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 { return pageHeaderSize + (int(unsafe.Sizeof(pgid(0))) * f.count()) } // 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 } // all returns a list of all free ids and all pending ids in one sorted list. func (f *freelist) all() []pgid { ids := make([]pgid, len(f.ids)) copy(ids, f.ids) for _, list := range f.pending { ids = append(ids, list...) } sort.Sort(pgids(ids)) return ids } // 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) { 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. f.ids = append(f.ids, ids...) delete(f.pending, tid) } } sort.Sort(pgids(f.ids)) } // 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. 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. ids := f.all() // 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. if len(ids) < 0xFFFF { p.count = uint16(len(ids)) copy(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[:], ids) } else { p.count = 0xFFFF ((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[0] = pgid(len(ids)) copy(((*[maxAllocSize]pgid)(unsafe.Pointer(&p.ptr)))[1:], ids) } 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) for _, id := range f.ids { f.cache[id] = true } for _, pendingIDs := range f.pending { for _, pendingID := range pendingIDs { f.cache[pendingID] = true } } }