On Fri, May 23, 2014 at 2:02 PM, Michael Haggerty <mhag...@alum.mit.edu> wrote:
> On 05/23/2014 06:14 PM, Ronnie Sahlberg wrote:
>> On Fri, May 23, 2014 at 6:49 AM, Michael Haggerty <mhag...@alum.mit.edu> 
>> wrote:
>>> [...]
>>> When I combine these two lines of thought, it suggests to me that we
>>> could do a better job of supporting *both* use cases.  What if the
>>> transaction object contained not an err strbuf but a string_list?  If an
>>> error occurs while building up the transaction, a message would be added
>>> to the string list and the function would return an error status.  The
>>> caller can monitor errors while it is building up the transaction and
>>> abort immediately if it wants, or it can ignore the return values and
>>> let the error messages accumulate in the string list.  When the caller
>>> attempts the commit, it would notice that the transaction failed, and at
>>> that time the caller could emit *all* of the accumulated error messages
>>> by reading them out of the string list; e.g.,
>>>     Error fetching from $REMOTE:   <- this is generated by caller
>>>         $ERR[0]    <- these come from the error string list,
>>>         $ERR[1]       printed with indentation by caller
>>>         $ERR[2]
>>>         $ERR[3]
>>> This style would have another advantage: we might have some back ends
>>> for which transactions have a high overhead.  Such a back end would
>>> probably choose not to do any checks while the transaction is being
>>> built up, e.g., to avoid a round-trip to a database.  When commit() is
>>> called, it would learn about all of the errors at once.  (1) It would
>>> need a way to return all of the errors to the caller.  (2) It would be
>>> nice for the caller to be able to treat such a back end the same as it
>>> treats a back end that is able to report errors immediately.  It seems
>>> to me that having a way to report multiple errors at the same time would
>>> solve both problems nicely.
>> Inretesting.
>> That would mean changing all functions to take a string_list provided
>> by the caller instead of a strbuf.
>> And then have _update/_create/_delete do actual work instead of
>> bailing out after the first error.
>> Users that want to check for error and log after each call to
>> _update/_create/_delete could do so and
>> just use the last entry added to the string list or otherwise they
>> could just wait until _commit time and if it fails log
>> all the strings.
> I still think we should consider storing the err string_list within the
> transaction object; otherwise it's annoying to have to pass that
> parameter around everywhere.  And if there were also a policy that any
> caller that checks and reports any error should report *all* of the
> accumulated errors and abort the transaction, then we don't have to
> worry about error messages being output multiple times or zero times.
> It might be nice to have a printf-style helper function like
>     ref_transaction_perror(transaction, fmt, ...)
> (or maybe ref_transaction_die()) that outputs the accumulated errors
> with msg as a header (like my example output above), to make error
> handling easy and uniform.

We can add this later once we get the basic transaction stuff in.
It will make it easier to experiment with different types of error
reporting then.

> Michael
> --
> Michael Haggerty
> mhag...@alum.mit.edu
> http://softwareswirl.blogspot.com/
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