more platform

projection/snapshot_unix.go

@@ -0,0 +1,61 @@
+//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-key — touching random record slots, not
+// streaming the file.
+package projection
+
+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("projection: mmap %s: %w", path, mmapErr)
+	}
+
+	// Binary-search by key = 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
+}