[8.19] Improve rolling appender test logic (#242095) (#242260)

# Backport

This will backport the following commits from `main` to `8.19`:
- [Improve rolling appender test logic
(#242095)](#242095)

<!--- Backport version: 9.6.6 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sorenlouv/backport)

<!--BACKPORT [{"author":{"name":"Gerard
Soldevila","email":"[email protected]"},"sourceCommit":{"committedDate":"2025-11-07T13:27:52Z","message":"Improve
rolling appender test logic (#242095)\n\n## Summary\n\nCloses
https://github.com/elastic/kibana/issues/232893\n\nThe previous
improvements were not enough,
[latest\nfailure](https://buildkite.com/elastic/kibana-on-merge/builds/81112/steps/canvas?jid=019a4a9c-77de-450f-a4e8-70d4efc34012)\nshows
that we can't guarantee how many times the rolling logic has
run:\n\n<img width=\"846\" height=\"272\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/9ccaac11-92bb-42cf-8b5a-60fd258c23d9\"\n/>\n\nThe
PR makes the test even more resilient by not assuming that the\nrolling
logid has run exactly twice, and instead it just makes\nassertions that
are valid for the 2 scenarios:\n* Assert there are multiple log files
(rollover has happened).\n* Assert that the rolled files are not
empty\n* Assert that when concatenating all log files in an ordered
fashion, no\nlog entries are
lost.","sha":"11a9589ea8068a734dcd0eb9845d462ab73330e2","branchLabelMapping":{"^v9.3.0$":"main","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["Team:Core","release_note:skip","test-failure-flaky","backport:all-open","v9.3.0"],"title":"Improve
rolling appender test
logic","number":242095,"url":"https://github.com/elastic/kibana/pull/242095","mergeCommit":{"message":"Improve
rolling appender test logic (#242095)\n\n## Summary\n\nCloses
https://github.com/elastic/kibana/issues/232893\n\nThe previous
improvements were not enough,
[latest\nfailure](https://buildkite.com/elastic/kibana-on-merge/builds/81112/steps/canvas?jid=019a4a9c-77de-450f-a4e8-70d4efc34012)\nshows
that we can't guarantee how many times the rolling logic has
run:\n\n<img width=\"846\" height=\"272\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/9ccaac11-92bb-42cf-8b5a-60fd258c23d9\"\n/>\n\nThe
PR makes the test even more resilient by not assuming that the\nrolling
logid has run exactly twice, and instead it just makes\nassertions that
are valid for the 2 scenarios:\n* Assert there are multiple log files
(rollover has happened).\n* Assert that the rolled files are not
empty\n* Assert that when concatenating all log files in an ordered
fashion, no\nlog entries are
lost.","sha":"11a9589ea8068a734dcd0eb9845d462ab73330e2"}},"sourceBranch":"main","suggestedTargetBranches":[],"targetPullRequestStates":[{"branch":"main","label":"v9.3.0","branchLabelMappingKey":"^v9.3.0$","isSourceBranch":true,"state":"MERGED","url":"https://github.com/elastic/kibana/pull/242095","number":242095,"mergeCommit":{"message":"Improve
rolling appender test logic (#242095)\n\n## Summary\n\nCloses
https://github.com/elastic/kibana/issues/232893\n\nThe previous
improvements were not enough,
[latest\nfailure](https://buildkite.com/elastic/kibana-on-merge/builds/81112/steps/canvas?jid=019a4a9c-77de-450f-a4e8-70d4efc34012)\nshows
that we can't guarantee how many times the rolling logic has
run:\n\n<img width=\"846\" height=\"272\"
alt=\"image\"\nsrc=\"https://github.com/user-attachments/assets/9ccaac11-92bb-42cf-8b5a-60fd258c23d9\"\n/>\n\nThe
PR makes the test even more resilient by not assuming that the\nrolling
logid has run exactly twice, and instead it just makes\nassertions that
are valid for the 2 scenarios:\n* Assert there are multiple log files
(rollover has happened).\n* Assert that the rolled files are not
empty\n* Assert that when concatenating all log files in an ordered
fashion, no\nlog entries are
lost.","sha":"11a9589ea8068a734dcd0eb9845d462ab73330e2"}}]}] BACKPORT-->

Co-authored-by: Gerard Soldevila <[email protected]>

Author: kibanamachine

src/core/server/integration_tests/logging/rolling_file_appender.test.ts

@@ -307,22 +307,30 @@ describe('RollingFileAppender', () => {
       logger.info(message(5));
       logger.info(message(6));
 
+      await waitForNextRollingTime();
+
       await flush();
 
       const files = await readdir(testDir);
 
-      const sortedLogFiles = files.sort();
-      expect(sortedLogFiles).toEqual(['kibana-1.log', 'kibana-2.log', 'kibana.log']);
+      // ['kibana.log', ['kibana-3.log',] 'kibana-2.log', 'kibana-1.log']
+      const sortedLogFiles = files.sort().reverse();
+      expect(sortedLogFiles).toContain('kibana.log');
+      // there could be a kibana-3.log already, but we cannot guarantee it
+      expect(sortedLogFiles).toContain('kibana-2.log');
+      expect(sortedLogFiles).toContain('kibana-1.log');
+
+      // kibana.log contains the newest entries, so it must go in the end
+      const kibanaLog = sortedLogFiles.shift()!;
+      sortedLogFiles.push(kibanaLog);
 
       // the "rolling time sync" mechanism above is not perfect, so the test is going to guarantee:
-      // - that something has been rolled over
-      // - that no log entries have been lost
-      const oldestEntries = await getFileContent('kibana-2.log');
-      const newerEntries = await getFileContent('kibana-1.log');
-      const newestEntries = await getFileContent('kibana.log');
-      const allEntries = oldestEntries + newerEntries + newestEntries;
-      expect(oldestEntries.length + newerEntries.length).toBeGreaterThan(40);
-      expect(allEntries).toEqual(expectedFileContent([1, 2, 3, 4, 5, 6]));
+      // A) that no log entries have been lost
+      const allEntries = await Promise.all(sortedLogFiles.map(getFileContent));
+      expect(allEntries.join('')).toEqual(expectedFileContent([1, 2, 3, 4, 5, 6]));
+
+      // B) that something has been rolled over
+      expect(allEntries.slice(-2).join('').length).toBeGreaterThan(40);
     });
   });
 });