[Sugar-devel] Problem using sugar-update-control to update large (100+ MB) activity bundles

[C. Scott Ananian]

On Wed, Jun 17, 2009 at 4:27 AM, Tomeu Vizoso<tomeu at sugarlabs.org> wrote:
> On Wed, Jun 17, 2009 at 03:15, C. Scott Ananian<cscott at cscott.net> wrote:
>> When I wrote the sugar updater, I included all the code necessary to
>> transfer only the changed portions of a large zip file.
>>  http://dev.laptop.org/git/users/cscott/sugar-update-control/tree/bitfrost/util/urlrange.py
>> contains the interesting bits.  The only missing piece was a proper
>> manifest file format, which would allow you to download only the
>> manifest to determine which bits have changed, so you could then
>> download only the changed parts inside the zip file.
>
> What about reading the zip index (central directory?), then deciding
> which files to download by the timestamp and file size?

That would work fine; we can read the zip index without downloading
the entire file.  It might require a bit of care on the part of the
bundle-preparer to ensure that the timestamps of unmodified files are
kept constant.  The zip index also has a CRC32 of the file (see
http://docs.python.org/library/zipfile.html).  It should be noted that
trusting the CRC32 to validate the file contents is insecure -- but
IIRC the security of the update scheme relies on other signatures
(probably still unimplemented -- do you know anything about that,
Michael?) so you shouldn't have to worry about it during download.
 --scott

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