Turns out that CommandStart actions do not have their exceptions caught,
which is why the giveup was causing a crash. Mostly these actions
do not do very much work on their own, but it does seem possible there
are other commands whose CommandStart also throws an exception.
So, my first attempt at a fix was to catch those exceptions. But,
--json-error-messages then causes a difficulty, because in order to output
a json error message, an action needs to have been started; that sets up
the json object that the error message will be included in a field of.
While it would be possible to output an object with just an error field,
this would be json output of a format that the user has no reason to
expect, that happens only in an exceptional circumstance. That is something
I have always wanted to avoid with the json output; while git-annex man
pages don't document what the json looks like, the output has always
been made to be self-describing. Eg, it includes "error-messages":[]
even when there's no errors.
With that ruled out, it doesn't seem a good idea to catch CommandStart
exceptions and display the error to stderr when --json-error-messages
is set. And so I don't know if it makes sense to catch exceptions from that
at all. Maybe I'd have a different opinion if --json-error-messages did not
exist though.
So instead, output a blank line like other batch commands do.
This also leaves open the possibility of implementing support for matching
object with metadata --json, which would also want to output a blank line
when the input didn't match.
Sponsored-by: Dartmouth College's DANDI project
