UnboundLocalError when SansIOHTTPPolicy handles an exception

[chlowell]

_SansIOHTTPPolicyRunner allows a SansIOHTTPPolicy to stop it propagating pipeline exceptions, by returning True from its on_exception method. However, when a policy does this, _SansIOHTTPPolicyRunner raises UnboundLocalError:

from azure.core.pipeline import Pipeline
from azure.core.pipeline.transport import HttpRequest
from azure.core.pipeline.policies import HTTPPolicy, SansIOHTTPPolicy

class ReproPolicy(SansIOHTTPPolicy):
    def on_exception(self, request):
        return True

class ErrorPolicy(HTTPPolicy):
    def send(self, request):
        raise Exception("oops")

pipeline = Pipeline(transport=object(), policies=[ReproPolicy(), ErrorPolicy()])
pipeline.run(HttpRequest("GET", "..."))

Traceback (most recent call last):
  ...
  File "c:\code\azure-sdk-for-python\sdk\core\azure-core\azure\core\pipeline\_base.py", line 77, in send
    return response
UnboundLocalError: local variable 'response' referenced before assignment

... which makes sense because an exception prevents the assignment:

azure-sdk-for-python/sdk/core/azure-core/azure/core/pipeline/_base.py

Lines 70 to 77 in 82175dc

	try:
	response = self.next.send(request)
	except Exception: # pylint: disable=broad-except
	if not _await_result(self._policy.on_exception, request):
	raise
	else:
	_await_result(self._policy.on_response, request, response)
	return response

I opened an issue rather than a PR because the fix requires some design. Should _SansIOHTTPPolicyRunner return a PipelineResponse with no http_response in this case?

[chlowell]

A fix for this should apply to BearerTokenCredentialPolicy as well. That policy began as a subclass of SansIOHTTPPolicy and now follows its behavior for compatibility reasons.