks to ensure that the pool
// is well-formed; it is disabled by default, and can be enabled during tests
// to catch additional bugs.
const consistencyCheck = false
// Pool is a pool of resources. It is not safe for concurrent use.
type Pool[V any] struct {
s []itemAndIndex[V]
}
type itemAndIndex[V any] struct {
// item is the element in the pool
item V
// index is the current location of this item in pool.s. It gets set to
// -1 when the item is removed from the pool.
index *int
}
// Handle is an opaque handle to a resource in a pool. It is used to delete an
// item from the pool, without requiring the item to be comparable.
type Handle[V any] struct {
idx *int // pointer to index; -1 if not in slice
}
// Len returns the current size of the pool.
func (p *Pool[V]) Len() int {
return len(p.s)
}
// Clear removes all items from the pool.
func (p *Pool[V]) Clear() {
p.s = nil
}
// AppendTakeAll removes all items from the pool, appending them to the
// provided slice (which can be nil) and returning them. The returned slice can
// be nil if the provided slice was nil and the pool was empty.
//
// This function does not free the backing storage for the pool; to do that,
// use the Clear function.
func (p *Pool[V]) AppendTakeAll(dst []V) []V {
ret := dst
for i := range p.s {
e := p.s[i]
if consistencyCheck && e.index == nil {
panic(fmt.Sprintf("pool: index is nil at %d", i))
}
if *e.index >= 0 {
ret = append(ret, p.s[i].item)
}
}
p.s = p.s[:0]
return ret
}
// Add adds an item to the pool and returns a handle to it. The handle can be
// used to delete the item from the pool with the Delete method.
func (p *Pool[V]) Add(item V) Handle[V] {
// Store the index in a pointer, so that we can pass it to both the
// handle and store it in the itemAndIndex.
idx := new(len(p.s))
p.s = append(p.s, itemAndIndex[V]{
item: item,
index: idx,
})
return Handle[V]{idx}
}
// Peek will return the item with the given handle without removing it from the
// pool.
//
// It will return ok=false if the item has been deleted or previously taken.
func (p *Pool[V]) Peek(h Handle[V]) (v V, ok bool) {
p.checkHandle(h)
idx := *h.idx
if idx < 0 {
var zero V
return zero, false
}
p.checkIndex(idx)
return p.s[idx].item, true
}
// Delete removes the item from the pool.
//
// It reports whether the element was deleted; it will return false if the item
// has been taken with the TakeRandom function, or if the item was already
// deleted.
func (p *Pool[V]) Delete(h Handle[V]) bool {
p.checkHandle(h)
idx := *h.idx
if idx < 0 {
return false
}
p.deleteIndex(idx)
return true
}
func (p *Pool[V]) deleteIndex(idx int) {
// Mark the item as deleted.
p.checkIndex(idx)
*(p.s[idx].index) = -1
// If this isn't the last element in the slice, overwrite the element
// at this item's index with the last element.
lastIdx := len(p.s) - 1
if idx < lastIdx {
last := p.s[lastIdx]
p.checkElem(lastIdx, last)
*last.index = idx
p.s[idx] = last
}
// Zero out last element (for GC) and truncate slice.
p.s[lastIdx] = itemAndIndex[V]{}
p.s = p.s[:lastIdx]
}
// Take will remove the item with the given handle from the pool and return it.
//
// It will return ok=false and the zero value if the item has been deleted or
// previously taken.
func (p *Pool[V]) Take(h Handle[V]) (v V, ok bool) {
p.checkHandle(h)
idx := *h.idx
if idx < 0 {
var zero V
return zero, false
}
e := p.s[idx]
p.deleteIndex(idx)
return e.item, true
}
// TakeRandom returns and removes a random element from p
// and reports whether there was one to take.
//
// It will return ok=false and the zero value if the pool is empty.
func (p *Pool[V]) TakeRandom() (v V, ok bool) {
if len(p.s) == 0 {
var zero V
return zero, false
}
pick := rand.IntN(len(p.s))
e := p.s[pick]
p.checkElem(pick, e)
p.deleteIndex(pick)
return e.item, true
}
// checkIndex verifies that the provided index is within the bounds of the
// pool's slice, and that the corresponding element has a non-nil index
// pointer, and panics if not.
func (p *Pool[V]) checkIndex(idx int) {
if !consistencyCheck {
return
}
if idx >= len(p.s) {
panic(fmt.Sprintf("pool: index %d out of range (len %d)", idx, len(p.s)))
}
if p.s[idx].index == nil {
panic(fmt.Sprintf("pool: index is nil at %d", idx))
}
}
// checkHandle verifies that the provided handle is not nil, and panics if it
// is.
func (p *Pool[V]) checkHandle(h Handle[V]) {
if !consistencyCheck {
return
}
if h.idx == nil {
panic("pool: nil handle")
}
}
// checkElem verifies that the provided itemAndIndex has a non-nil index, and
// that the stored index matches the expected position within the slice.
func (p *Pool[V]) checkElem(idx int, e itemAndIndex[V]) {
if !consistencyCheck {
return
}
if e.index == nil {
panic("pool: index is nil")
}
if got := *e.index; got != idx {
panic(fmt.Sprintf("pool: index is incorrect: want %d, got %d", idx, got))
}
}
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