Method allocate

db.go:1165–1220

allocate returns a contiguous block of memory starting at a given page.

(txid common.Txid, count int)

// allocate returns a contiguous block of memory starting at a given page.
func (db *DB) allocate(txid common.Txid, count int) (*common.Page, error) {
	// Allocate a temporary buffer for the page.
	var buf []byte
	if count == 1 {
		buf = db.pagePool.Get().([]byte)
	} else {
		buf = make([]byte, count*db.pageSize)
	}
	p := (*common.Page)(unsafe.Pointer(&buf[0]))
	p.SetOverflow(uint32(count - 1))

	// Use pages from the freelist if they are available.
	p.SetId(db.freelist.Allocate(txid, count))
	if p.Id() != 0 {
		return p, nil
	}

	// Resize mmap() if we're at the end.
	p.SetId(db.rwtx.meta.Pgid())
	var minsz = int((p.Id()+common.Pgid(count))+1) * db.pageSize
	if db.MaxSize > 0 {
		nextAllocSize := minsz
		nextMmapSize, err := db.mmapSize(minsz)
		if err != nil {
			return nil, fmt.Errorf("mmap size calculation error: %w", err)
		}
		if runtime.GOOS == "windows" {
			// nextAllocSize may not exactly match nextMmapSize.
			// On Windows, this mismatch may cause the file size to slightly exceed maxSize,
			// while it is harmless on other platforms.
			nextAllocSize = nextMmapSize
		} else {
			// On non-Windows platforms, the database file is only grown explicitly in grow calls.
			nextAllocSize = db.growSize(nextMmapSize, nextAllocSize)
		}
		if nextAllocSize > db.MaxSize {
			db.Logger().Errorf("[GOOS: %s, GOARCH: %s] maximum db size reached, minSize: %d (allocSize: %d), db.MaxSize: %d", runtime.GOOS, runtime.GOARCH, minsz, nextAllocSize, db.MaxSize)
			return nil, berrors.ErrMaxSizeReached
		}
	}
	if minsz >= db.datasz {
		if err := db.mmap(minsz); err != nil {
			if err == berrors.ErrMaxSizeReached {
				return nil, err
			} else {
				return nil, fmt.Errorf("mmap allocate error: %s", err)
			}
		}
	}

	// Move the page id high water mark.
	curPgid := db.rwtx.meta.Pgid()
	db.rwtx.meta.SetPgid(curPgid + common.Pgid(count))

	return p, nil
}

// grow grows the size of the database to the given sz.