fix(span): handle errors on spans when a custom exception overrides __str__

[wantsui]

Today when a custom exception overrides __str__ by raising an exception, the tracer cannot apply an error message on it because str(exc_val) doesn't work (it raises the exception again). As a result the span doesn't close correctly because it's an exception, not a string and thus the span can't get sent.

If this condition occurs, this PR sets the error message to the class name of the custom exception so the spans can still get sent and there's a clue to what the exception was.

The current logic is this:

dd-trace-py/ddtrace/_trace/span.py

Line 523 in 6b5f13c

self._meta[ERROR_MSG] = str(exc_val)

When we run the current logic against a custom exception that overrides __str__, we can see from the failed job that str(exc_val) leads to an error:

_____________________ test_set_exc_info_with_str_override ______________________
    def test_set_exc_info_with_str_override():
        span = Span("span")
    
        class CustomException(Exception):
            def __str__(self):
                raise Exception("A custom exception")
    
        try:
>           raise CustomException()
E           tests.tracer.test_span.test_set_exc_info_with_str_override.<locals>.CustomException: <exception str() failed>
tests/tracer/test_span.py:839: CustomException
During handling of the above exception, another exception occurred:
    def test_set_exc_info_with_str_override():
        span = Span("span")
    
        class CustomException(Exception):
            def __str__(self):
                raise Exception("A custom exception")
    
        try:
            raise CustomException()
        except Exception:
            type_, value_, traceback_ = sys.exc_info()
>           span.set_exc_info(type_, value_, traceback_)
tests/tracer/test_span.py:842: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
ddtrace/_trace/span.py:523: in set_exc_info
    self._meta[ERROR_MSG] = str(exc_val)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
self = CustomException()
    def __str__(self):
>       raise Exception("A custom exception")
E       Exception: A custom exception
tests/tracer/test_span.py:836: Exception

The fix is to try to catch this exception if str(exc_val) and just add the class name as a placeholder for the error message since there isn't an easy way to turn the exception into a string (since it just keeps raising the exception). This way, spans can at least be closed and sent.

Thanks to @wiggzz for demonstrating the issue in https://github.com/wiggzz/dd-trace-debug!

This is for APMS-14834 .

Checklist

• PR author has checked that all the criteria below are met
• The PR description includes an overview of the change
• The PR description articulates the motivation for the change
• The change includes tests OR the PR description describes a testing strategy
• The PR description notes risks associated with the change, if any
• Newly-added code is easy to change
• The change follows the library release note guidelines
• The change includes or references documentation updates if necessary
• Backport labels are set (if applicable)

Reviewer Checklist

• Reviewer has checked that all the criteria below are met
• Title is accurate
• All changes are related to the pull request's stated goal
• Avoids breaking API changes
• Testing strategy adequately addresses listed risks
• Newly-added code is easy to change
• Release note makes sense to a user of the library
• If necessary, author has acknowledged and discussed the performance implications of this PR as reported in the benchmarks PR comment
• Backport labels are set in a manner that is consistent with the release branch maintenance policy

[github-actions]

CODEOWNERS have been resolved as:

releasenotes/notes/fix-custom-error-spans-with-__str__-override-a05726045f8751d6.yaml  @DataDog/apm-python
ddtrace/_trace/span.py                                                  @DataDog/apm-sdk-api-python
tests/tracer/test_span.py                                               @DataDog/apm-sdk-api-python

[datadog-dd-trace-py-rkomorn]

Datadog Report

Branch report: wantsui/capture-raise-exceptions-in-__str__
Commit report: 556ed73
Test service: dd-trace-py

✅ 0 Failed, 130 Passed, 1184 Skipped, 3m 30.82s Total duration (25m 39.64s time saved)

[pr-commenter]

Benchmarks

Benchmark execution time: 2025-02-13 15:08:16

Comparing candidate commit 556ed73 in PR branch wantsui/capture-raise-exceptions-in-__str__ with baseline commit 47f3bb2 in branch main.

Found 1 performance improvements and 1 performance regressions! Performance is the same for 392 metrics, 2 unstable metrics.

scenario:iast_aspects-ljust_aspect

• 🟥 execution_time [+1.053µs; +1.156µs] or [+7.436%; +8.162%]

scenario:iast_aspects-ospathdirname_aspect

• 🟩 execution_time [-651.355ns; -592.588ns] or [-15.118%; -13.754%]

[github-actions]

The backport to 2.21 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-2.21 2.21
# Navigate to the new working tree
cd .worktrees/backport-2.21
# Create a new branch
git switch --create backport-12307-to-2.21
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 bada1cd7974515cc63a033b9d2e7361dedaef998
# Push it to GitHub
git push --set-upstream origin backport-12307-to-2.21
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-2.21

Then, create a pull request where the base branch is 2.21 and the compare/head branch is backport-12307-to-2.21.