//go:build unix

// mmap-on-unix loader for SnapshotReader. The file handle stays alive for
// the lifetime of the mmap region (the kernel needs it pinned). Closer
// munmaps AND closes the fd. Uses MADV_RANDOM because the dominant access
// pattern is binary-search-by-SKU — touching random record slots, not
// streaming the file.
package catalog

import (
	"fmt"
	"os"

	"golang.org/x/sys/unix"
)

// loadSnapshotBytes returns the file contents (mmap'd) + a closer that munmaps
// and closes the fd.
func loadSnapshotBytes(path string) (data []byte, closer func() error, err error) {
	f, err := os.Open(path)
	if err != nil {
		return nil, nil, err
	}
	info, err := f.Stat()
	if err != nil {
		_ = f.Close()
		return nil, nil, err
	}
	size := info.Size()
	if size == 0 {
		_ = f.Close()
		return []byte{}, func() error { return nil }, nil
	}
	if size < int64(headerSize) {
		_ = f.Close()
		return nil, nil, ErrSnapshotTruncated
	}

	data, mmapErr := unix.Mmap(int(f.Fd()), 0, int(size), unix.PROT_READ, unix.MAP_SHARED)
	if mmapErr != nil {
		_ = f.Close()
		return nil, nil, fmt.Errorf("catalog: mmap %s: %w", path, mmapErr)
	}

	// Binary-search by SKU = random page access; MADV_RANDOM tells the
	// kernel NOT to do read-ahead — saves I/O on cold layers.
	if maErr := unix.Madvise(data, unix.MADV_RANDOM); maErr != nil {
		// non-fatal; degrade gracefully
		_ = maErr
	}

	closer = func() error {
		mErr := unix.Munmap(data)
		cErr := f.Close()
		if mErr != nil {
			return mErr
		}
		return cErr
	}
	return data, closer, nil
}