To paraphrase @peff:
You can get both size and type from a packed object reasonably cheaply.
If you have:
* An object that is not a delta; both type and size are available in the
packfile header.
* An object that is a delta. The packfile type will be OBJ_*_DELTA, and
you have to resolve back to the base to find the real type. That means
potentially a lot of packfile index lookups, but each one is
relatively cheap. For the size, you inflate the first few bytes of the
delta, whose header will tell you the resulting size of applying the
delta to the base.
For simplicity, we just decompress the whole delta for now.
A mmap-window is not guaranteed to give you the whole object, but the
indexer currently assumes so.
Loop asking for more data until we've successfully CRC'd all of the
packed data.
Up to now, deltas needed to be enterily in the packfile, and we tried
to decompress then in their entirety over and over again.
Adjust the logic so we read them as they come, just as we do for full
objects. This also allows us to simplify the logic and have less
nested code. The delta resolving phase still needs to decompress the
whole object into memory, as there is not yet any streaming
delta-apply support, but it helps in speeding up the downloading
process and reduces the amount of memory allocations we need to do.
The new API allows us to read the object bit by bit from the packfile,
instead of needing it all at once in the packfile. This also allows us
to hash the object as it comes in from the network instead of having
to try to read it all and failing repeatedly for larger objects.
This is only the first step, but it already shows huge improvements
when dealing with objects over a few megabytes in size. It reduces the
memory needs in some cases, but delta objects still need to be
completely in memory and the old inefficent method is still used for
that.
