From fefd8ecfe397eb5f27e1f0c13ed574f1e9e54a7e Mon Sep 17 00:00:00 2001
From: Ayoub Mrini <ayoubmrini424@gmail.com>
Date: Mon, 30 Dec 2024 12:17:47 +0100
Subject: [PATCH] [BUGFIX] avoid closing the query engine until it is
 guaranteed to no longer be in use. (#15724)

Backport of #15723

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 cd0775bbdd..249c6d5490 100644
--- a/cmd/prometheus/main.go
+++ b/cmd/prometheus/main.go
@@ -988,18 +988,12 @@ func main() {
 	listeners, err := webHandler.Listeners()
 	if err != nil {
 		logger.Error("Unable to start web listener", "err", err)
-		if err := queryEngine.Close(); err != nil {
-			logger.Warn("Closing query engine failed", "err", err)
-		}
 		os.Exit(1)
 	}
 
 	err = toolkit_web.Validate(*webConfig)
 	if err != nil {
 		logger.Error("Unable to validate web configuration file", "err", err)
-		if err := queryEngine.Close(); err != nil {
-			logger.Warn("Closing query engine failed", "err", err)
-		}
 		os.Exit(1)
 	}
 
@@ -1021,9 +1015,6 @@ func main() {
 				case <-cancel:
 					reloadReady.Close()
 				}
-				if err := queryEngine.Close(); err != nil {
-					logger.Warn("Closing query engine failed", "err", err)
-				}
 				return nil
 			},
 			func(err error) {
diff --git a/promql/engine.go b/promql/engine.go
index a23e899d04..bbd8410268 100644
--- a/promql/engine.go
+++ b/promql/engine.go
@@ -436,6 +436,8 @@ 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