Currently, with a default of strict_exception_groups=false, run_process will currently almost always return singular exceptions, only giving multiple exceptions (afaik) if it encounters problems with reading from input/output streams.

with #2886 that will change, where any exception encountered when running the user-specified deliver_cancel, plus stream errors, will be wrapped. @Zac-HD initially favored reraising groups as TrioInternalError with the group as a cause - but after discussion with @richardsheridan I'm punting it to a separate issue & PR.

Possible solutions include:

• Be open about the fact that there's a nested nursery, and you will get an ExceptionGroup if there's a problem in it. [Status Quo in change strict_exception_groups default to True #2886]
• specify strict_exception_groups=false to the nested nursery a la Fix regressions introduced when hiding Nursery.start implementation-detail nursery #2845 - but that still leaves the theoretical possibility of getting exceptiongroups in weird cases (unless those are then caught and reraised as a different exceptions), and if we're to deprecate loose exception groups then it seems bad to use it here.
• catch ExceptionGroups from the nursery, and reraise as TrioInternalError, unless they're all Cancelled so as not to mess with Cancellations.
• Do special handling of exceptions in deliver_cancel, marking them and extracting them from the exceptiongroup to be raised.
• If there's only a single exception in the group, extract and raise it.
• Wrap all other exceptions in exceptiongroups.
• ... likely much better solutions I'm not aware of