tsdb: fix division by zero in stale series compaction (#17952)

Guard the stale series ratio calculation by checking numSeries > 0
before computing the ratio. This prevents division by zero when
the head has no series.

Fixes #17949

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>

tsdb/db.go

@@ -1172,22 +1172,23 @@ func (db *DB) run(ctx context.Context) {
 			db.head.mmapHeadChunks()
 
 			numStaleSeries, numSeries := db.Head().NumStaleSeries(), db.Head().NumSeries()
-			staleSeriesRatio := float64(numStaleSeries) / float64(numSeries)
-			if db.autoCompact && db.opts.staleSeriesCompactionThreshold.Load() > 0 &&
-				staleSeriesRatio >= db.opts.staleSeriesCompactionThreshold.Load() {
-				nextCompactionIsSoon := false
-				if !db.lastHeadCompactionTime.IsZero() {
-					compactionInterval := time.Duration(db.head.chunkRange.Load()) * time.Millisecond
-					nextEstimatedCompactionTime := db.lastHeadCompactionTime.Add(compactionInterval)
-					if time.Now().Add(10 * time.Minute).After(nextEstimatedCompactionTime) {
-						// Next compaction is starting within next 10 mins.
-						nextCompactionIsSoon = true
+			if db.autoCompact && numSeries > 0 && db.opts.staleSeriesCompactionThreshold.Load() > 0 {
+				staleSeriesRatio := float64(numStaleSeries) / float64(numSeries)
+				if staleSeriesRatio >= db.opts.staleSeriesCompactionThreshold.Load() {
+					nextCompactionIsSoon := false
+					if !db.lastHeadCompactionTime.IsZero() {
+						compactionInterval := time.Duration(db.head.chunkRange.Load()) * time.Millisecond
+						nextEstimatedCompactionTime := db.lastHeadCompactionTime.Add(compactionInterval)
+						if time.Now().Add(10 * time.Minute).After(nextEstimatedCompactionTime) {
+							// Next compaction is starting within next 10 mins.
+							nextCompactionIsSoon = true
+						}
 					}
-				}
 
-				if !nextCompactionIsSoon {
-					if err := db.CompactStaleHead(); err != nil {
-						db.logger.Error("immediate stale series compaction failed", "err", err)
+					if !nextCompactionIsSoon {
+						if err := db.CompactStaleHead(); err != nil {
+							db.logger.Error("immediate stale series compaction failed", "err", err)
+						}
 					}
 				}
 			}

tsdb/db_test.go

@@ -9561,3 +9561,27 @@ func TestStaleSeriesCompaction(t *testing.T) {
 		verifyHeadBlock()
 	}
 }
+
+// TestStaleSeriesCompactionWithZeroSeries verifies that CompactStaleHead handles
+// an empty head (0 series) gracefully without division by zero or incorrectly
+// triggering compaction. This is a regression test for issue #17949.
+func TestStaleSeriesCompactionWithZeroSeries(t *testing.T) {
+	opts := DefaultOptions()
+	opts.MinBlockDuration = 1000
+	opts.MaxBlockDuration = 1000
+	db := newTestDB(t, withOpts(opts))
+	db.DisableCompactions()
+	t.Cleanup(func() {
+		require.NoError(t, db.Close())
+	})
+
+	// Verify the head is empty.
+	require.Equal(t, uint64(0), db.Head().NumSeries())
+	require.Equal(t, uint64(0), db.Head().NumStaleSeries())
+
+	// CompactStaleHead should handle zero series gracefully (no panic, no error).
+	require.NoError(t, db.CompactStaleHead())
+
+	// Should still have no blocks since there was nothing to compact.
+	require.Empty(t, db.Blocks())
+}