[Bro-Dev] Consistent error handling for scripting mistakes

[Jon Siwek]

Trying to broaden the scope of:

https://github.com/bro/bro/issues/208

I recently noticed there's a range of behaviors in how various
scripting mistakes are treated.  They may (1) abort, as in case of bad
subnet mask or incompatible vector element assignment (2) skip over
evaluating (sub)expression(s), but otherwise continue current function
body, as in case of non-existing table index access or (3) exit the
current function body, as in the classic case of uninitialized record
field access.

1st question: should these be made more consistent? I'd say yes.

2nd question: what is the expected way for these to be handled?  I'd
argue that (3) is close to expected behavior, but it's still weird
that it's only the *current function body* (yes, *function*, not
event) that exits early -- hard to reason about what sort of arbitrary
code was depending on that function to be fully evaluated and what
other sort of inconsistent state is caused by exiting early.

I propose, for 2.7, to aim for consistent error handling for scripting
mistakes and that the expected behavior is to unwind all the way to
exiting the current event handler (all its function bodies).  That
makes it easier to explain how to write event handlers such that they
won't enter too wild/inconsistent of a state should a scripting error
occur: "always write an event handler such that it makes no
assumptions about order/priority of other events handlers".  That's
already close to current suggestions/approaches.

One exception may be within bro_init(), if an error happens at that
time, I'd say it's fine to completely abort -- it's unlikely or hard
to say whether Bro would operate well if it proceeded after an error
that early in initialization.

Thoughts?

- Jon