fix(cli): bound how much log a single drain reads into memory [PC-4873]

[robert-ursu]

Draft — third in the stack. Base is feat/python-job-cancellation (#1842), so the diff here is only the bounding work. Paired with UiPath/hdens#7711.

Summary

JobLogTailer._drain called f.read() with no limit, so a single pass pulled everything written since the last poll into memory — and the decoded str costs more again.

The 250 ms poll normally keeps that tiny. It does not when:

• a job logs heavily between polls,
• the tailer is starved (a stop can hold the loop for up to STOP_GRACE_SECONDS),
• or the final drain runs after a long job.

Same read-it-all-only-to-forward-it shape that has OOMed pods on the handler side (UiPath/hdens#7711).

Each pass now reads at most LOG_READ_CHUNK_BYTES (256 KiB) and loops until caught up, so peak memory is the window, not the backlog.

Two cases only reachable once the read is bounded

• A chunk boundary landing mid-line — trimmed back to the last newline. That is also what keeps the decode correct: \n never appears inside a multi-byte UTF-8 sequence, so cutting there can't split a character.
• A single line longer than the window — emitted as a fragment rather than held. Buffering it defeats the point, and stalling on it would wedge the tailer permanently (the old code's cut == -1 → return becomes an infinite stall once reads are bounded).

Testing

Two new tests for exactly those cases: catch-up across multiple windows with a shrunk LOG_READ_CHUNK_BYTES, and a 500-byte line through a 64-byte window. The existing 14 tailer tests are unchanged and still pass — ordering, no-replay, partial-line hold-back, and final-drain behaviour are all preserved.

Full tests/cli green; ruff, format, mypy clean.

Jira

PC-4873

🤖 Generated with Claude Code

[github-actions]

🚨 Heads up: uipath-langchain cross-tests are FAILING 🚨

Your changes may break the uipath-langchain-python integration.

⚠️ These checks are NOT enforced by branch protection rules. Please review the failures before merging.

🔍 Inspect the failed run →