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freelist.go
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freelist.go
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package bolt
import (
"fmt"
"sort"
"unsafe"
)
// freelist represents a list of all pages that are available for allocation.
// It also tracks pages that have been freed but are still in use by open transactions.
type freelist struct {
ids []pgid // all free and available free page ids.
pending map[txid][]pgid // mapping of soon-to-be free page ids by tx.
cache map[pgid]bool // fast lookup of all free and pending page ids.
}
// newFreelist returns an empty, initialized freelist.
func newFreelist() *freelist {
return &freelist{
pending: make(map[txid][]pgid),
cache: make(map[pgid]bool),
}
}
// size returns the size of the page after serialization.
func (f *freelist) size() int {
n := f.count()
if n >= 0xFFFF {
// The first element will be used to store the count. See freelist.write.
n++
}
return pageHeaderSize + (int(unsafe.Sizeof(pgid(0))) * n)
}
// count returns count of pages on the freelist
func (f *freelist) count() int {
return f.free_count() + f.pending_count()
}
// free_count returns count of free pages
func (f *freelist) free_count() int {
return len(f.ids)
}
// pending_count returns count of pending pages
func (f *freelist) pending_count() int {
var count int
for _, list := range f.pending {
count += len(list)
}
return count
}
// copyall copies into dst a list of all free ids and all pending ids in one sorted list.
// f.count returns the minimum length required for dst.
func (f *freelist) copyall(dst []pgid) {
m := make(pgids, 0, f.pending_count())
for _, list := range f.pending {
m = append(m, list...)
}
sort.Sort(m)
mergepgids(dst, f.ids, m)
}
// allocate returns the starting page id of a contiguous list of pages of a given size.
// If a contiguous block cannot be found then 0 is returned.
func (f *freelist) allocate(n int) pgid {
if len(f.ids) == 0 {
return 0
}
var initial, previd pgid
for i, id := range f.ids {
if id <= 1 {
panic(fmt.Sprintf("invalid page allocation: %d", id))
}
// Reset initial page if this is not contiguous.
if previd == 0 || id-previd != 1 {
initial = id
}
// If we found a contiguous block then remove it and return it.
if (id-initial)+1 == pgid(n) {
// If we're allocating off the beginning then take the fast path
// and just adjust the existing slice. This will use extra memory
// temporarily but the append() in free() will realloc the slice
// as is necessary.
if (i + 1) == n {
f.ids = f.ids[i+1:]
} else {
copy(f.ids[i-n+1:], f.ids[i+1:])
f.ids = f.ids[:len(f.ids)-n]
}
// Remove from the free cache.
for i := pgid(0); i < pgid(n); i++ {
delete(f.cache, initial+i)
}
return initial
}
previd = id
}
return 0
}
// free releases a page and its overflow for a given transaction id.
// If the page is already free then a panic will occur.
func (f *freelist) free(txid txid, p *page) {
if p.id <= 1 {
panic(fmt.Sprintf("cannot free page 0 or 1: %d", p.id))
}
// Free page and all its overflow pages.
var ids = f.pending[txid]
for id := p.id; id <= p.id+pgid(p.overflow); id++ {
// Verify that page is not already free.
if f.cache[id] {
panic(fmt.Sprintf("page %d already freed", id))
}
// Add to the freelist and cache.
ids = append(ids, id)
f.cache[id] = true
}
f.pending[txid] = ids
}
// release moves all page ids for a transaction id (or older) to the freelist.
func (f *freelist) release(txid txid) {
m := make(pgids, 0)
for tid, ids := range f.pending {
if tid <= txid {
// Move transaction's pending pages to the available freelist.
// Don't remove from the cache since the page is still free.
m = append(m, ids...)
delete(f.pending, tid)
}
}
sort.Sort(m)
f.ids = pgids(f.ids).merge(m)
}
// rollback removes the pages from a given pending tx.
func (f *freelist) rollback(txid txid) {
// Remove page ids from cache.
for _, id := range f.pending[txid] {
delete(f.cache, id)
}
// Remove pages from pending list.
delete(f.pending, txid)
}
// freed returns whether a given page is in the free list.
func (f *freelist) freed(pgid pgid) bool {
return f.cache[pgid]
}
// read initializes the freelist from a freelist page.
func (f *freelist) read(p *page) {
// If the page.count is at the max uint16 value (64k) then it's considered
// an overflow and the size of the freelist is stored as the first element.
idx, count := 0, int(p.count)
if count == 0xFFFF {
idx = 1
count = int(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[0])
}
// Copy the list of page ids from the freelist.
if count == 0 {
f.ids = nil
} else {
ids := ((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[idx:count]
f.ids = make([]pgid, len(ids))
copy(f.ids, ids)
// Make sure they're sorted.
sort.Sort(pgids(f.ids))
}
// Rebuild the page cache.
f.reindex()
}
// write writes the page ids onto a freelist page. All free and pending ids are
// saved to disk since in the event of a program crash, all pending ids will
// become free.
func (f *freelist) write(p *page) error {
// Combine the old free pgids and pgids waiting on an open transaction.
// Update the header flag.
p.flags |= freelistPageFlag
// The page.count can only hold up to 64k elements so if we overflow that
// number then we handle it by putting the size in the first element.
lenids := f.count()
if lenids == 0 {
p.count = uint16(lenids)
} else if lenids < 0xFFFF {
p.count = uint16(lenids)
f.copyall(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[:])
} else {
p.count = 0xFFFF
((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[0] = pgid(lenids)
f.copyall(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[1:])
}
return nil
}
// reload reads the freelist from a page and filters out pending items.
func (f *freelist) reload(p *page) {
f.read(p)
// Build a cache of only pending pages.
pcache := make(map[pgid]bool)
for _, pendingIDs := range f.pending {
for _, pendingID := range pendingIDs {
pcache[pendingID] = true
}
}
// Check each page in the freelist and build a new available freelist
// with any pages not in the pending lists.
var a []pgid
for _, id := range f.ids {
if !pcache[id] {
a = append(a, id)
}
}
f.ids = a
// Once the available list is rebuilt then rebuild the free cache so that
// it includes the available and pending free pages.
f.reindex()
}
// reindex rebuilds the free cache based on available and pending free lists.
func (f *freelist) reindex() {
f.cache = make(map[pgid]bool, len(f.ids))
for _, id := range f.ids {
f.cache[id] = true
}
for _, pendingIDs := range f.pending {
for _, pendingID := range pendingIDs {
f.cache[pendingID] = true
}
}
}