From bd0a83766c3424cea12878e880a41b58d4aabc35 Mon Sep 17 00:00:00 2001
From: machine424 <ayoubmrini424@gmail.com>
Date: Mon, 30 Dec 2024 05:14:44 +0100
Subject: [PATCH] fix(main.go): avoid closing the query engine until it is
 guaranteed to no longer be in use.

partially reverts https://github.com/prometheus/prometheus/pull/14064

fixes https://github.com/prometheus/prometheus/issues/15232

supersedes https://github.com/prometheus/prometheus/pull/15533

reusing Engine.Close() outside of tests will require more consideration.

Signed-off-by: machine424 <ayoubmrini424@gmail.com>
---
 cmd/prometheus/main.go | 9 ---------
 promql/engine.go       | 2 ++
 2 files changed, 2 insertions(+), 9 deletions(-)

diff --git a/cmd/prometheus/main.go b/cmd/prometheus/main.go
index b3bcb78b78bb..a021259f9a0b 100644
--- a/cmd/prometheus/main.go
+++ b/cmd/prometheus/main.go
@@ -1000,18 +1000,12 @@ func main() {
 	listeners, err := webHandler.Listeners()
 	if err != nil {
 		level.Error(logger).Log("msg", "Unable to start web listeners", "err", err)
-		if err := queryEngine.Close(); err != nil {
-			level.Warn(logger).Log("msg", "Closing query engine failed", "err", err)
-		}
 		os.Exit(1)
 	}
 
 	err = toolkit_web.Validate(*webConfig)
 	if err != nil {
 		level.Error(logger).Log("msg", "Unable to validate web configuration file", "err", err)
-		if err := queryEngine.Close(); err != nil {
-			level.Warn(logger).Log("msg", "Closing query engine failed", "err", err)
-		}
 		os.Exit(1)
 	}
 
@@ -1033,9 +1027,6 @@ func main() {
 				case <-cancel:
 					reloadReady.Close()
 				}
-				if err := queryEngine.Close(); err != nil {
-					level.Warn(logger).Log("msg", "Closing query engine failed", "err", err)
-				}
 				return nil
 			},
 			func(err error) {
diff --git a/promql/engine.go b/promql/engine.go
index 6cc4c69cf172..7ec73b20f306 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -434,6 +434,8 @@ func NewEngine(opts EngineOpts) *Engine {
 }
 
 // Close closes ng.
+// Callers must ensure the engine is really no longer in use before calling this to avoid
+// issues failures like in https://github.com/prometheus/prometheus/issues/15232
 func (ng *Engine) Close() error {
 	if ng == nil {
 		return nil