fix: Lazily instantiate the KV-IR writer when using disk buffering to prevent uploading files that only contain IR preamble.

[davemarco]

Description

I noticed a subtle behavioral change introduced by the KV-IR PR related to when the IR preamble gets written to the tag’s IR file.

Consider the expected behavior from the old implementation.

Suppose logs are written for a specific tag and the plugin crashes. During recovery, we upload that tag’s IR file to S3 and then truncate the file so it becomes empty. If no additional logs arrive for that tag and the plugin crashes again, the file remains empty and is deleted on startup. Nothing further is uploaded.

This worked because of how the old writer used an intermediate buffer. Even though the IR file itself already existed on disk, the writer did not write the preamble directly into the file. Instead, the preamble and subsequent bytes were written into a buffer, and that buffer was only flushed into the IR file when the first log record was actually written. If no logs arrived after recovery, nothing ever triggered a flush, so the truncated file stayed truly empty.

With the KV-IR changes, this behavior is different. The writer now writes the IR preamble directly into the IR file as soon as the writer is instantiated. As a result, after recovery and truncation, the new IR file is no longer truly empty — it immediately contains the preamble. If no logs are written for that tag and the plugin crashes again, this preamble-only file is treated as non-empty and gets uploaded to S3.

To address this, I changed the code to lazily instantiate the writer so that the IR preamble is only written once there is at least one log record to write, restoring the previous behavior.

Note a buffered writer wrapper could also solve this problem, but for now decided just to use the exported IR interface directly.

Checklist

• The PR satisfies the contribution guidelines.
• This is a breaking change and that has been indicated in the PR title, OR this isn't a
  breaking change.
• Necessary docs have been updated, OR no docs need to be updated.

Validation performed

Sent some logs to plugin with disk buffer. Then closed and restarted plugin multiple times. Only the first restart actually sent logs. Later restarts sent nothing. Tmp folders were empty as well.

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