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restic/internal/index/indexmap.go
2022-10-08 21:15:34 +02:00

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package index
import (
"hash/maphash"
"github.com/restic/restic/internal/restic"
)
// An indexMap is a chained hash table that maps blob IDs to indexEntries.
// It allows storing multiple entries with the same key.
//
// IndexMap uses some optimizations that are not compatible with supporting
// deletions.
//
// The buckets in this hash table contain only pointers, rather than inlined
// key-value pairs like the standard Go map. This way, only a pointer array
// needs to be resized when the table grows, preventing memory usage spikes.
type indexMap struct {
// The number of buckets is always a power of two and never zero.
buckets []*indexEntry
numentries uint
mh maphash.Hash
free *indexEntry // Free list.
}
const (
growthFactor = 2 // Must be a power of 2.
maxLoad = 4 // Max. number of entries per bucket.
)
// add inserts an indexEntry for the given arguments into the map,
// using id as the key.
func (m *indexMap) add(id restic.ID, packIdx int, offset, length uint32, uncompressedLength uint32) {
switch {
case m.numentries == 0: // Lazy initialization.
m.init()
case m.numentries >= maxLoad*uint(len(m.buckets)):
m.grow()
}
h := m.hash(id)
e := m.newEntry()
e.id = id
e.next = m.buckets[h] // Prepend to existing chain.
e.packIndex = packIdx
e.offset = offset
e.length = length
e.uncompressedLength = uncompressedLength
m.buckets[h] = e
m.numentries++
}
// foreach calls fn for all entries in the map, until fn returns false.
func (m *indexMap) foreach(fn func(*indexEntry) bool) {
for _, e := range m.buckets {
for e != nil {
if !fn(e) {
return
}
e = e.next
}
}
}
// foreachWithID calls fn for all entries with the given id.
func (m *indexMap) foreachWithID(id restic.ID, fn func(*indexEntry)) {
if len(m.buckets) == 0 {
return
}
h := m.hash(id)
for e := m.buckets[h]; e != nil; e = e.next {
if e.id != id {
continue
}
fn(e)
}
}
// get returns the first entry for the given id.
func (m *indexMap) get(id restic.ID) *indexEntry {
if len(m.buckets) == 0 {
return nil
}
h := m.hash(id)
for e := m.buckets[h]; e != nil; e = e.next {
if e.id == id {
return e
}
}
return nil
}
func (m *indexMap) grow() {
old := m.buckets
m.buckets = make([]*indexEntry, growthFactor*len(m.buckets))
for _, e := range old {
for e != nil {
h := m.hash(e.id)
next := e.next
e.next = m.buckets[h]
m.buckets[h] = e
e = next
}
}
}
func (m *indexMap) hash(id restic.ID) uint {
// We use maphash to prevent backups of specially crafted inputs
// from degrading performance.
// While SHA-256 should be collision-resistant, for hash table indices
// we use only a few bits of it and finding collisions for those is
// much easier than breaking the whole algorithm.
m.mh.Reset()
_, _ = m.mh.Write(id[:])
h := uint(m.mh.Sum64())
return h & uint(len(m.buckets)-1)
}
func (m *indexMap) init() {
const initialBuckets = 64
m.buckets = make([]*indexEntry, initialBuckets)
}
func (m *indexMap) len() uint { return m.numentries }
func (m *indexMap) newEntry() *indexEntry {
// We keep a free list of objects to speed up allocation and GC.
// There's an obvious trade-off here: allocating in larger batches
// means we allocate faster and the GC has to keep fewer bits to track
// what we have in use, but it means we waste some space.
//
// Then again, allocating each indexEntry separately also wastes space
// on 32-bit platforms, because the Go malloc has no size class for
// exactly 52 bytes, so it puts the indexEntry in a 64-byte slot instead.
// See src/runtime/sizeclasses.go in the Go source repo.
//
// The batch size of 4 means we hit the size classes for 4×64=256 bytes
// (64-bit) and 4×52=208 bytes (32-bit), wasting nothing in malloc on
// 64-bit and relatively little on 32-bit.
const entryAllocBatch = 4
e := m.free
if e != nil {
m.free = e.next
} else {
free := new([entryAllocBatch]indexEntry)
e = &free[0]
for i := 1; i < len(free)-1; i++ {
free[i].next = &free[i+1]
}
m.free = &free[1]
}
return e
}
type indexEntry struct {
id restic.ID
next *indexEntry
packIndex int // Position in containing Index's packs field.
offset uint32
length uint32
uncompressedLength uint32
}