fix: perform drain when file is deleted in tail package

[kalleep]

PR Description

After the refactor of tail package, and maybe even before that, we have the potential to miss log lines when files are rotated.

If we got to EOF and we detected a delete event we would ignore whats left for that file. It could be that a partial line read before was fully flushed or even more lines.

So to handle this I added bufferedLines. If we detect a delete event we use the open fd we have and drain all remaning lines into that and then perform the reopen.

On reads we check if we have any lines buffered and reads from that first.

Which issue(s) this PR fixes

Notes to the Reviewer

PR Checklist

• Documentation added
• Tests updated
• Config converters updated

BEGIN_COMMIT_OVERRIDE
fix: Perform drain when file is deleted in tail package (#5139)
END_COMMIT_OVERRIDE

[Copilot]

Pull request overview

This PR addresses a potential data loss issue during file rotation in the tail package. The main change introduces a drain() function that reads all remaining lines from an old file handle before it's closed during rotation, storing them in a buffer to be returned before reading from the new file.

Key changes:

• Added buffering mechanism (bufferedLines) to preserve lines from rotated files
• Implemented drain() function to read remaining lines when file deletion is detected
• Enhanced logging consistency by adding path to logger context and standardizing error log keys

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 7 comments.

File	Description
internal/component/loki/source/file/internal/tail/file.go	Core implementation of drain functionality, bufferedLines field, and improved debug logging for file events
internal/component/loki/source/file/internal/tail/file_test.go	Added test case to verify drain behavior during file rotation and helper function for rotation simulation
internal/component/loki/source/file/tailer.go	Logging improvements: added path to logger context and standardized error log key from "err" to "error"