Tracing crashes with UnicodeEncodeError on surrogate escapes in hook arguments/results

Problem

When pluggy tracing is enabled (e.g. pytest's --debug flag), hook arguments or return values containing surrogate escape characters (like \ud800) cause a UnicodeEncodeError crash. This is because _format_message in _tracing.py uses str() to format values, and the resulting string with literal surrogates cannot be written to most output targets.

Reported originally as pytest-dev/pytest#13750.

Analysis

_format_message currently formats two kinds of data identically using str():

1. Structural labels passed as positional args — e.g. "finish", "-->", hook names — which are always safe ASCII strings
2. Python values — hook kwargs values (rendered via the extra dict) and hook return values — which may contain arbitrary data including surrogates

The fix needs to apply repr() only to the value positions, not to structural labels.

Intended Solution

1. In _tracing.py line 38 — use {value!r} for the extra/kwargs dict values:

   lines.append(f"{indent}    {name}: {value!r}\n")
   

   This makes kwargs values in trace output show their type (e.g. 'lfplugin' instead of lfplugin, PosixPath('/foo') instead of /foo) and safely escapes surrogates.

2. In _manager.py line 506 — use repr() on the hook result before passing it as a trace arg:

   hooktrace("finish", hook_name, "-->", repr(outcome.get_result()))
   

   This ensures the result value is safely formatted, while "finish", hook_name, and "-->" remain plain str()-formatted (via _format_message's existing map(str, args)).

3. Do NOT change the content = " ".join(map(str, args)) line in _format_message. Keeping str() there preserves readable structural output without quoting labels. The caller (_manager.py) is responsible for pre-formatting any unsafe values with repr().

This avoids the double-repr problem and keeps trace output readable:

  finish pytest_runtest_call --> '\ud800' [hook]
      config: <Config object at 0x...>
      plugin_name: 'lfplugin'

Rather than the over-quoted version that blanket repr() in _format_message would produce:

  'finish' 'pytest_runtest_call' '-->' "'\\ ud800'" [hook]

References

• pytest-dev/pytest#13750
• #627 (PR with discussion leading to this distilled approach)

Start with _format_message in _tracing.py around line 38 and the hook result tracing call in _manager.py around line 506. Verify tracing with surrogate-containing hook arguments and results no longer raises UnicodeEncodeError, while structural labels remain readable and values are safely represented.