Method WriteAt

vfs.go:1542–1598

(b []byte, off int64)

}

func (f *VFSFile) WriteAt(b []byte, off int64) (n int, err error) {
	f.logger.Debug("write at", "off", off, "len", len(b))

	pageSize, err := f.pageSizeBytes()
	if err != nil {
		return 0, err
	}

	// Calculate page number and offset within page
	pgno := uint32(off/int64(pageSize)) + 1
	pageOffset := int(off % int64(pageSize))

	// Skip lock page
	if pgno == ltx.LockPgno(pageSize) {
		return 0, fmt.Errorf("cannot write to lock page")
	}

	f.mu.Lock()
	defer f.mu.Unlock()

	// If write support is not enabled, return read-only error
	if !f.writeEnabled {
		return 0, sqlite3vfs.ReadOnlyError
	}

	// Get page data - either from buffer file (if dirty) or from cache/remote
	page := make([]byte, pageSize)
	if bufferOff, ok := f.dirty[pgno]; ok {
		// Page is already dirty - read from buffer file
		if _, err := f.bufferFile.ReadAt(page, bufferOff); err != nil {
			return 0, fmt.Errorf("read dirty page from buffer: %w", err)
		}
	} else {
		// Page is not dirty - read from cache/remote
		if err := f.readPageForWrite(pgno, page); err != nil {
			// If page doesn't exist, use zero-filled page
			f.logger.Debug("page not found, using empty page", "pgno", pgno)
		}
	}

	// Apply write to page
	n = copy(page[pageOffset:], b)

	// Update commit count if this extends the database
	if pgno > f.commit {
		f.commit = pgno
	}

	// Write to buffer for durability (this updates f.dirty with the offset)
	if err := f.writeToBuffer(pgno, page); err != nil {
		f.logger.Error("failed to write to buffer", "error", err)
		return 0, fmt.Errorf("write to buffer: %w", err)
	}

	f.logger.Debug("wrote to dirty page", "pgno", pgno, "offset", pageOffset, "len", n, "commit", f.commit)
	return n, nil
}