From a5f86c3fb6c394ba1c3e4d9934e4af5601ba33a4 Mon Sep 17 00:00:00 2001
From: Arve Knudsen <arve.knudsen@gmail.com>
Date: Tue, 27 Jan 2026 10:02:16 +0100
Subject: [PATCH] cmd/prometheus: fix flaky TestQueryLog race condition
 (#17933)

Add waitForQueryLog helper that polls for query log entries to appear
before asserting, rather than reading the file immediately after making
a query. This fixes a race condition where the query log wasn't flushed
to disk before the test read the file.

The helper uses a 5 second timeout with 100ms polling intervals, which
is generous enough to handle slow CI environments while keeping the test
responsive.

Signed-off-by: Arve Knudsen <arve.knudsen@gmail.com>
---
 cmd/prometheus/query_log_test.go | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/cmd/prometheus/query_log_test.go b/cmd/prometheus/query_log_test.go
index 5e5a9ac3b7..e410f836a9 100644
--- a/cmd/prometheus/query_log_test.go
+++ b/cmd/prometheus/query_log_test.go
@@ -334,7 +334,8 @@ func (p *queryLogTest) run(t *testing.T) {
 
 	p.query(t)
 
-	ql := readQueryLog(t, queryLogFile.Name())
+	// Wait for query log entry to be written (avoid race with file I/O).
+	ql := waitForQueryLog(t, queryLogFile.Name(), 1)
 	qc := len(ql)
 	if p.exactQueryCount() {
 		require.Equal(t, 1, qc)
@@ -361,7 +362,8 @@ func (p *queryLogTest) run(t *testing.T) {
 	p.query(t)
 	qc++
 
-	ql = readQueryLog(t, queryLogFile.Name())
+	// Wait for query log entry to be written (avoid race with file I/O).
+	ql = waitForQueryLog(t, queryLogFile.Name(), qc)
 	if p.exactQueryCount() {
 		require.Len(t, ql, qc)
 	} else {
@@ -392,7 +394,8 @@ func (p *queryLogTest) run(t *testing.T) {
 
 	qc++
 
-	ql = readQueryLog(t, newFile.Name())
+	// Wait for query log entry to be written (avoid race with file I/O).
+	ql = waitForQueryLog(t, newFile.Name(), qc)
 	if p.exactQueryCount() {
 		require.Len(t, ql, qc)
 	} else {
@@ -404,7 +407,8 @@ func (p *queryLogTest) run(t *testing.T) {
 
 	p.query(t)
 
-	ql = readQueryLog(t, queryLogFile.Name())
+	// Wait for query log entry to be written (avoid race with file I/O).
+	ql = waitForQueryLog(t, queryLogFile.Name(), 1)
 	qc = len(ql)
 	if p.exactQueryCount() {
 		require.Equal(t, 1, qc)
@@ -446,6 +450,18 @@ func readQueryLog(t *testing.T, path string) []queryLogLine {
 	return ql
 }
 
+// waitForQueryLog waits for the query log to contain at least minEntries entries,
+// polling at regular intervals until the timeout is reached.
+func waitForQueryLog(t *testing.T, path string, minEntries int) []queryLogLine {
+	t.Helper()
+	var ql []queryLogLine
+	require.Eventually(t, func() bool {
+		ql = readQueryLog(t, path)
+		return len(ql) >= minEntries
+	}, 5*time.Second, 100*time.Millisecond, "timed out waiting for query log to have at least %d entries, got %d", minEntries, len(ql))
+	return ql
+}
+
 func TestQueryLog(t *testing.T) {
 	if testing.Short() {
 		t.Skip("skipping test in short mode.")