From 5895510a2e09a1d64ce0b680ee9f9af6443a44b0 Mon Sep 17 00:00:00 2001
From: Michael Hoffmann <mhoffmann@cloudflare.com>
Date: Thu, 9 Jan 2025 09:24:01 +0000
Subject: [PATCH] query: fix panic when selecting non-default engine

Fix duplicate metrics registration when selecting non-default engine

Signed-off-by: Michael Hoffmann <mhoffmann@cloudflare.com>
---
 pkg/api/query/v1.go | 29 +++++++++++------------------
 1 file changed, 11 insertions(+), 18 deletions(-)

diff --git a/pkg/api/query/v1.go b/pkg/api/query/v1.go
index 0eaec2de72..049dc482a7 100644
--- a/pkg/api/query/v1.go
+++ b/pkg/api/query/v1.go
@@ -133,26 +133,10 @@ type QueryEngineFactory struct {
 }
 
 func (f *QueryEngineFactory) GetPrometheusEngine() Engine {
-	f.createPrometheusEngine.Do(func() {
-		if f.prometheus != nil {
-			return
-		}
-		f.prometheus = &prometheusEngineAdapter{engine: promql.NewEngine(f.engineOpts.EngineOpts)}
-	})
 	return f.prometheus
 }
 
 func (f *QueryEngineFactory) GetThanosEngine() Engine {
-	f.createThanosEngine.Do(func() {
-		if f.thanos != nil {
-			return
-		}
-		if f.remoteEngineEndpoints == nil {
-			f.thanos = engine.New(f.engineOpts)
-		} else {
-			f.thanos = engine.NewDistributedEngine(f.engineOpts, f.remoteEngineEndpoints)
-		}
-	})
 	return f.thanos
 }
 
@@ -160,9 +144,18 @@ func NewQueryEngineFactory(
 	engineOpts engine.Opts,
 	remoteEngineEndpoints promqlapi.RemoteEndpoints,
 ) *QueryEngineFactory {
+	promEngine := promql.NewEngine(engineOpts.EngineOpts)
+	// set engine here to avoid duplicate registration of metrics
+	engineOpts.Engine = promEngine
+	var thanosEngine Engine
+	if remoteEngineEndpoints == nil {
+		thanosEngine = engine.New(engineOpts)
+	} else {
+		thanosEngine = engine.NewDistributedEngine(engineOpts, remoteEngineEndpoints)
+	}
 	return &QueryEngineFactory{
-		engineOpts:            engineOpts,
-		remoteEngineEndpoints: remoteEngineEndpoints,
+		prometheus: &prometheusEngineAdapter{promEngine},
+		thanos:     thanosEngine,
 	}
 }