fix(shells): decode background-shell output with errors=replace to avoid pipe deadlock

[Yuren0312]

Fixes #13.

Problem

subprocess.Popen is started with text=True but without errors=, so decoding defaults to strict. When a command emits non-UTF-8 bytes, the reader thread raises UnicodeDecodeError, its except Exception: pass swallows it, and the reader stops draining stdout. Once the child fills the 64KB OS pipe buffer, it blocks in write(), and _reader's final proc.wait() hangs forever — freezing background shells and security scanners (strings, fuzzers, compiled-binary tests) indefinitely.

Fix

Decode with errors="replace" so invalid bytes become U+FFFD instead of raising, and the reader keeps consuming the pipe until EOF:

             stderr=subprocess.STDOUT,
             text=True,
+            errors="replace",
             bufsize=1,

Verification

Added test_non_utf8_output_does_not_deadlock in tests/test_shells.py, which spawns python3 -c '... sys.stdout.buffer.write(b"\xff" * 200000)' and asserts the shell exits (no deadlock), returncode == 0, and the invalid bytes are decoded as replacement characters rather than dropped.

Reproduced the hang before the fix (shell stays running forever, child never reaped) and confirmed it exits cleanly after.

[greptile-apps]

Confidence Score: 5/5

[Medium impact] The PR appears safe to merge.

Summary

Background shells now replace invalid output bytes instead of stopping the reader and leaving the child blocked.

• Background shells keep reading output with invalid bytes.

_{Reviews (1) · Last reviewed commit: "fix(shells): decode background-shell out..." · Reviewed by Greptile}