From 678e3b376332bb9074c8e01e8b2ba1f971f54370 Mon Sep 17 00:00:00 2001
From: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
Date: Wed, 22 Apr 2026 10:08:23 +0200
Subject: [PATCH] promql: fix spurious reset when prevST is invalid in
 isStartTimestampReset

Signed-off-by: Julien Pivotto <291750+roidelapluie@users.noreply.github.com>
---
 promql/functions.go                              |  6 ++++++
 promql/promqltest/testdata/start_timestamps.test | 16 ++++++++++------
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/promql/functions.go b/promql/functions.go
index 7b3fd2d363..6ca32c4a81 100644
--- a/promql/functions.go
+++ b/promql/functions.go
@@ -447,6 +447,12 @@ func isStartTimestampReset(prevStartTimestamp, prevTimestamp, currStartTimestamp
 	// This should be treated as a reset for deltas, but it is not a reset for cumulative series
 	// with unknown start timestamp. Thus we have to check whether the start timestamp
 	// of the previous datapoint is known.
+	//
+	// A previous start timestamp greater than the previous sample timestamp is invalid; treat it
+	// as unknown to avoid a spurious reset.
+	if prevStartTimestamp > prevTimestamp {
+		return false
+	}
 	return prevStartTimestamp != 0
 }
 
diff --git a/promql/promqltest/testdata/start_timestamps.test b/promql/promqltest/testdata/start_timestamps.test
index c7148e5cf3..3ad31e85ba 100644
--- a/promql/promqltest/testdata/start_timestamps.test
+++ b/promql/promqltest/testdata/start_timestamps.test
@@ -94,17 +94,21 @@ clear
 
 # Test for cumulative with unknow start timestamp, which is described in OTel spec. It is easy to incorrectly treat
 # second datapoint in such timeseries as delta, producing incorrect result. These can be distinguished by:
-#   * ST_curr == T_prev && ST_prev == 0 -> cumulative with unknown start, no reset
-#   * ST_curr == T_prev && ST_prev != 0 -> delta datapoint, treat as a reset
+#   * ST_curr == T_prev && ST_prev == 0  -> cumulative with unknown start, no reset
+#   * ST_curr == T_prev && ST_prev != 0  -> delta datapoint, treat as a reset
+#   * ST_curr == T_prev && ST_prev > T_prev (invalid) -> must not be mistaken for the delta case above
 load 1m
-    series{type="otel_cumulative_unknown_start"}@st  _ _   _ -1m
-    series{type="otel_cumulative_unknown_start"}     _ _   1   1
-    series{type="otel_delta"}@st                     _ _ -1m -1m
-    series{type="otel_delta"}                        _ _   1   1
+    series{type="otel_cumulative_unknown_start"}@st  _ _    _ -1m
+    series{type="otel_cumulative_unknown_start"}     _ _    1   1
+    series{type="otel_delta"}@st                     _ _ -1m  -1m
+    series{type="otel_delta"}                        _ _    1    1
+    series{type="invalid_prev_st"}@st                _ _ +1m  -1m
+    series{type="invalid_prev_st"}                   _ _    1    1
 
 eval instant at 3m round(increase(series[1m1ms]))
     {type="otel_cumulative_unknown_start"}  0
     {type="otel_delta"}                     1
+    {type="invalid_prev_st"}                0
 
 clear