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tailscale/util/deephash/deephash.go

735 lines
22 KiB
Go

// Copyright (c) Tailscale Inc & AUTHORS
// SPDX-License-Identifier: BSD-3-Clause
// Package deephash hashes a Go value recursively, in a predictable order,
// without looping. The hash is only valid within the lifetime of a program.
// Users should not store the hash on disk or send it over the network.
// The hash is sufficiently strong and unique such that
// Hash(&x) == Hash(&y) is an appropriate replacement for x == y.
//
// The definition of equality is identical to reflect.DeepEqual except:
// - Floating-point values are compared based on the raw bits,
// which means that NaNs (with the same bit pattern) are treated as equal.
// - time.Time are compared based on whether they are the same instant in time
// and also in the same zone offset. Monotonic measurements and zone names
// are ignored as part of the hash.
// - netip.Addr are compared based on a shallow comparison of the struct.
//
// WARNING: This package, like most of the tailscale.com Go module,
// should be considered Tailscale-internal; we make no API promises.
//
// # Cycle detection
//
// This package correctly handles cycles in the value graph,
// but in a way that is potentially pathological in some situations.
//
// The algorithm for cycle detection operates by
// pushing a pointer onto a stack whenever deephash is visiting a pointer and
// popping the pointer from the stack after deephash is leaving the pointer.
// Before visiting a new pointer, deephash checks whether it has already been
// visited on the pointer stack. If so, it hashes the index of the pointer
// on the stack and avoids visiting the pointer.
//
// This algorithm is guaranteed to detect cycles, but may expand pointers
// more often than a potential alternate algorithm that remembers all pointers
// ever visited in a map. The current algorithm uses O(D) memory, where D
// is the maximum depth of the recursion, while the alternate algorithm
// would use O(P) memory where P is all pointers ever seen, which can be a lot,
// and most of which may have nothing to do with cycles.
// Also, the alternate algorithm has to deal with challenges of producing
// deterministic results when pointers are visited in non-deterministic ways
// such as when iterating through a Go map. The stack-based algorithm avoids
// this challenge since the stack is always deterministic regardless of
// non-deterministic iteration order of Go maps.
//
// To concretely see how this algorithm can be pathological,
// consider the following data structure:
//
// var big *Item = ... // some large data structure that is slow to hash
// var manyBig []*Item
// for i := 0; i < 1000; i++ {
// manyBig = append(manyBig, &big)
// }
// deephash.Hash(manyBig)
//
// Here, the manyBig data structure is not even cyclic.
// We have the same big *Item being stored multiple times in a []*Item.
// When deephash hashes []*Item, it hashes each individual *Item
// not realizing that it had just done the computation earlier.
// To avoid the pathological situation, Item should implement [SelfHasher] and
// memoize attempts to hash itself.
package deephash
// TODO: Add option to teach deephash to memoize the Hash result of particular types?
import (
"crypto/sha256"
"encoding/binary"
"encoding/hex"
"fmt"
"reflect"
"sync"
"time"
"tailscale.com/util/hashx"
"tailscale.com/util/set"
)
// There is much overlap between the theory of serialization and hashing.
// A hash (useful for determining equality) can be produced by printing a value
// and hashing the output. The format must:
// * be deterministic such that the same value hashes to the same output, and
// * be parsable such that the same value can be reproduced by the output.
//
// The logic below hashes a value by printing it to a hash.Hash.
// To be parsable, it assumes that we know the Go type of each value:
// * scalar types (e.g., bool or int32) are directly printed as their
// underlying memory representation.
// * list types (e.g., strings and slices) are prefixed by a
// fixed-width length field, followed by the contents of the list.
// * slices, arrays, and structs print each element/field consecutively.
// * interfaces print with a 1-byte prefix indicating whether it is nil.
// If non-nil, it is followed by a fixed-width field of the type index,
// followed by the format of the underlying value.
// * pointers print with a 1-byte prefix indicating whether the pointer is
// 1) nil, 2) previously seen, or 3) newly seen. Previously seen pointers are
// followed by a fixed-width field with the index of the previous pointer.
// Newly seen pointers are followed by the format of the underlying value.
// * maps print with a 1-byte prefix indicating whether the map pointer is
// 1) nil, 2) previously seen, or 3) newly seen. Previously seen pointers
// are followed by a fixed-width field of the index of the previous pointer.
// Newly seen maps are printed with a fixed-width length field, followed by
// a fixed-width field with the XOR of the hash of every map entry.
// With a sufficiently strong hash, this value is theoretically "parsable"
// by looking up the hash in a magical map that returns the set of entries
// for that given hash.
// SelfHasher is implemented by types that can compute their own hash
// by writing values through the provided [Hasher] parameter.
// Implementations must not leak the provided [Hasher].
//
// If the implementation of SelfHasher recursively calls [deephash.Hash],
// then infinite recursion is quite likely to occur.
// To avoid this, use a type definition to drop methods before calling [deephash.Hash]:
//
// func (v *MyType) Hash(h deephash.Hasher) {
// v.hashMu.Lock()
// defer v.hashMu.Unlock()
// if v.dirtyHash {
// type MyTypeWithoutMethods MyType // type define MyType to drop Hash method
// v.dirtyHash = false // clear out dirty bit to avoid hashing over it
// v.hashSum = deephash.Sum{} // clear out hashSum to avoid hashing over it
// v.hashSum = deephash.Hash((*MyTypeWithoutMethods)(v))
// }
// h.HashSum(v.hashSum)
// }
//
// In the above example, we acquire a lock since it is possible that deephash
// is called in a concurrent manner, which implies that MyType.Hash may also
// be called in a concurrent manner. Whether this lock is necessary is
// application-dependent and left as an exercise to the reader.
// Also, the example assumes that dirtyHash is set elsewhere by application
// logic whenever a mutation is made to MyType that would alter the hash.
type SelfHasher interface {
Hash(Hasher)
}
// Hasher is a value passed to [SelfHasher.Hash] that allow implementations
// to hash themselves in a structured manner.
type Hasher struct{ h *hashx.Block512 }
// HashBytes hashes a sequence of bytes b.
// The length of b is not explicitly hashed.
func (h Hasher) HashBytes(b []byte) { h.h.HashBytes(b) }
// HashString hashes the string data of s
// The length of s is not explicitly hashed.
func (h Hasher) HashString(s string) { h.h.HashString(s) }
// HashUint8 hashes a uint8.
func (h Hasher) HashUint8(n uint8) { h.h.HashUint8(n) }
// HashUint16 hashes a uint16.
func (h Hasher) HashUint16(n uint16) { h.h.HashUint16(n) }
// HashUint32 hashes a uint32.
func (h Hasher) HashUint32(n uint32) { h.h.HashUint32(n) }
// HashUint64 hashes a uint64.
func (h Hasher) HashUint64(n uint64) { h.h.HashUint64(n) }
// HashSum hashes a [Sum].
func (h Hasher) HashSum(s Sum) {
// NOTE: Avoid calling h.HashBytes since it escapes b,
// which would force s to be heap allocated.
h.h.HashUint64(binary.LittleEndian.Uint64(s.sum[0:8]))
h.h.HashUint64(binary.LittleEndian.Uint64(s.sum[8:16]))
h.h.HashUint64(binary.LittleEndian.Uint64(s.sum[16:24]))
h.h.HashUint64(binary.LittleEndian.Uint64(s.sum[24:32]))
}
// hasher is reusable state for hashing a value.
// Get one via hasherPool.
type hasher struct {
hashx.Block512
visitStack visitStack
}
var hasherPool = &sync.Pool{
New: func() any { return new(hasher) },
}
func (h *hasher) reset() {
if h.Block512.Hash == nil {
h.Block512.Hash = sha256.New()
}
h.Block512.Reset()
}
// hashType hashes a reflect.Type.
// The hash is only consistent within the lifetime of a program.
func (h *hasher) hashType(t reflect.Type) {
// This approach relies on reflect.Type always being backed by a unique
// *reflect.rtype pointer. A safer approach is to use a global sync.Map
// that maps reflect.Type to some arbitrary and unique index.
// While safer, it requires global state with memory that can never be GC'd.
rtypeAddr := reflect.ValueOf(t).Pointer() // address of *reflect.rtype
h.HashUint64(uint64(rtypeAddr))
}
func (h *hasher) sum() (s Sum) {
h.Sum(s.sum[:0])
return s
}
// Sum is an opaque checksum type that is comparable.
type Sum struct {
sum [sha256.Size]byte
}
func (s1 *Sum) xor(s2 Sum) {
for i := 0; i < sha256.Size; i++ {
s1.sum[i] ^= s2.sum[i]
}
}
func (s Sum) String() string {
// Note: if we change this, keep in sync with AppendTo
return hex.EncodeToString(s.sum[:])
}
// AppendTo appends the string encoding of this sum (as returned by the String
// method) to the provided byte slice and returns the extended buffer.
func (s Sum) AppendTo(b []byte) []byte {
// TODO: switch to upstream implementation if accepted:
// https://github.com/golang/go/issues/53693
var lb [len(s.sum) * 2]byte
hex.Encode(lb[:], s.sum[:])
return append(b, lb[:]...)
}
var (
seedOnce sync.Once
seed uint64
)
func initSeed() {
seed = uint64(time.Now().UnixNano())
}
// Hash returns the hash of v.
func Hash[T any](v *T) Sum {
h := hasherPool.Get().(*hasher)
defer hasherPool.Put(h)
h.reset()
seedOnce.Do(initSeed)
h.HashUint64(seed)
// Always treat the Hash input as if it were an interface by including
// a hash of the type. This ensures that hashing of two different types
// but with the same value structure produces different hashes.
t := reflect.TypeOf(v).Elem()
h.hashType(t)
if v == nil {
h.HashUint8(0) // indicates nil
} else {
h.HashUint8(1) // indicates visiting pointer element
p := pointerOf(reflect.ValueOf(v))
hash := lookupTypeHasher(t)
hash(h, p)
}
return h.sum()
}
// Option is an optional argument to HasherForType.
type Option interface {
isOption()
}
type fieldFilterOpt struct {
t reflect.Type
fields set.Set[string]
includeOnMatch bool // true to include fields, false to exclude them
}
func (fieldFilterOpt) isOption() {}
func (f fieldFilterOpt) filterStructField(sf reflect.StructField) (include bool) {
if f.fields.Contains(sf.Name) {
return f.includeOnMatch
}
return !f.includeOnMatch
}
// IncludeFields returns an option that modifies the hashing for T to only
// include the named struct fields.
//
// T must be a struct type, and must match the type of the value passed to
// HasherForType.
func IncludeFields[T any](fields ...string) Option {
return newFieldFilter[T](true, fields)
}
// ExcludeFields returns an option that modifies the hashing for T to include
// all struct fields of T except those provided in fields.
//
// T must be a struct type, and must match the type of the value passed to
// HasherForType.
func ExcludeFields[T any](fields ...string) Option {
return newFieldFilter[T](false, fields)
}
func newFieldFilter[T any](include bool, fields []string) Option {
var zero T
t := reflect.TypeOf(&zero).Elem()
fieldSet := set.Set[string]{}
for _, f := range fields {
if _, ok := t.FieldByName(f); !ok {
panic(fmt.Sprintf("unknown field %q for type %v", f, t))
}
fieldSet.Add(f)
}
return fieldFilterOpt{t, fieldSet, include}
}
// HasherForType returns a hash that is specialized for the provided type.
//
// HasherForType panics if the opts are invalid for the provided type.
//
// Currently, at most one option can be provided (IncludeFields or
// ExcludeFields) and its type must match the type of T. Those restrictions may
// be removed in the future, along with documentation about their precedence
// when combined.
func HasherForType[T any](opts ...Option) func(*T) Sum {
var v *T
seedOnce.Do(initSeed)
if len(opts) > 1 {
panic("HasherForType only accepts one optional argument") // for now
}
t := reflect.TypeOf(v).Elem()
var hash typeHasherFunc
for _, o := range opts {
switch o := o.(type) {
default:
panic(fmt.Sprintf("unknown HasherOpt %T", o))
case fieldFilterOpt:
if t.Kind() != reflect.Struct {
panic("HasherForStructTypeWithFieldFilter requires T of kind struct")
}
if t != o.t {
panic(fmt.Sprintf("field filter for type %v does not match HasherForType type %v", o.t, t))
}
hash = makeStructHasher(t, o.filterStructField)
}
}
if hash == nil {
hash = lookupTypeHasher(t)
}
return func(v *T) (s Sum) {
// This logic is identical to Hash, but pull out a few statements.
h := hasherPool.Get().(*hasher)
defer hasherPool.Put(h)
h.reset()
h.HashUint64(seed)
h.hashType(t)
if v == nil {
h.HashUint8(0) // indicates nil
} else {
h.HashUint8(1) // indicates visiting pointer element
p := pointerOf(reflect.ValueOf(v))
hash(h, p)
}
return h.sum()
}
}
// Update sets last to the hash of v and reports whether its value changed.
func Update[T any](last *Sum, v *T) (changed bool) {
sum := Hash(v)
changed = sum != *last
if changed {
*last = sum
}
return changed
}
// typeHasherFunc hashes the value pointed at by p for a given type.
// For example, if t is a bool, then p is a *bool.
// The provided pointer must always be non-nil.
type typeHasherFunc func(h *hasher, p pointer)
var typeHasherCache sync.Map // map[reflect.Type]typeHasherFunc
func lookupTypeHasher(t reflect.Type) typeHasherFunc {
if v, ok := typeHasherCache.Load(t); ok {
return v.(typeHasherFunc)
}
hash := makeTypeHasher(t)
v, _ := typeHasherCache.LoadOrStore(t, hash)
return v.(typeHasherFunc)
}
func makeTypeHasher(t reflect.Type) typeHasherFunc {
// Types with specific hashing.
switch t {
case timeTimeType:
return hashTime
case netipAddrType:
return hashAddr
}
// Types that implement their own hashing.
if t.Kind() != reflect.Pointer && t.Kind() != reflect.Interface {
// A method can be implemented on either the value receiver or pointer receiver.
if t.Implements(selfHasherType) || reflect.PointerTo(t).Implements(selfHasherType) {
return makeSelfHasher(t)
}
}
// Types that can have their memory representation directly hashed.
if typeIsMemHashable(t) {
return makeMemHasher(t.Size())
}
switch t.Kind() {
case reflect.String:
return hashString
case reflect.Array:
return makeArrayHasher(t)
case reflect.Slice:
return makeSliceHasher(t)
case reflect.Struct:
return makeStructHasher(t, keepAllStructFields)
case reflect.Map:
return makeMapHasher(t)
case reflect.Pointer:
return makePointerHasher(t)
case reflect.Interface:
return makeInterfaceHasher(t)
default: // Func, Chan, UnsafePointer
return func(*hasher, pointer) {}
}
}
func hashTime(h *hasher, p pointer) {
// Include the zone offset (but not the name) to keep
// Hash(t1) == Hash(t2) being semantically equivalent to
// t1.Format(time.RFC3339Nano) == t2.Format(time.RFC3339Nano).
t := *p.asTime()
_, offset := t.Zone()
h.HashUint64(uint64(t.Unix()))
h.HashUint32(uint32(t.Nanosecond()))
h.HashUint32(uint32(offset))
}
func hashAddr(h *hasher, p pointer) {
// The formatting of netip.Addr covers the
// IP version, the address, and the optional zone name (for v6).
// This is equivalent to a1.MarshalBinary() == a2.MarshalBinary().
ip := *p.asAddr()
switch {
case !ip.IsValid():
h.HashUint64(0)
case ip.Is4():
b := ip.As4()
h.HashUint64(4)
h.HashUint32(binary.LittleEndian.Uint32(b[:]))
case ip.Is6():
b := ip.As16()
z := ip.Zone()
h.HashUint64(16 + uint64(len(z)))
h.HashUint64(binary.LittleEndian.Uint64(b[:8]))
h.HashUint64(binary.LittleEndian.Uint64(b[8:]))
h.HashString(z)
}
}
func makeSelfHasher(t reflect.Type) typeHasherFunc {
return func(h *hasher, p pointer) {
p.asValue(t).Interface().(SelfHasher).Hash(Hasher{&h.Block512})
}
}
func hashString(h *hasher, p pointer) {
s := *p.asString()
h.HashUint64(uint64(len(s)))
h.HashString(s)
}
func makeMemHasher(n uintptr) typeHasherFunc {
return func(h *hasher, p pointer) {
h.HashBytes(p.asMemory(n))
}
}
func makeArrayHasher(t reflect.Type) typeHasherFunc {
var once sync.Once
var hashElem typeHasherFunc
init := func() {
hashElem = lookupTypeHasher(t.Elem())
}
n := t.Len() // number of array elements
nb := t.Elem().Size() // byte size of each array element
return func(h *hasher, p pointer) {
once.Do(init)
for i := 0; i < n; i++ {
hashElem(h, p.arrayIndex(i, nb))
}
}
}
func makeSliceHasher(t reflect.Type) typeHasherFunc {
nb := t.Elem().Size() // byte size of each slice element
if typeIsMemHashable(t.Elem()) {
return func(h *hasher, p pointer) {
pa := p.sliceArray()
if pa.isNil() {
h.HashUint8(0) // indicates nil
return
}
h.HashUint8(1) // indicates visiting slice
n := p.sliceLen()
b := pa.asMemory(uintptr(n) * nb)
h.HashUint64(uint64(n))
h.HashBytes(b)
}
}
var once sync.Once
var hashElem typeHasherFunc
init := func() {
hashElem = lookupTypeHasher(t.Elem())
if typeIsRecursive(t) {
hashElemDefault := hashElem
hashElem = func(h *hasher, p pointer) {
if idx, ok := h.visitStack.seen(p.p); ok {
h.HashUint8(2) // indicates cycle
h.HashUint64(uint64(idx))
return
}
h.HashUint8(1) // indicates visiting slice element
h.visitStack.push(p.p)
defer h.visitStack.pop(p.p)
hashElemDefault(h, p)
}
}
}
return func(h *hasher, p pointer) {
pa := p.sliceArray()
if pa.isNil() {
h.HashUint8(0) // indicates nil
return
}
once.Do(init)
h.HashUint8(1) // indicates visiting slice
n := p.sliceLen()
h.HashUint64(uint64(n))
for i := 0; i < n; i++ {
pe := pa.arrayIndex(i, nb)
hashElem(h, pe)
}
}
}
func keepAllStructFields(keepField reflect.StructField) bool { return true }
func makeStructHasher(t reflect.Type, keepField func(reflect.StructField) bool) typeHasherFunc {
type fieldHasher struct {
idx int // index of field for reflect.Type.Field(n); negative if memory is directly hashable
keep bool
hash typeHasherFunc // only valid if idx is not negative
offset uintptr
size uintptr
}
var once sync.Once
var fields []fieldHasher
init := func() {
for i, numField := 0, t.NumField(); i < numField; i++ {
sf := t.Field(i)
f := fieldHasher{i, keepField(sf), nil, sf.Offset, sf.Type.Size()}
if f.keep && typeIsMemHashable(sf.Type) {
f.idx = -1
}
// Combine with previous field if both contiguous and mem-hashable.
if f.idx < 0 && len(fields) > 0 {
if last := &fields[len(fields)-1]; last.idx < 0 && last.offset+last.size == f.offset {
last.size += f.size
continue
}
}
fields = append(fields, f)
}
for i, f := range fields {
if f.idx >= 0 {
fields[i].hash = lookupTypeHasher(t.Field(f.idx).Type)
}
}
}
return func(h *hasher, p pointer) {
once.Do(init)
for _, field := range fields {
if !field.keep {
continue
}
pf := p.structField(field.idx, field.offset, field.size)
if field.idx < 0 {
h.HashBytes(pf.asMemory(field.size))
} else {
field.hash(h, pf)
}
}
}
}
func makeMapHasher(t reflect.Type) typeHasherFunc {
var once sync.Once
var hashKey, hashValue typeHasherFunc
var isRecursive bool
init := func() {
hashKey = lookupTypeHasher(t.Key())
hashValue = lookupTypeHasher(t.Elem())
isRecursive = typeIsRecursive(t)
}
return func(h *hasher, p pointer) {
v := p.asValue(t).Elem() // reflect.Map kind
if v.IsNil() {
h.HashUint8(0) // indicates nil
return
}
once.Do(init)
if isRecursive {
pm := v.UnsafePointer() // underlying pointer of map
if idx, ok := h.visitStack.seen(pm); ok {
h.HashUint8(2) // indicates cycle
h.HashUint64(uint64(idx))
return
}
h.visitStack.push(pm)
defer h.visitStack.pop(pm)
}
h.HashUint8(1) // indicates visiting map entries
h.HashUint64(uint64(v.Len()))
mh := mapHasherPool.Get().(*mapHasher)
defer mapHasherPool.Put(mh)
// Hash a map in a sort-free manner.
// It relies on a map being a an unordered set of KV entries.
// So long as we hash each KV entry together, we can XOR all the
// individual hashes to produce a unique hash for the entire map.
k := mh.valKey.get(v.Type().Key())
e := mh.valElem.get(v.Type().Elem())
mh.sum = Sum{}
mh.h.visitStack = h.visitStack // always use the parent's visit stack to avoid cycles
for iter := v.MapRange(); iter.Next(); {
k.SetIterKey(iter)
e.SetIterValue(iter)
mh.h.reset()
hashKey(&mh.h, pointerOf(k.Addr()))
hashValue(&mh.h, pointerOf(e.Addr()))
mh.sum.xor(mh.h.sum())
}
h.HashBytes(mh.sum.sum[:])
}
}
func makePointerHasher(t reflect.Type) typeHasherFunc {
var once sync.Once
var hashElem typeHasherFunc
var isRecursive bool
init := func() {
hashElem = lookupTypeHasher(t.Elem())
isRecursive = typeIsRecursive(t)
}
return func(h *hasher, p pointer) {
pe := p.pointerElem()
if pe.isNil() {
h.HashUint8(0) // indicates nil
return
}
once.Do(init)
if isRecursive {
if idx, ok := h.visitStack.seen(pe.p); ok {
h.HashUint8(2) // indicates cycle
h.HashUint64(uint64(idx))
return
}
h.visitStack.push(pe.p)
defer h.visitStack.pop(pe.p)
}
h.HashUint8(1) // indicates visiting a pointer element
hashElem(h, pe)
}
}
func makeInterfaceHasher(t reflect.Type) typeHasherFunc {
return func(h *hasher, p pointer) {
v := p.asValue(t).Elem() // reflect.Interface kind
if v.IsNil() {
h.HashUint8(0) // indicates nil
return
}
h.HashUint8(1) // indicates visiting an interface value
v = v.Elem()
t := v.Type()
h.hashType(t)
va := reflect.New(t).Elem()
va.Set(v)
hashElem := lookupTypeHasher(t)
hashElem(h, pointerOf(va.Addr()))
}
}
type mapHasher struct {
h hasher
valKey valueCache
valElem valueCache
sum Sum
}
var mapHasherPool = &sync.Pool{
New: func() any { return new(mapHasher) },
}
type valueCache map[reflect.Type]reflect.Value
// get returns an addressable reflect.Value for the given type.
func (c *valueCache) get(t reflect.Type) reflect.Value {
v, ok := (*c)[t]
if !ok {
v = reflect.New(t).Elem()
if *c == nil {
*c = make(valueCache)
}
(*c)[t] = v
}
return v
}