Xtrigger function arg validation. #6894

Job	Run time
1/4	14m 45s
2/4	12m 52s
3/4	14m 3s
4/4	12m 11s
py-3-latest	20m 32s
flaky	6m 49s
_remote_background_indep_poll	5m 42s
_remote_background_indep_tcp	7m 23s
macos 1/5	20m 20s
macos 2/5	24m 17s
codecov	7s
	2h 19m 1s

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