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