fix(runtime): decode monitor() output with replacement 🤖🤖🤖

[sushant-mishra-dtu]

What this fixes

producers.monitor() decodes subprocess output with a bare bytes.decode(), which is
strict UTF-8:

async for line in proc.stdout:
    yield line.decode().rstrip("\n")      # src/nooa/runtime/producers.py:87

monitor() streams arbitrary command output — it is exposed to every agent as
self.producers.monitor via the nemo.producers entry point. Any tool that emits a
byte sequence which is not valid UTF-8 (a legacy OEM/ANSI code page, a stray byte in a
build log, binary noise on stderr — which is merged into stdout here) raises
UnicodeDecodeError out of the async generator. The producer dies mid-stream and
every subsequent line is lost.

Reproduction

A command emitting one clean line, one latin-1 line, then one more clean line:

monitor() RAISED UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe9 in position 3: invalid continuation byte
lines delivered: ['clean line']
-> 'after' delivered: False

after and everything after it are gone. The failure is not confined to the bad line —
it terminates the stream.

The fix

One argument, matching what this codebase already does for the other subprocess line
reader. src/nooa/tools/_bash_session.py:549 reads:

line = raw.decode("utf-8", errors="replace").rstrip("\n")

monitor() is the one place that reads subprocess output without it. This change makes
the two consistent:

yield line.decode("utf-8", errors="replace").rstrip("\n")

errors="replace" is the established convention here — it is used at
_bash_session.py:387, 423, 530, 531, 549, 586, unifiedllm/http_logging.py:282, 370,
runtime/sandbox/guards.py:198 and throughout nooa-cli's file tools.

No behaviour change for output that is already valid UTF-8 — same bytes in, same
strings out. The only difference is that undecodable bytes now become U+FFFD instead of
killing the producer. The docstring is updated to state this.

Test

One regression test, TestMonitorOutputDecoding::test_undecodable_byte_does_not_end_the_stream.
It spawns a Python emitter that writes first / an invalid UTF-8 byte / last and
asserts the stream survives to last. It is platform-independent (raw
sys.stdout.buffer writes, no shell builtins, no newline translation), so it is not
vacuous on the Linux CI that runs it.

Verified to fail on the unfixed tree before being kept:

E   UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe9 in position 3: invalid continuation byte
FAILED tests/runtime/test_producers.py::TestMonitorOutputDecoding::test_undecodable_byte_does_not_end_the_stream

Scope

Deliberately limited to the decode. producers.py has a separate, unrelated defect in
monitor()'s cleanup path (os.killpg is POSIX-only and AttributeError escapes the
finally); that is a platform question that overlaps #98/#137, and is raised separately as
#288 rather than folded in here -- the obvious one-line guard turns the crash into an
unbounded hang on cancel, so it needs a maintainer decision, not a drive-by patch.

Summary by CodeRabbit

• Bug Fixes

  • Command output containing invalid UTF-8 bytes is now handled safely, replacing undecodable characters instead of terminating the output stream.
  • Subsequent valid output remains available after encountering malformed text.

• Documentation

  • Added documentation describing how invalid UTF-8 output is handled.

• Tests

  • Added regression coverage to verify stream continuity and replacement-character behavior for undecodable command output.

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📝 Walkthrough

Walkthrough

monitor now decodes command output as UTF-8 with replacement for invalid bytes. A regression test verifies that streaming continues and preserves subsequent output.

Changes

Monitor output decoding

Layer / File(s)	Summary
UTF-8 replacement and streaming validation src/nooa/runtime/producers.py, tests/runtime/test_producers.py	monitor documents and applies replacement decoding. The regression test verifies the replacement character and subsequent output.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Merge Risk: ⚪ Minimal · up to a892a

Command output with invalid UTF-8 is now represented with replacement characters instead of ending the output stream; regression coverage confirms later output remains available. The change is ready to merge.

Suggested reviewers: rdasilveiracabral

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: decoding monitor() output with replacement handling. The emojis add minor noise but do not make the title unclear or unrelated.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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Actionable comments posted: 1

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Inline comments:
In `@tests/runtime/test_producers.py`:
- Line 156: Update the assertion in the relevant producer test to compare the
complete expected second line, including the replacement character, rather than
using startswith("caf"). Preserve the expected surrounding content so the test
verifies errors="replace" and continued stream output.

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