Xtrigger function arg validation. #6881

Job	Run time
1/4	14m 13s
2/4	12m 39s
3/4	11m 33s
4/4	11m 18s
py-3-latest	20m 41s
flaky	6m 16s
_remote_background_indep_poll	5m 31s
_remote_background_indep_tcp	7m 35s
macos 1/5	19m 14s
macos 2/5	24m 27s
codecov	10s
	2h 13m 37s

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