From 855f3a9844aea433c858bc0dea630505815c68dd Mon Sep 17 00:00:00 2001
From: pawarpranav83 <pawarpranav@gmail.com>
Date: Wed, 27 Dec 2023 16:22:18 +0530
Subject: [PATCH] Added negative offset check for caching queries

Signed-off-by: pawarpranav83 <pawarpranav@gmail.com>
---
 CHANGELOG.md                                  |  1 +
 .../querier/queryrange/results_cache.go       | 44 +++++++++++++++++++
 .../querier/queryrange/results_cache_test.go  | 39 ++++++++++++++++
 3 files changed, 84 insertions(+)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index b4d407ee276..2fe31eec01d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -12,6 +12,7 @@ We use *breaking :warning:* to mark changes that are not backward compatible (re
 
 ### Fixed
 
+- [#7011] (https://github.com/thanos-io/thanos/pull/7011) Query Frontend: queries with negative offset should check whether it is cacheable or not.
 - [#6874](https://github.com/thanos-io/thanos/pull/6874) Sidecar: fix labels returned by 'api/v1/series' in presence of conflicting external and inner labels.
 - [#7009](https://github.com/thanos-io/thanos/pull/7009) Rule: Fix spacing error in URL.
 
diff --git a/internal/cortex/querier/queryrange/results_cache.go b/internal/cortex/querier/queryrange/results_cache.go
index d29f5c33050..266bcd2b605 100644
--- a/internal/cortex/querier/queryrange/results_cache.go
+++ b/internal/cortex/querier/queryrange/results_cache.go
@@ -282,6 +282,9 @@ func (s resultsCache) shouldCacheResponse(ctx context.Context, req Request, r Re
 	if !s.isAtModifierCachable(req, maxCacheTime) {
 		return false
 	}
+	if !s.isOffsetCachable(req) {
+		return false
+	}
 
 	if s.cacheGenNumberLoader == nil {
 		return true
@@ -357,6 +360,47 @@ func (s resultsCache) isAtModifierCachable(r Request, maxCacheTime int64) bool {
 	return atModCachable
 }
 
+var negativeOffset = errors.New("negative offset")
+
+// isOffsetCachable returns true if the offset is positive, result is safe to cache.
+// and false when offset is negative, result is not cached.
+func (s resultsCache) isOffsetCachable(r Request) bool {
+	query := r.GetQuery()
+	if !strings.Contains(query, "offset") {
+		return true
+	}
+	expr, err := parser.ParseExpr(query)
+	if err != nil {
+		level.Warn(s.logger).Log("msg", "failed to parse query, considering offset as not cachable", "query", query, "err", err)
+		return false
+	}
+
+	offsetCachable := true
+	parser.Inspect(expr, func(n parser.Node, _ []parser.Node) error {
+		switch e := n.(type) {
+		case *parser.VectorSelector:
+			if e.OriginalOffset < 0 {
+				offsetCachable = false
+				return negativeOffset
+			}
+		case *parser.MatrixSelector:
+			offset := e.VectorSelector.(*parser.VectorSelector).OriginalOffset
+			if offset < 0 {
+				offsetCachable = false
+				return negativeOffset
+			}
+		case *parser.SubqueryExpr:
+			if e.OriginalOffset < 0 {
+				offsetCachable = false
+				return negativeOffset
+			}
+		}
+		return nil
+	})
+
+	return offsetCachable
+}
+
 func getHeaderValuesWithName(r Response, headerName string) (headerValues []string) {
 	for _, hv := range r.GetHeaders() {
 		if hv.GetName() != headerName {
diff --git a/internal/cortex/querier/queryrange/results_cache_test.go b/internal/cortex/querier/queryrange/results_cache_test.go
index cb7d04fc95b..cb12d5cb231 100644
--- a/internal/cortex/querier/queryrange/results_cache_test.go
+++ b/internal/cortex/querier/queryrange/results_cache_test.go
@@ -532,6 +532,45 @@ func TestShouldCache(t *testing.T) {
 			input:    Response(&PrometheusResponse{}),
 			expected: false,
 		},
+		// offset on vector selectors.
+		{
+			name:     "positive offset on vector selector",
+			request:  &PrometheusRequest{Query: "metric offset 10ms", End: 125000},
+			input:    Response(&PrometheusResponse{}),
+			expected: true,
+		},
+		{
+			name:     "negative offset on vector selector",
+			request:  &PrometheusRequest{Query: "metric offset -10ms", End: 125000},
+			input:    Response(&PrometheusResponse{}),
+			expected: false,
+		},
+		// offset on matrix selectors.
+		{
+			name:     "positive offset on matrix selector",
+			request:  &PrometheusRequest{Query: "rate(metric[5m] offset 10ms)", End: 125000},
+			input:    Response(&PrometheusResponse{}),
+			expected: true,
+		},
+		{
+			name:     "negative offset on matrix selector",
+			request:  &PrometheusRequest{Query: "rate(metric[5m] offset -10ms)", End: 125000},
+			input:    Response(&PrometheusResponse{}),
+			expected: false,
+		},
+		// offset on subqueries.
+		{
+			name:     "positive offset on subqueries",
+			request:  &PrometheusRequest{Query: "sum_over_time(rate(metric[1m])[10m:1m] offset 10ms)", End: 125000},
+			input:    Response(&PrometheusResponse{}),
+			expected: true,
+		},
+		{
+			name:     "negative offset on subqueries",
+			request:  &PrometheusRequest{Query: "sum_over_time(rate(metric[1m])[10m:1m] offset -10ms)", End: 125000},
+			input:    Response(&PrometheusResponse{}),
+			expected: false,
+		},
 	} {
 		{
 			t.Run(tc.name, func(t *testing.T) {