LoggingHandler.emit() can deadlock when _translate() triggers internal logging (e.g., _clean_extended_attribute calling _logger.warning() for invalid attribute types). The warning re-enters the handler chain on the same thread, which tries to acquire the same handler lock, causing a permanent deadlock that freezes all worker threads.

This adds a threading.local() guard that detects re-entrant calls to emit() on the same thread and skips the recursive call, preventing the deadlock while preserving all application logs.

Fixes open-telemetry/opentelemetry-python#3858
Relates to open-telemetry/opentelemetry-python#3560
Relates to open-telemetry/opentelemetry-python#4585

Description

As suggested by @xrmx in opentelemetry-python#4954, opening this fix against the contrib repo since LoggingHandler in opentelemetry-sdk is deprecated.

LoggingHandler.emit() can deadlock when _translate() triggers internal logging. The call chain is:

1. emit() → _translate() → BoundedAttributes() → _clean_extended_attribute()
2. _clean_extended_attribute() finds a non-primitive attribute and calls _logger.warning("Invalid type...")
3. This warning re-enters the logging handler chain on the same thread
4. The handler tries to acquire the same lock already held by step 1
5. Deadlock — the thread waits for itself, lock is never released

Once one thread deadlocks, every other thread that attempts to log blocks behind the same lock. In production (Gunicorn with 4 workers × 5 threads), this caused 100% request failure (Nginx 499/504) and complete service outage.

Thread dump captured via py-spy during production outage confirmed all 20 request threads stuck at acquire (logging/__init__.py:927):

acquire (logging/__init__.py:927)                       ← STUCK
warning (logging/__init__.py:1501)                      ← OTel internal warning
_clean_extended_attribute (attributes/__init__.py:209)
_translate (_logs/_internal/__init__.py:637)
emit (_logs/_internal/__init__.py:656)                  ← OTel LoggingHandler
handle (logging/__init__.py:978)                        ← Lock first acquired here

While #4808 (in 1.39.0) reduces the most common trigger by converting unknown types to str(), the deadlock can still occur via the invalid key path (_logger.warning("invalid key...")) and when str(value) itself raises. This recursion guard protects against all current and future _logger.warning() calls in the pipeline as a defense-in-depth measure.

Fixes open-telemetry/opentelemetry-python#3858

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

4 new unit tests added in test_logging_handler_recursion.py:

• test_recursive_emit_is_skipped — verifies guard blocks recursive calls
• test_normal_emit_works — verifies normal logging is not affected
• test_guard_is_reset_after_emit — verifies guard resets after emit completes
• test_guard_is_thread_local — verifies guard on one thread doesn't block other threads

Does This PR Require a Core Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated