From f1719fa1d4e56303f608031fb809c6e8b7b945b8 Mon Sep 17 00:00:00 2001
From: Bryan Boreham <bjboreham@gmail.com>
Date: Wed, 7 Jan 2026 14:01:02 +0000
Subject: [PATCH] [BUGFIX] Agent: fix crash from invalid type in pool (#17802)

We have separate pools for Appender and AppenderV2 objects, and must not
put another kind of object into them.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
---
 tsdb/agent/db.go                | 16 ++++++++++++----
 tsdb/agent/db_append_v2.go      | 10 ++++++++++
 tsdb/agent/db_append_v2_test.go |  4 ++++
 tsdb/agent/db_test.go           |  3 +++
 4 files changed, 29 insertions(+), 4 deletions(-)

diff --git a/tsdb/agent/db.go b/tsdb/agent/db.go
index a0f7a93b6d..3e9d7ed714 100644
--- a/tsdb/agent/db.go
+++ b/tsdb/agent/db.go
@@ -1124,13 +1124,22 @@ func (a *appender) AppendSTZeroSample(ref storage.SeriesRef, l labels.Labels, t,
 }
 
 // Commit submits the collected samples and purges the batch.
-func (a *appenderBase) Commit() error {
+func (a *appender) Commit() error {
+	defer a.appenderPool.Put(a)
+	return a.commit()
+}
+
+func (a *appender) Rollback() error {
+	defer a.appenderPool.Put(a)
+	return a.rollback()
+}
+
+func (a *appenderBase) commit() error {
 	if err := a.log(); err != nil {
 		return err
 	}
 
 	a.clearData()
-	a.appenderPool.Put(a)
 
 	if a.writeNotified != nil {
 		a.writeNotified.Notify()
@@ -1244,7 +1253,7 @@ func (a *appenderBase) clearData() {
 	a.floatHistogramSeries = a.floatHistogramSeries[:0]
 }
 
-func (a *appenderBase) Rollback() error {
+func (a *appenderBase) rollback() error {
 	// Series are created in-memory regardless of rollback. This means we must
 	// log them to the WAL, otherwise subsequent commits may reference a series
 	// which was never written to the WAL.
@@ -1253,7 +1262,6 @@ func (a *appenderBase) Rollback() error {
 	}
 
 	a.clearData()
-	a.appenderPool.Put(a)
 	return nil
 }
 
diff --git a/tsdb/agent/db_append_v2.go b/tsdb/agent/db_append_v2.go
index f356a4feae..bb2601e1e3 100644
--- a/tsdb/agent/db_append_v2.go
+++ b/tsdb/agent/db_append_v2.go
@@ -127,6 +127,16 @@ func (a *appenderV2) Append(ref storage.SeriesRef, ls labels.Labels, st, t int64
 	return storage.SeriesRef(s.ref), partialErr
 }
 
+func (a *appenderV2) Commit() error {
+	defer a.appenderV2Pool.Put(a)
+	return a.commit()
+}
+
+func (a *appenderV2) Rollback() error {
+	defer a.appenderV2Pool.Put(a)
+	return a.rollback()
+}
+
 func (a *appenderV2) appendExemplars(s *memSeries, exemplar []exemplar.Exemplar) error {
 	var errs []error
 	for _, e := range exemplar {
diff --git a/tsdb/agent/db_append_v2_test.go b/tsdb/agent/db_append_v2_test.go
index 6a85e93c35..3e10a1163b 100644
--- a/tsdb/agent/db_append_v2_test.go
+++ b/tsdb/agent/db_append_v2_test.go
@@ -224,6 +224,10 @@ func TestCommit_AppendV2(t *testing.T) {
 	require.Equal(t, numSeries*numDatapoints, walExemplarsCount, "unexpected number of exemplars")
 	require.Equal(t, numSeries*numHistograms*2, walHistogramCount, "unexpected number of histograms")
 	require.Equal(t, numSeries*numHistograms*2, walFloatHistogramCount, "unexpected number of float histograms")
+
+	// Check that we can still create both kinds of Appender - see https://github.com/prometheus/prometheus/issues/17800.
+	_ = s.Appender(context.TODO())
+	_ = s.AppenderV2(context.TODO())
 }
 
 func TestRollback_AppendV2(t *testing.T) {
diff --git a/tsdb/agent/db_test.go b/tsdb/agent/db_test.go
index 94e84fa2eb..d2e005c175 100644
--- a/tsdb/agent/db_test.go
+++ b/tsdb/agent/db_test.go
@@ -259,6 +259,9 @@ func TestCommit(t *testing.T) {
 	require.Equal(t, numSeries*numDatapoints, walExemplarsCount, "unexpected number of exemplars")
 	require.Equal(t, numSeries*numHistograms*2, walHistogramCount, "unexpected number of histograms")
 	require.Equal(t, numSeries*numHistograms*2, walFloatHistogramCount, "unexpected number of float histograms")
+
+	// Check that we can get another appender after this - see https://github.com/prometheus/prometheus/issues/17800.
+	_ = s.Appender(context.TODO())
 }
 
 func TestRollback(t *testing.T) {