[Bro-Dev] Consistent error handling for scripting mistakes

[Jim Mellander]

Along the same vein of sensible Bro script error handling, I'm resending an
issue I found in January:

I was tinkering with the sumstats code, and inadvertantly deleted the final
"}" closing out the last function.  When running the code, the misleading
error message is received:

error in bro/share/bro/base/frameworks/tunnels/./main.bro, line 8: syntax
error, at or near "module"

presumably due to the function still being open when the next policy script
is loaded.  Wouldn't it be more reasonable to check at the end of each
script when loaded that there are no dangling functions, expressions, etc.
????

==========================

There are also silent fails which probably should give a warning, such as
failing to include the fully-qualified event name silently preventing the
event from being triggered.


==========================

The above are more in the area of parsing vs runtime.

My idea on runtime scripting errors would be to apply a sensible default to
the offending expression (null or 0, as the case may be, might be
sufficient), log the error, and continue....


Jim



On Mon, Nov 12, 2018 at 10:27 AM, Jon Siwek <jsiwek at corelight.com> wrote:

> 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
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