mirror of
https://github.com/tailscale/tailscale.git
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util/pool: add package for storing and using a pool of items
This can be used to implement a persistent pool (i.e. one that isn't
cleared like sync.Pool is) of items–e.g. database connections.
Some benchmarks vs. a naive implementation that uses a single map
iteration show a pretty meaningful improvement:
$ benchstat -col /impl ./bench.txt
goos: darwin
goarch: arm64
pkg: tailscale.com/util/pool
│ Pool │ map │
│ sec/op │ sec/op vs base │
Pool_AddDelete-10 10.56n ± 2% 15.11n ± 1% +42.97% (p=0.000 n=10)
Pool_TakeRandom-10 56.75n ± 4% 1899.50n ± 20% +3246.84% (p=0.000 n=10)
geomean 24.49n 169.4n +591.74%
Updates tailscale/corp#19900
Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: Ie509cb65573c4726cfc3da9a97093e61c216ca18
This commit is contained in:
Andrew Dunham
parent
87ee559b6f
commit
8e4a29433f
213
util/pool/pool.go
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213
util/pool/pool.go
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@ -0,0 +1,213 @@
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// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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// Package pool contains a generic type for managing a pool of resources; for
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// example, connections to a database, or to a remote service.
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//
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// Unlike sync.Pool from the Go standard library, this pool does not remove
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// items from the pool when garbage collection happens, nor is it safe for
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// concurrent use like sync.Pool.
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package pool
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import (
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"fmt"
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"math/rand/v2"
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"tailscale.com/types/ptr"
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)
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// consistencyCheck enables additional runtime checks to ensure that the pool
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// is well-formed; it is disabled by default, and can be enabled during tests
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// to catch additional bugs.
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const consistencyCheck = false
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// Pool is a pool of resources. It is not safe for concurrent use.
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type Pool[V any] struct {
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s []itemAndIndex[V]
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}
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type itemAndIndex[V any] struct {
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// item is the element in the pool
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item V
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// index is the current location of this item in pool.s. It gets set to
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// -1 when the item is removed from the pool.
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index *int
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}
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// Handle is an opaque handle to a resource in a pool. It is used to delete an
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// item from the pool, without requiring the item to be comparable.
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type Handle[V any] struct {
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idx *int // pointer to index; -1 if not in slice
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}
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// Len returns the current size of the pool.
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func (p *Pool[V]) Len() int {
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return len(p.s)
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}
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// Clear removes all items from the pool.
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func (p *Pool[V]) Clear() {
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p.s = nil
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}
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// AppendTakeAll removes all items from the pool, appending them to the
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// provided slice (which can be nil) and returning them. The returned slice can
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// be nil if the provided slice was nil and the pool was empty.
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//
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// This function does not free the backing storage for the pool; to do that,
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// use the Clear function.
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func (p *Pool[V]) AppendTakeAll(dst []V) []V {
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ret := dst
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for i := range p.s {
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e := p.s[i]
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if consistencyCheck && e.index == nil {
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panic(fmt.Sprintf("pool: index is nil at %d", i))
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}
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if *e.index >= 0 {
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ret = append(ret, p.s[i].item)
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}
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}
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p.s = p.s[:0]
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return ret
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}
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// Add adds an item to the pool and returns a handle to it. The handle can be
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// used to delete the item from the pool with the Delete method.
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func (p *Pool[V]) Add(item V) Handle[V] {
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// Store the index in a pointer, so that we can pass it to both the
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// handle and store it in the itemAndIndex.
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idx := ptr.To(len(p.s))
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p.s = append(p.s, itemAndIndex[V]{
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item: item,
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index: idx,
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})
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return Handle[V]{idx}
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}
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// Peek will return the item with the given handle without removing it from the
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// pool.
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//
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// It will return ok=false if the item has been deleted or previously taken.
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func (p *Pool[V]) Peek(h Handle[V]) (v V, ok bool) {
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p.checkHandle(h)
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idx := *h.idx
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if idx < 0 {
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var zero V
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return zero, false
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}
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p.checkIndex(idx)
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return p.s[idx].item, true
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}
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// Delete removes the item from the pool.
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//
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// It reports whether the element was deleted; it will return false if the item
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// has been taken with the TakeRandom function, or if the item was already
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// deleted.
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func (p *Pool[V]) Delete(h Handle[V]) bool {
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p.checkHandle(h)
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idx := *h.idx
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if idx < 0 {
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return false
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}
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p.deleteIndex(idx)
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return true
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}
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func (p *Pool[V]) deleteIndex(idx int) {
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// Mark the item as deleted.
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p.checkIndex(idx)
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*(p.s[idx].index) = -1
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// If this isn't the last element in the slice, overwrite the element
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// at this item's index with the last element.
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lastIdx := len(p.s) - 1
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if idx < lastIdx {
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last := p.s[lastIdx]
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p.checkElem(lastIdx, last)
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*last.index = idx
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p.s[idx] = last
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}
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// Zero out last element (for GC) and truncate slice.
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p.s[lastIdx] = itemAndIndex[V]{}
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p.s = p.s[:lastIdx]
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}
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// Take will remove the item with the given handle from the pool and return it.
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//
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// It will return ok=false and the zero value if the item has been deleted or
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// previously taken.
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func (p *Pool[V]) Take(h Handle[V]) (v V, ok bool) {
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p.checkHandle(h)
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idx := *h.idx
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if idx < 0 {
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var zero V
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return zero, false
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}
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e := p.s[idx]
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p.deleteIndex(idx)
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return e.item, true
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}
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// TakeRandom returns and removes a random element from p
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// and reports whether there was one to take.
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//
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// It will return ok=false and the zero value if the pool is empty.
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func (p *Pool[V]) TakeRandom() (v V, ok bool) {
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if len(p.s) == 0 {
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var zero V
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return zero, false
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}
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pick := rand.IntN(len(p.s))
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e := p.s[pick]
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p.checkElem(pick, e)
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p.deleteIndex(pick)
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return e.item, true
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}
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// checkIndex verifies that the provided index is within the bounds of the
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// pool's slice, and that the corresponding element has a non-nil index
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// pointer, and panics if not.
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func (p *Pool[V]) checkIndex(idx int) {
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if !consistencyCheck {
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return
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}
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if idx >= len(p.s) {
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panic(fmt.Sprintf("pool: index %d out of range (len %d)", idx, len(p.s)))
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}
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if p.s[idx].index == nil {
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panic(fmt.Sprintf("pool: index is nil at %d", idx))
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}
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}
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// checkHandle verifies that the provided handle is not nil, and panics if it
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// is.
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func (p *Pool[V]) checkHandle(h Handle[V]) {
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if !consistencyCheck {
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return
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}
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if h.idx == nil {
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panic("pool: nil handle")
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}
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}
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// checkElem verifies that the provided itemAndIndex has a non-nil index, and
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// that the stored index matches the expected position within the slice.
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func (p *Pool[V]) checkElem(idx int, e itemAndIndex[V]) {
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if !consistencyCheck {
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return
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}
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if e.index == nil {
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panic("pool: index is nil")
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}
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if got := *e.index; got != idx {
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panic(fmt.Sprintf("pool: index is incorrect: want %d, got %d", idx, got))
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}
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}
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203
util/pool/pool_test.go
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203
util/pool/pool_test.go
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@ -0,0 +1,203 @@
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// Copyright (c) Tailscale Inc & AUTHORS
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// SPDX-License-Identifier: BSD-3-Clause
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package pool
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import (
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"slices"
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"testing"
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)
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func TestPool(t *testing.T) {
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p := Pool[int]{}
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if got, want := p.Len(), 0; got != want {
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t.Errorf("got initial length %v; want %v", got, want)
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}
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h1 := p.Add(101)
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h2 := p.Add(102)
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h3 := p.Add(103)
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h4 := p.Add(104)
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if got, want := p.Len(), 4; got != want {
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t.Errorf("got length %v; want %v", got, want)
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}
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tests := []struct {
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h Handle[int]
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want int
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}{
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{h1, 101},
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{h2, 102},
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{h3, 103},
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{h4, 104},
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}
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for i, test := range tests {
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got, ok := p.Peek(test.h)
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if !ok {
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t.Errorf("test[%d]: did not find item", i)
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continue
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}
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if got != test.want {
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t.Errorf("test[%d]: got %v; want %v", i, got, test.want)
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}
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}
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if deleted := p.Delete(h2); !deleted {
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t.Errorf("h2 not deleted")
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}
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if deleted := p.Delete(h2); deleted {
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t.Errorf("h2 should not be deleted twice")
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}
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if got, want := p.Len(), 3; got != want {
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t.Errorf("got length %v; want %v", got, want)
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}
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if _, ok := p.Peek(h2); ok {
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t.Errorf("h2 still in pool")
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}
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// Remove an item by handle
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got, ok := p.Take(h4)
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if !ok {
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t.Errorf("h4 not found")
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}
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if got != 104 {
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t.Errorf("got %v; want 104", got)
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}
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// Take doesn't work on previously-taken or deleted items.
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if _, ok := p.Take(h4); ok {
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t.Errorf("h4 should not be taken twice")
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}
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if _, ok := p.Take(h2); ok {
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t.Errorf("h2 should not be taken after delete")
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}
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// Remove all items and return them
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items := p.AppendTakeAll(nil)
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want := []int{101, 103}
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if !slices.Equal(items, want) {
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t.Errorf("got items %v; want %v", items, want)
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}
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if got := p.Len(); got != 0 {
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t.Errorf("got length %v; want 0", got)
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}
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// Insert and then clear should result in no items.
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p.Add(105)
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p.Clear()
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if got := p.Len(); got != 0 {
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t.Errorf("got length %v; want 0", got)
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}
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}
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func TestTakeRandom(t *testing.T) {
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p := Pool[int]{}
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for i := 0; i < 10; i++ {
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p.Add(i + 100)
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}
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seen := make(map[int]bool)
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for i := 0; i < 10; i++ {
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item, ok := p.TakeRandom()
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if !ok {
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t.Errorf("unexpected empty pool")
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break
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}
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if seen[item] {
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t.Errorf("got duplicate item %v", item)
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}
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seen[item] = true
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}
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// Verify that the pool is empty
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if _, ok := p.TakeRandom(); ok {
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t.Errorf("expected empty pool")
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}
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for i := 0; i < 10; i++ {
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want := 100 + i
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if !seen[want] {
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t.Errorf("item %v not seen", want)
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}
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}
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if t.Failed() {
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t.Logf("seen: %+v", seen)
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}
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}
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func BenchmarkPool_AddDelete(b *testing.B) {
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b.Run("impl=Pool", func(b *testing.B) {
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p := Pool[int]{}
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// Warm up/force an initial allocation
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h := p.Add(0)
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p.Delete(h)
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b.ResetTimer()
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for i := 0; i < b.N; i++ {
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h := p.Add(i)
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p.Delete(h)
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}
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})
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b.Run("impl=map", func(b *testing.B) {
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p := make(map[int]bool)
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// Force initial allocation
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p[0] = true
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delete(p, 0)
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b.ResetTimer()
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for i := 0; i < b.N; i++ {
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p[i] = true
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delete(p, i)
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}
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})
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}
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func BenchmarkPool_TakeRandom(b *testing.B) {
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b.Run("impl=Pool", func(b *testing.B) {
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p := Pool[int]{}
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// Insert the number of items we'll be taking, then reset the timer.
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for i := 0; i < b.N; i++ {
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p.Add(i)
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}
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b.ResetTimer()
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// Now benchmark taking all the items.
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for i := 0; i < b.N; i++ {
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p.TakeRandom()
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}
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if p.Len() != 0 {
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b.Errorf("pool not empty")
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}
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})
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b.Run("impl=map", func(b *testing.B) {
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p := make(map[int]bool)
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// Insert the number of items we'll be taking, then reset the timer.
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for i := 0; i < b.N; i++ {
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p[i] = true
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}
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b.ResetTimer()
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// Now benchmark taking all the items.
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for i := 0; i < b.N; i++ {
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// Taking a random item is simulated by a single map iteration.
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for k := range p {
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delete(p, k) // "take" the item by removing it
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break
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}
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}
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if len(p) != 0 {
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b.Errorf("map not empty")
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}
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})
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}
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