Ensure that the exception dictionaries are not mutated when generating a html report

[mboutet]

This PR fixes the following issue:

[2021-06-09 20:36:13,702] master1/CRITICAL/locust.runners: Unhandled exception in greenlet: <Greenlet at 0x7ff41a2846a0: <bound method MasterRunner.client_listener of <locust.runners.MasterRunner object at 0x7ff3ea9d4b80>>>
Traceback (most recent call last):
  File "src/gevent/greenlet.py", line 906, in gevent._gevent_cgreenlet.Greenlet.run
  File "/home/adminuser/.venv/lib/python3.8/site-packages/locust/runners.py", line 842, in client_listener
    self.log_exception(msg.node_id, msg.data["msg"], msg.data["traceback"])
  File "/home/adminuser/.venv/lib/python3.8/site-packages/locust/runners.py", line 422, in log_exception
    row["nodes"].add(node_id)
AttributeError: 'str' object has no attribute 'add'

If a user generated a html report during a load test run, then the nodes field in every exception dict was mutated to a string.

This also caused the following when hovering on one of the exception in the web UI:

[mboutet]

PS: I will incorporate this fix in the distribution branch because it is causing my load tests to crash (I use the html report feature quite a lot).

[cyberw]

Why not just do something like exceptions_statistics.append(", ".join(exc["nodes"])) instead? No need to copy runner.exceptions if exceptions is never used again, right?

[mboutet]

The exceptions_statistics must also contain the other fields of the exception dictionary as the html template also needs the other properties:

locust/locust/html.py

Lines 66 to 80 in 60cde73

	res = render_template(
	"report.html",
	int=int,
	round=round,
	requests_statistics=requests_statistics,
	failures_statistics=failures_statistics,
	exceptions_statistics=exceptions_statistics,
	start_time=start_time,
	end_time=end_time,
	host=host,
	history=history,
	static_js="\n".join(static_js),
	static_css="\n".join(static_css),
	show_download_link=show_download_link,
	)

If you prefer, it could be something like this instead of the for-loop:

exceptions_statistics = [
    {**exc, "nodes": ", ".join(exc["nodes"])}
    for exc in environment.runner.exceptions.values()
]

[mboutet]

I changed it. Looks better now in my opinion.

[cyberw]

Thx!