Method ReadAt

vfs.go:1422–1540

(p []byte, off int64)

}

func (f *VFSFile) ReadAt(p []byte, off int64) (n int, err error) {
	f.logger.Debug("reading at", "off", off, "len", len(p))
	pageSize, err := f.pageSizeBytes()
	if err != nil {
		return 0, err
	}

	pgno := uint32(off/int64(pageSize)) + 1
	pageOffset := int(off % int64(pageSize))

	// Check dirty pages first (takes priority over cache and remote)
	f.mu.Lock()
	if f.writeEnabled {
		if bufferOff, ok := f.dirty[pgno]; ok {
			// Read page from buffer file
			data := make([]byte, pageSize)
			if _, err := f.bufferFile.ReadAt(data, bufferOff); err != nil {
				f.mu.Unlock()
				return 0, fmt.Errorf("read dirty page from buffer: %w", err)
			}
			n = copy(p, data[pageOffset:])
			f.mu.Unlock()
			f.logger.Debug("dirty page hit", "page", pgno, "n", n)

			// Update the first page to pretend like we are in journal mode.
			if off == 0 && len(p) >= 28 {
				p[18], p[19] = 0x01, 0x01
				_, _ = rand.Read(p[24:28])
			}

			return n, nil
		}
	}
	f.mu.Unlock()

	// If hydration complete, read from local file
	if f.hydrator != nil && f.hydrator.Complete() {
		return f.hydrator.ReadAt(p, off)
	}

	// Check cache (cache is thread-safe)
	if data, ok := f.cache.Get(pgno); ok {
		n = copy(p, data[pageOffset:])
		f.logger.Debug("cache hit", "page", pgno, "n", n)

		// Update the first page to pretend like we are in journal mode.
		if off == 0 {
			p[18], p[19] = 0x01, 0x01
			_, _ = rand.Read(p[24:28])
		}

		return n, nil
	}

	// Get page index element
	f.mu.Lock()
	elem, ok := f.index[pgno]
	writeEnabled := f.writeEnabled // capture while holding lock to avoid data race