Re: sha-1 check in rev-list --verify-objects redundant?

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Nguyen Thai Ngoc Duy <pclouds@xxxxxxxxx> writes:

> On the well-formedness, unless I'm mistaken, --verify-objects is
> _always_ used in conjunction with index-pack.

Hmm, you are making my head hurt.  Is the above "always" a typo of
"never"?

The static check_everything_connected() function in builtin/fetch.c is a
direct callsite of "rev-list --verify-objects", and the function is used
in two codepaths:

 * store_updated_refs() that is used after we receive and store objects
   from the other end.  We may or may not have run index-pack in this
   codepath; in either case we need to make sure the other side did send
   everything that is needed to complete the history between what we used
   to have and what they claimed to supply us, to protect us from a broken
   remote side.

 * quickfetch() that is called even before we get any object from the
   other end, to optimize the transfer when we already have what we need.

The latter is the original use to protect against unconnected island of
chain I explained in the previous message, but the former is also abot the
same protection, in a different callchain.

In both cases, the check by --verify-objects is about completeness of the
history (is everything connected to the tips of refs we have?), and is
different from integrity of individual objects (is each individual object
well formed and hash correctly?).  Both kinds of sanity need to be
checked, as they are orthogonal concepts.

In order to check the history completeness, we need to read the objects
that we walk during the check. I wouldn't be surprised if the codepath to
do this is written overly defensive, taking a belt-and-suspender approach,
and check the well-formedness of an object before it reads it to find out
the other objects pointed by it.

If we _know_ that we have checked the integrity of all the necessary
individual objects before we start reading them in order to check the
completeness of the history, there is an opportunity to optimize by
teaching --verify-objects paths to optionally be looser than it currently
is, to avoid checking the object integrity twice.
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