[dajiaohuang]

Fixes #338.

Summary

• Only count a size-only backup when the filename portion before its numeric suffix matches the configured base filename.
• Add a synthetic regression test for a prefix-sharing unrelated sibling.
• Leave static-name and date-rolling backup handling unchanged.

Verification

• The regression test failed on the frozen baseline (CurrentSizeRollBackups was 5 instead of 0) and passed with this change.
• Restored the solution using a task-local NuGet configuration with the public nuget.org source, then ran dotnet test src/log4net.sln --no-restore: 462 log4net.Tests passed, 1 skipped; 52 log4net.Ext.Mail.Tests passed. Restore reported NU1902/NU1903 advisories for System.Security.Cryptography.Pkcs 10.0.0 in log4net.Ext.Mail.Tests; this PR does not change that dependency.

AI/tool disclosure: prepared with OpenAI Codex gpt-6-luna, operated by dajiaohuang. The tests above were executed by the agent; this disclosure does not claim independent human-authored verification.

[FreeAndNil]

Thanks @dajiaohuang, the bug is real and your fix is right.

We reshaped it a little before merging:

• one guard instead of two: the static name check from count only the current date's backups when RollingFileAppender starts #334 now also covers a name without dates
• the test moved to RollingFileAppenderTest, next to the other InitializeRollBackups tests, and calls it directly, so it needs no subclass and no temp folder
• a positive case: log.txt.5 still counts as backup 5
• a changelog entry

Here is the commit on top of your branch: 2fbda3d

Could you pull it into your branch and push? It applies without conflicts, and we squash when merging the PR.

git switch fix/338-prefix-sibling-size-backup
git pull https://github.com/apache/logging-log4net.git Feature/338-size-backup-prefix-sibling
git push