Exception raised in `set_context()`

[zry656565]

Describe the bug: ...

[host: noj-59dfd94498-45jdw|pid: 24294|timestamp: 1564716068.956608] [ERROR|elasticapm.errors:188] Exception during timing of request
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/dist-packages/elasticapm/contrib/django/middleware/__init__.py", line 185, in process_response
    elasticapm.set_context(lambda: self.client.get_user_info(request), "user")
  File "/usr/local/lib/python3.7/dist-packages/elasticapm/traces.py", line 410, in set_context
    transaction.context[key].update(data)
TypeError: 'function' object is not iterable

Related codes:

apm-agent-python/elasticapm/traces.py

Lines 615 to 631 in ae1c777

	def set_context(data, key="custom"):
	transaction = execution_context.get_transaction()
	if not transaction:
	return
	if callable(data) and transaction.is_sampled:
	data = data()
	# remove invalid characters from key names
	if not callable(data): # if transaction wasn't sampled, data is still a callable here and can be ignored
	for k in list(data.keys()):
	if TAG_RE.search(k):
	data[TAG_RE.sub("_", k)] = data.pop(k)
	if key in transaction.context:
	transaction.context[key].update(data)
	else:
	transaction.context[key] = data

And when I print data when meeting exception, the output is:

<function TracingMiddleware.process_response.<locals>.<lambda> at 0x7f39e36a00e0>

I guess maybe we have to deal with the situation that data is callable and transaction.is_sampled == False.

Expected behavior:

set_context() should handle data in lambda type well.

Environment (please complete the following information)

• OS: Linux
• Python version: 3.7.4
• Framework and version: Django 2.1.9
• APM Server version: -
• Agent version: 4.2.2

[beniwohli]

@zry656565 thanks for opening this super detailed issue! I had a look, and as far I can tell, this happens if set_context() is called twice in an unsampled transaction, first with a dictionary, then with a callable. Something like this:

apm-agent-python/tests/instrumentation/transactions_store_tests.py

Lines 424 to 434 in 187cc2b

	def test_callable_context_ignored_when_not_sampled(elasticapm_client):
	callable_data = mock.Mock()
	callable_data.return_value = {"a": "b"}
	transaction = elasticapm_client.begin_transaction("test")
	transaction.is_sampled = False
	elasticapm.set_context({"c": "d"})
	elasticapm.set_context(callable_data)
	elasticapm_client.end_transaction("test", "OK")
	transaction = elasticapm_client.events[TRANSACTION][0]
	assert callable_data.call_count == 0
	assert "context" not in transaction

I opened #542 to address this issue. With that change, we will bail out early in set_context if the transaction is not sampled.

[zry656565]

Cool, happy to know this. 👍