fix(diff): prevent event loop blocking and watcher crash during file write (#1607)

[RealKai42]

Related Issue

Resolve #1607

Description

Fix the Write/Replace tool freezing issue reported in #1607. Root cause analysis identified three problems:

A. Event loop blocking — build_diff_blocks() runs Python's difflib.SequenceMatcher (O(n²)) synchronously inside an async function, blocking the event loop for large files. Fix: offload to asyncio.to_thread().

B. Silent watcher death — _watch_root_wire_hub() in Shell has zero exception handling, unlike its counterpart in wire/server.py. Any exception in _handle_root_hub_message kills the watcher silently, permanently breaking the approval flow. Fix: add try/except matching the wire server pattern, plus QueueShutDown handling for graceful shutdown.

C. Large file performance — For files >2000 lines, enable autojunk=True for faster (slightly less precise) diffs. For files >10000 lines, skip diff entirely and show a summary block. Also short-circuit immediately when old_text == new_text.

Changes

• src/kimi_cli/utils/diff.py — Refactor build_diff_blocks into sync _build_diff_blocks_sync + async wrapper with to_thread; add large/huge file thresholds and early-exit for unchanged content
• src/kimi_cli/tools/file/write.py / replace.py — await the now-async build_diff_blocks
• src/kimi_cli/ui/shell/__init__.py — Add exception resilience and QueueShutDown handling to _watch_root_wire_hub
• New test files for all three fixes with full pyright and ruff compliance

---

[Copilot]

Pull request overview

Fixes the write/replace tool freezing issue (#1607) by moving CPU-heavy diff computation off the event loop and hardening the root hub watcher against crashes and shutdown signals.

Changes:

• Refactors diff generation to run in a background thread, adding large/huge file fast paths and thresholds.
• Updates file write/replace tools to await the new async diff builder.
• Makes the shell’s root hub watcher resilient to handler exceptions and queue shutdown, with added test coverage and release note updates.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/kimi_cli/utils/diff.py	Splits diff building into sync + async wrapper using asyncio.to_thread; adds thresholds and early-exit logic.
src/kimi_cli/tools/file/write.py	Awaits async build_diff_blocks to avoid blocking the event loop.
src/kimi_cli/tools/file/replace.py	Awaits async build_diff_blocks to avoid blocking the event loop.
src/kimi_cli/ui/shell/__init__.py	Prevents watcher death by handling QueueShutDown and logging handler exceptions.
tests/utils/test_diff_utils.py	Adjusts tests to call the new sync helper for deterministic diff assertions.
tests/utils/test_diff_render.py	Updates diff-block offset assertions and uses sync diff helper.
tests/utils/test_diff_nonblocking.py	Adds tests ensuring thread delegation and large/huge file behavior.
tests/ui_and_conv/test_root_hub_watcher_resilience.py	Adds tests verifying watcher survives handler exceptions and exits on shutdown.
docs/zh/release-notes/changelog.md	Adds unreleased notes for diff non-blocking + watcher resilience + huge-file summary behavior.
docs/en/release-notes/changelog.md	Same as above (English).
CHANGELOG.md	Same as above (top-level changelog).

---

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[Copilot]

except Exception: here will also catch asyncio.CancelledError (Python >=3.12), which can prevent the watcher task from being cancelled cleanly (e.g., on shutdown) if cancellation happens while awaiting _handle_root_hub_message. Please special-case asyncio.CancelledError (re-raise it) before logging other exceptions.

Suggested change

	await self._handle_root_hub_message(msg)
	await self._handle_root_hub_message(msg)
	except asyncio.CancelledError:
	# Allow task cancellation to propagate cleanly.
	raise

[Copilot]

These imports from kimi_cli.utils.diff are split across two separate from ... import (...) blocks (and one uses parentheses for a single name). This is likely to be reformatted by ruff/isort and may fail import-sorting checks; consider combining them into a single import statement.

Suggested change

	)
	from kimi_cli.utils.diff import (

[Copilot]

The old_text == new_text short-circuit currently lives inside _build_diff_blocks_sync, so build_diff_blocks() still schedules a to_thread() call even when there is no diff. If this fast-path is expected to be common, consider checking old_text == new_text in the async wrapper before calling asyncio.to_thread to avoid unnecessary threadpool work.

Suggested change

	"""
	"""
	if old_text == new_text:
	return []

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep hook order stable in DiffContent

DiffContent now returns early for is_summary before the useEffect/useMemo hooks later in the component, which makes hook execution conditional. If a live update reuses this component instance and toggles is_summary (e.g., when display data is replaced during streaming), React will hit a hook-order mismatch (Rendered fewer hooks than expected) and break rendering for that tool output. Please keep hooks unconditional by branching in JSX after hooks, or split summary/non-summary into separate components.

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