From 55effbfc3aa2a6b678252fa1ef925f7e3bf05c51 Mon Sep 17 00:00:00 2001
From: Carrie Edwards <edwrdscarrie@gmail.com>
Date: Tue, 5 May 2026 09:52:37 -0700
Subject: [PATCH] Fix for appendHistogram

Signed-off-by: Carrie Edwards <edwrdscarrie@gmail.com>
---
 tsdb/head_append.go | 16 +++++++++++--
 tsdb/head_test.go   | 57 +++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 71 insertions(+), 2 deletions(-)

diff --git a/tsdb/head_append.go b/tsdb/head_append.go
index c7143d8d96..e6d243f074 100644
--- a/tsdb/head_append.go
+++ b/tsdb/head_append.go
@@ -1873,7 +1873,13 @@ func (s *memSeries) appendHistogram(st, t int64, h *histogram.Histogram, appendI
 	// chunk reference afterwards and mmap used up chunks.
 
 	// Ignoring ok is ok, since we don't want to compare to the wrong previous appender anyway.
-	prevApp, _ := s.app.(*chunkenc.HistogramAppender)
+	var prevApp *chunkenc.HistogramAppender
+	switch p := s.app.(type) {
+	case *chunkenc.HistogramAppender:
+		prevApp = p
+	case *chunkenc.HistogramSTAppender:
+		prevApp = &p.HistogramAppender
+	}
 
 	c, sampleInOrder, chunkCreated := s.histogramsAppendPreprocessor(t, chunkenc.ValHistogram.ChunkEncoding(o.useXOR2), o)
 	if !sampleInOrder {
@@ -1930,7 +1936,13 @@ func (s *memSeries) appendFloatHistogram(st, t int64, fh *histogram.FloatHistogr
 	// chunk reference afterwards and mmap used up chunks.
 
 	// Ignoring ok is ok, since we don't want to compare to the wrong previous appender anyway.
-	prevApp, _ := s.app.(*chunkenc.FloatHistogramAppender)
+	var prevApp *chunkenc.FloatHistogramAppender
+	switch p := s.app.(type) {
+	case *chunkenc.FloatHistogramAppender:
+		prevApp = p
+	case *chunkenc.FloatHistogramSTAppender:
+		prevApp = &p.FloatHistogramAppender
+	}
 
 	c, sampleInOrder, chunkCreated := s.histogramsAppendPreprocessor(t, chunkenc.ValFloatHistogram.ChunkEncoding(o.useXOR2), o)
 	if !sampleInOrder {
diff --git a/tsdb/head_test.go b/tsdb/head_test.go
index 1000b292ea..7f8eb84286 100644
--- a/tsdb/head_test.go
+++ b/tsdb/head_test.go
@@ -5179,6 +5179,63 @@ func TestHistogramCounterResetHeader(t *testing.T) {
 	}
 }
 
+func TestHistogramSTCounterResetHeaderOnChunkCut(t *testing.T) {
+	for _, floatHisto := range []bool{false, true} {
+		t.Run(fmt.Sprintf("floatHistogram=%t", floatHisto), func(t *testing.T) {
+			l := labels.FromStrings("a", "b")
+			opts := newTestHeadDefaultOptions(2, false)
+			opts.EnableXOR2Encoding.Store(true)
+			head, _ := newTestHeadWithOptions(t, compression.None, opts)
+			t.Cleanup(func() {
+				require.NoError(t, head.Close())
+			})
+			require.NoError(t, head.Init(0))
+
+			appendHistogram := func(ts int64, h *histogram.Histogram) {
+				app := head.Appender(context.Background())
+				var err error
+				if floatHisto {
+					_, err = app.AppendHistogram(0, l, ts, nil, h.ToFloat(nil))
+				} else {
+					_, err = app.AppendHistogram(0, l, ts, h.Copy(), nil)
+				}
+				require.NoError(t, err)
+				require.NoError(t, app.Commit())
+			}
+
+			h1 := tsdbutil.GenerateTestHistogram(0)
+			h2 := tsdbutil.GenerateTestHistogram(1)
+
+			// Chunk range is 2, so appending at t=1 and t=2 cuts a new chunk on the second append.
+			appendHistogram(1, h1)
+			appendHistogram(2, h2)
+
+			ms, _, err := head.getOrCreate(l.Hash(), l, false)
+			require.NoError(t, err)
+			require.NotNil(t, ms.headChunks)
+			require.NotNil(t, ms.headChunks.prev)
+
+			if floatHisto {
+				prevChunk, ok := ms.headChunks.prev.chunk.(*chunkenc.FloatHistogramSTChunk)
+				require.True(t, ok)
+				require.Equal(t, chunkenc.UnknownCounterReset, prevChunk.GetCounterResetHeader())
+
+				headChunk, ok := ms.headChunks.chunk.(*chunkenc.FloatHistogramSTChunk)
+				require.True(t, ok)
+				require.Equal(t, chunkenc.NotCounterReset, headChunk.GetCounterResetHeader())
+			} else {
+				prevChunk, ok := ms.headChunks.prev.chunk.(*chunkenc.HistogramSTChunk)
+				require.True(t, ok)
+				require.Equal(t, chunkenc.UnknownCounterReset, prevChunk.GetCounterResetHeader())
+
+				headChunk, ok := ms.headChunks.chunk.(*chunkenc.HistogramSTChunk)
+				require.True(t, ok)
+				require.Equal(t, chunkenc.NotCounterReset, headChunk.GetCounterResetHeader())
+			}
+		})
+	}
+}
+
 func TestOOOHistogramCounterResetHeaders(t *testing.T) {
 	for _, floatHisto := range []bool{true, false} {
 		t.Run(fmt.Sprintf("floatHistogram=%t", floatHisto), func(t *testing.T) {