On 2016-01-20 18:41, John Van Ostrand wrote:
I think this is a summary of where we left of:
1. dc_context_t pointer doesn't need to be passed to cochran serial open a
setup functions. (fixed with patch pending.)
Patch looks fine to me.
2. Cochran can store more log entries than profiles. So when the profile
ring buffer wraps earlier logs will point to profile data from other dives.
Jef suggests processing dives in reverse chronological order, adding up
profile data used and not processing profiles after it's processed a full
ring-buffer of data. I have yet to work on this or consider alternatives.
The dc_device_foreach() function must return the dives in reverse chronological order. This is a requirement for the download only new dives feature. When dives are returned in reverse chronological order, an application (or libdivecomputer itself) can simply stop downloading dives as soon as a previously downloaded dive is recognized. Very simple and efficient.
Thus if we need to return the dives in reverse chronological order, it makes sense to process (and also download) the data in this order. Otherwise you'll end up with a rather inefficient implementation.
I have to read through your documentation again, but processing the dives in reverse order might also make the recovery of corrupt dives easier. If the tail of a dive is missing, then it can run at most until the start of the next dive. And due the reverse order we already have that one.
3. Corrupt dive handling. In some cases (like a low battery especially in
cold water) the computer resets during a dive. This results in a
"start-dive" block written but no valid "end-dive" block written. We know
information from the start of a dive (like date/time, gasses, profile start
pointer, etc.) but we don't know information accumulated during or at the
end of a dive (like end-profile pointer, max depth, min temp, etc.) I've
taken to guessing the end of a dive by starting with the next dive's
pre-dive-profile-pointer and backing up until we think we have the previous
dive's end. We haven't resolved our differences on this. It seems to down
to the question: Do we present a partial or broken profile in the interest
of giving the diver something or do we give nothing in the interest of
being accurate?
That's a difficult question. In general, I prefer to be very strict and simply fail on unexpected data. Usually this is the correct thing to do, because such unexpected data often turns out to be an wrong assumption in the code. So being strict helps finding bugs. But sometimes the data is really wrong (due to a firmware bug, running out of battery during the dive, etc), and if it happens frequently, then a workaround might indeed be necessary.
It also depends on where the data "corruption" is located. If the information needed to move from one dive to another is good, but we are unsure about the contents of the dive, then we can return the bogus dive to the application and let the parser deal with it. You might get incorrect data for that particular dive, or even a failure to parse the dive. This would ensure that we can still download the other dives. But if the primary structure is damaged and we can no longer safely move to the next dive, then I think we should fail.
Jef