Make head block ULIDs descriptive (#13100)

* Make head block ULIDs descriptive

As far as I understand, these ULIDs aren't persisted anywhere, so it
should be safe to change them.

When debugging an issue, seeing an ULID like
`2ZBXFNYVVFDXFPGSB1CHFNYQTZ` or `33DXR7JA39CHDKMQ9C40H6YVVF` isn't very
helpful, so I propose to make them readable in their ULID string
version.

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

* Set a different ULID for RangeHead

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

---------

Signed-off-by: Oleg Zaytsev <mail@olegzaytsev.com>

tsdb/head.go

@@ -1407,11 +1407,13 @@ func (h *RangeHead) NumSeries() uint64 {
 	return h.head.NumSeries()
 }
 
+var rangeHeadULID = ulid.MustParse("0000000000XXXXXXXRANGEHEAD")
+
 func (h *RangeHead) Meta() BlockMeta {
 	return BlockMeta{
 		MinTime: h.MinTime(),
 		MaxTime: h.MaxTime(),
-		ULID:    h.head.Meta().ULID,
+		ULID:    rangeHeadULID,
 		Stats: BlockStats{
 			NumSeries: h.NumSeries(),
 		},
@@ -1537,15 +1539,15 @@ func (h *Head) NumSeries() uint64 {
 	return h.numSeries.Load()
 }
 
+var headULID = ulid.MustParse("0000000000XXXXXXXXXXXXHEAD")
+
 // Meta returns meta information about the head.
 // The head is dynamic so will return dynamic results.
 func (h *Head) Meta() BlockMeta {
-	var id [16]byte
-	copy(id[:], "______head______")
 	return BlockMeta{
 		MinTime: h.MinTime(),
 		MaxTime: h.MaxTime(),
-		ULID:    ulid.ULID(id),
+		ULID:    headULID,
 		Stats: BlockStats{
 			NumSeries: h.NumSeries(),
 		},

tsdb/ooo_head.go

@@ -17,6 +17,8 @@ import (
 	"fmt"
 	"sort"
 
+	"github.com/oklog/ulid"
+
 	"github.com/prometheus/prometheus/tsdb/chunkenc"
 	"github.com/prometheus/prometheus/tsdb/tombstones"
 )
@@ -135,13 +137,13 @@ func (oh *OOORangeHead) Tombstones() (tombstones.Reader, error) {
 	return tombstones.NewMemTombstones(), nil
 }
 
+var oooRangeHeadULID = ulid.MustParse("0000000000XXXX000RANGEHEAD")
+
 func (oh *OOORangeHead) Meta() BlockMeta {
-	var id [16]byte
-	copy(id[:], "____ooo_head____")
 	return BlockMeta{
 		MinTime: oh.mint,
 		MaxTime: oh.maxt,
-		ULID:    id,
+		ULID:    oooRangeHeadULID,
 		Stats: BlockStats{
 			NumSeries: oh.head.NumSeries(),
 		},

tsdb/ooo_head_read.go

@@ -18,6 +18,7 @@ import (
 	"errors"
 	"math"
 
+	"github.com/oklog/ulid"
 	"golang.org/x/exp/slices"
 
 	"github.com/prometheus/prometheus/model/labels"
@@ -371,13 +372,13 @@ func (ch *OOOCompactionHead) Tombstones() (tombstones.Reader, error) {
 	return tombstones.NewMemTombstones(), nil
 }
 
+var oooCompactionHeadULID = ulid.MustParse("0000000000XX000COMPACTHEAD")
+
 func (ch *OOOCompactionHead) Meta() BlockMeta {
-	var id [16]byte
-	copy(id[:], "copy(id[:], \"ooo_compact_head\")")
 	return BlockMeta{
 		MinTime: ch.mint,
 		MaxTime: ch.maxt,
-		ULID:    id,
+		ULID:    oooCompactionHeadULID,
 		Stats: BlockStats{
 			NumSeries: uint64(len(ch.postings)),
 		},