+++ /dev/null
-// Copyright 2020 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-// Package intern lets you make smaller comparable values by boxing
-// a larger comparable value (such as a 16 byte string header) down
-// into a globally unique 8 byte pointer.
-//
-// The globally unique pointers are garbage collected with weak
-// references and finalizers. This package hides that.
-package intern
-
-import (
- "internal/godebug"
- "runtime"
- "sync"
- "unsafe"
-)
-
-// A Value pointer is the handle to an underlying comparable value.
-// See func Get for how Value pointers may be used.
-type Value struct {
- _ [0]func() // prevent people from accidentally using value type as comparable
- cmpVal any
- // resurrected is guarded by mu (for all instances of Value).
- // It is set true whenever v is synthesized from a uintptr.
- resurrected bool
-}
-
-// Get returns the comparable value passed to the Get func
-// that returned v.
-func (v *Value) Get() any { return v.cmpVal }
-
-// key is a key in our global value map.
-// It contains type-specialized fields to avoid allocations
-// when converting common types to empty interfaces.
-type key struct {
- s string
- cmpVal any
- // isString reports whether key contains a string.
- // Without it, the zero value of key is ambiguous.
- isString bool
-}
-
-// keyFor returns a key to use with cmpVal.
-func keyFor(cmpVal any) key {
- if s, ok := cmpVal.(string); ok {
- return key{s: s, isString: true}
- }
- return key{cmpVal: cmpVal}
-}
-
-// Value returns a *Value built from k.
-func (k key) Value() *Value {
- if k.isString {
- return &Value{cmpVal: k.s}
- }
- return &Value{cmpVal: k.cmpVal}
-}
-
-var (
- // mu guards valMap, a weakref map of *Value by underlying value.
- // It also guards the resurrected field of all *Values.
- mu sync.Mutex
- valMap = map[key]uintptr{} // to uintptr(*Value)
- valSafe = safeMap() // non-nil in safe+leaky mode
-)
-
-var intern = godebug.New("#intern")
-
-// safeMap returns a non-nil map if we're in safe-but-leaky mode,
-// as controlled by GODEBUG=intern=leaky
-func safeMap() map[key]*Value {
- if intern.Value() == "leaky" {
- return map[key]*Value{}
- }
- return nil
-}
-
-// Get returns a pointer representing the comparable value cmpVal.
-//
-// The returned pointer will be the same for Get(v) and Get(v2)
-// if and only if v == v2, and can be used as a map key.
-func Get(cmpVal any) *Value {
- return get(keyFor(cmpVal))
-}
-
-// GetByString is identical to Get, except that it is specialized for strings.
-// This avoids an allocation from putting a string into an interface{}
-// to pass as an argument to Get.
-func GetByString(s string) *Value {
- return get(key{s: s, isString: true})
-}
-
-// We play unsafe games that violate Go's rules (and assume a non-moving
-// collector). So we quiet Go here.
-// See the comment below Get for more implementation details.
-//
-//go:nocheckptr
-func get(k key) *Value {
- mu.Lock()
- defer mu.Unlock()
-
- var v *Value
- if valSafe != nil {
- v = valSafe[k]
- } else if addr, ok := valMap[k]; ok {
- v = (*Value)(unsafe.Pointer(addr))
- v.resurrected = true
- }
- if v != nil {
- return v
- }
- v = k.Value()
- if valSafe != nil {
- valSafe[k] = v
- } else {
- // SetFinalizer before uintptr conversion (theoretical concern;
- // see https://github.com/go4org/intern/issues/13)
- runtime.SetFinalizer(v, finalize)
- valMap[k] = uintptr(unsafe.Pointer(v))
- }
- return v
-}
-
-func finalize(v *Value) {
- mu.Lock()
- defer mu.Unlock()
- if v.resurrected {
- // We lost the race. Somebody resurrected it while we
- // were about to finalize it. Try again next round.
- v.resurrected = false
- runtime.SetFinalizer(v, finalize)
- return
- }
- delete(valMap, keyFor(v.cmpVal))
-}
-
-// Interning is simple if you don't require that unused values be
-// garbage collectable. But we do require that; we don't want to be
-// DOS vector. We do this by using a uintptr to hide the pointer from
-// the garbage collector, and using a finalizer to eliminate the
-// pointer when no other code is using it.
-//
-// The obvious implementation of this is to use a
-// map[interface{}]uintptr-of-*interface{}, and set up a finalizer to
-// delete from the map. Unfortunately, this is racy. Because pointers
-// are being created in violation of Go's unsafety rules, it's
-// possible to create a pointer to a value concurrently with the GC
-// concluding that the value can be collected. There are other races
-// that break the equality invariant as well, but the use-after-free
-// will cause a runtime crash.
-//
-// To make this work, the finalizer needs to know that no references
-// have been unsafely created since the finalizer was set up. To do
-// this, values carry a "resurrected" sentinel, which gets set
-// whenever a pointer is unsafely created. If the finalizer encounters
-// the sentinel, it clears the sentinel and delays collection for one
-// additional GC cycle, by re-installing itself as finalizer. This
-// ensures that the unsafely created pointer is visible to the GC, and
-// will correctly prevent collection.
-//
-// This technique does mean that interned values that get reused take
-// at least 3 GC cycles to fully collect (1 to clear the sentinel, 1
-// to clean up the unsafe map, 1 to be actually deleted).
-//
-// @ianlancetaylor commented in
-// https://github.com/golang/go/issues/41303#issuecomment-717401656
-// that it is possible to implement weak references in terms of
-// finalizers without unsafe. Unfortunately, the approach he outlined
-// does not work here, for two reasons. First, there is no way to
-// construct a strong pointer out of a weak pointer; our map stores
-// weak pointers, but we must return strong pointers to callers.
-// Second, and more fundamentally, we must return not just _a_ strong
-// pointer to callers, but _the same_ strong pointer to callers. In
-// order to return _the same_ strong pointer to callers, we must track
-// it, which is exactly what we cannot do with strong pointers.
-//
-// See https://github.com/inetaf/netaddr/issues/53 for more
-// discussion, and https://github.com/go4org/intern/issues/2 for an
-// illustration of the subtleties at play.
+++ /dev/null
-// Copyright 2020 The Go Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package intern
-
-import (
- "fmt"
- "runtime"
- "testing"
-)
-
-func TestBasics(t *testing.T) {
- clearMap()
- foo := Get("foo")
- bar := Get("bar")
- empty := Get("")
- nilEface := Get(nil)
- i := Get(0x7777777)
- foo2 := Get("foo")
- bar2 := Get("bar")
- empty2 := Get("")
- nilEface2 := Get(nil)
- i2 := Get(0x7777777)
- foo3 := GetByString("foo")
- empty3 := GetByString("")
-
- if foo.Get() != foo2.Get() {
- t.Error("foo/foo2 values differ")
- }
- if foo.Get() != foo3.Get() {
- t.Error("foo/foo3 values differ")
- }
- if foo.Get() != "foo" {
- t.Error("foo.Get not foo")
- }
- if foo != foo2 {
- t.Error("foo/foo2 pointers differ")
- }
- if foo != foo3 {
- t.Error("foo/foo3 pointers differ")
- }
-
- if bar.Get() != bar2.Get() {
- t.Error("bar values differ")
- }
- if bar.Get() != "bar" {
- t.Error("bar.Get not bar")
- }
- if bar != bar2 {
- t.Error("bar pointers differ")
- }
-
- if i.Get() != i.Get() {
- t.Error("i values differ")
- }
- if i.Get() != 0x7777777 {
- t.Error("i.Get not 0x7777777")
- }
- if i != i2 {
- t.Error("i pointers differ")
- }
-
- if empty.Get() != empty2.Get() {
- t.Error("empty/empty2 values differ")
- }
- if empty.Get() != empty.Get() {
- t.Error("empty/empty3 values differ")
- }
- if empty.Get() != "" {
- t.Error("empty.Get not empty string")
- }
- if empty != empty2 {
- t.Error("empty/empty2 pointers differ")
- }
- if empty != empty3 {
- t.Error("empty/empty3 pointers differ")
- }
-
- if nilEface.Get() != nilEface2.Get() {
- t.Error("nilEface values differ")
- }
- if nilEface.Get() != nil {
- t.Error("nilEface.Get not nil")
- }
- if nilEface != nilEface2 {
- t.Error("nilEface pointers differ")
- }
-
- if n := mapLen(); n != 5 {
- t.Errorf("map len = %d; want 4", n)
- }
-
- wantEmpty(t)
-}
-
-func wantEmpty(t testing.TB) {
- t.Helper()
- const gcTries = 5000
- for try := 0; try < gcTries; try++ {
- runtime.GC()
- n := mapLen()
- if n == 0 {
- break
- }
- if try == gcTries-1 {
- t.Errorf("map len = %d after (%d GC tries); want 0, contents: %v", n, gcTries, mapKeys())
- }
- }
-}
-
-func TestStress(t *testing.T) {
- iters := 10000
- if testing.Short() {
- iters = 1000
- }
- var sink []byte
- for i := 0; i < iters; i++ {
- _ = Get("foo")
- sink = make([]byte, 1<<20)
- }
- _ = sink
-}
-
-func BenchmarkStress(b *testing.B) {
- done := make(chan struct{})
- defer close(done)
- go func() {
- for {
- select {
- case <-done:
- return
- default:
- }
- runtime.GC()
- }
- }()
-
- clearMap()
- v1 := Get("foo")
- b.ReportAllocs()
- b.RunParallel(func(pb *testing.PB) {
- for pb.Next() {
- v2 := Get("foo")
- if v1 != v2 {
- b.Fatal("wrong value")
- }
- // And also a key we don't retain:
- _ = Get("bar")
- }
- })
- runtime.GC()
- wantEmpty(b)
-}
-
-func mapLen() int {
- mu.Lock()
- defer mu.Unlock()
- return len(valMap)
-}
-
-func mapKeys() (keys []string) {
- mu.Lock()
- defer mu.Unlock()
- for k := range valMap {
- keys = append(keys, fmt.Sprint(k))
- }
- return keys
-}
-
-func clearMap() {
- mu.Lock()
- defer mu.Unlock()
- clear(valMap)
-}
-
-var (
- globalString = "not a constant"
- sink string
-)
-
-func TestGetByStringAllocs(t *testing.T) {
- allocs := int(testing.AllocsPerRun(100, func() {
- GetByString(globalString)
- }))
- if allocs != 0 {
- t.Errorf("GetString allocated %d objects, want 0", allocs)
- }
-}
-
-func BenchmarkGetByString(b *testing.B) {
- b.ReportAllocs()
- for i := 0; i < b.N; i++ {
- v := GetByString(globalString)
- sink = v.Get().(string)
- }
-}