receive: fix multitsdb flush with one sample in head

CHANGELOG.md

@@ -16,6 +16,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 
 - [#6172](https://github.com/thanos-io/thanos/pull/6172) query-frontend: return JSON formatted errors for invalid PromQL expression in the split by interval middleware.
 - [#6171](https://github.com/thanos-io/thanos/pull/6171) Store: fix error handling on limits.
+- [#6183](https://github.com/thanos-io/thanos/pull/6183) Receiver: fix off by one in multitsdb flush that will result in empty blocks if the head only contains one sample
 
 ### Changed
 - [#6168](https://github.com/thanos-io/thanos/pull/6168) Receiver: Make ketama hashring fail early when configured with number of nodes lower than the replication factor.

pkg/receive/multitsdb.go

@@ -235,7 +235,7 @@ func (t *MultiTSDB) Flush() error {
 		wg.Add(1)
 		go func() {
 			head := db.Head()
-			if err := db.CompactHead(tsdb.NewRangeHead(head, head.MinTime(), head.MaxTime()-1)); err != nil {
+			if err := db.CompactHead(tsdb.NewRangeHead(head, head.MinTime(), head.MaxTime())); err != nil {
 				errmtx.Lock()
 				merr.Add(err)
 				errmtx.Unlock()

pkg/receive/multitsdb_test.go

@@ -162,6 +162,35 @@ func TestMultiTSDB(t *testing.T) {
 
 		testMulitTSDBSeries(t, m)
 	})
+
+	t.Run("flush with one sample produces a block", func(t *testing.T) {
+		const testTenant = "test_tenant"
+		m := NewMultiTSDB(
+			dir, logger, prometheus.NewRegistry(), &tsdb.Options{
+				MinBlockDuration:  (2 * time.Hour).Milliseconds(),
+				MaxBlockDuration:  (2 * time.Hour).Milliseconds(),
+				RetentionDuration: (6 * time.Hour).Milliseconds(),
+				NoLockfile:        true,
+			},
+			labels.FromStrings("replica", "01"),
+			"tenant_id",
+			nil,
+			false,
+			metadata.NoneFunc,
+		)
+		defer func() { testutil.Ok(t, m.Close()) }()
+
+		testutil.Ok(t, m.Flush())
+		testutil.Ok(t, m.Open())
+		testutil.Ok(t, appendSample(m, testTenant, time.Now()))
+
+		tenant := m.tenants[testTenant]
+		db := tenant.readyStorage().Get()
+
+		testutil.Equals(t, 0, len(db.Blocks()))
+		testutil.Ok(t, m.Flush())
+		testutil.Equals(t, 1, len(db.Blocks()))
+	})
 }
 
 var (