fix: panic when checkpoint directory is empty (#9687)

Calling `wal.NewSegmentBufReader()` without any segments would cause a
`panic` resulting in prometheus crashing. This patch fixes the panic by
making segmentBufReader return a EOF if there are not segments.

This also means an empty checkpoint directory which should never be the
case unless it has been tampered with (or has issues due to the
underlying filesystem e.g. NFS) would be ignored by Prometheus and would
continue to run instead of the current behaviour which is to panic.

Fixes: https://github.com/prometheus/prometheus/issues/9605

Signed-off-by: Sunil Thaha <sthaha@redhat.com>

tsdb/wal/wal.go

@@ -878,6 +878,10 @@ type segmentBufReader struct {
 
 // nolint:revive // TODO: Consider exporting segmentBufReader
 func NewSegmentBufReader(segs ...*Segment) *segmentBufReader {
+	if len(segs) == 0 {
+		return &segmentBufReader{}
+	}
+
 	return &segmentBufReader{
 		buf:  bufio.NewReaderSize(segs[0], 16*pageSize),
 		segs: segs,
@@ -886,9 +890,10 @@ func NewSegmentBufReader(segs ...*Segment) *segmentBufReader {
 
 // nolint:revive
 func NewSegmentBufReaderWithOffset(offset int, segs ...*Segment) (sbr *segmentBufReader, err error) {
-	if offset == 0 {
+	if offset == 0 || len(segs) == 0 {
 		return NewSegmentBufReader(segs...), nil
 	}
+
 	sbr = &segmentBufReader{
 		buf:  bufio.NewReaderSize(segs[0], 16*pageSize),
 		segs: segs,
@@ -910,6 +915,10 @@ func (r *segmentBufReader) Close() (err error) {
 
 // Read implements io.Reader.
 func (r *segmentBufReader) Read(b []byte) (n int, err error) {
+	if len(r.segs) == 0 {
+		return 0, io.EOF
+	}
+
 	n, err = r.buf.Read(b)
 	r.off += n