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If we have a file on disk and the hashes are the same in the new Release
file and the old one we have on disk we know that if we ask the server
for the file, we will at best get an IMS hit – at worse the server
doesn't support this and sends us the (unchanged) file and we have to
run all our checks on it again for nothing. So, we can save ourselves
(and the servers) some unneeded requests if we figure this out on our
own.
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If we e.g. fail on hash verification for Packages.xz its highly unlikely
that it will be any better with Packages.gz, so we just waste download
bandwidth and time. It also causes us always to fallback to the
uncompressed Packages file for which the error will finally be reported,
which in turn confuses users as the file usually doesn't exist on the
mirrors, so a bug in apt is suspected for even trying it…
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Not all servers we are talking to support If-Modified-Since and some are
not even sending Last-Modified for us, so in an effort to detect such
hits we run a hashsum check on the 'old' compared to the 'new' file, we
got the hashes for the 'new' already for "free" from the methods anyway
and hence just need to calculated the old ones.
This allows us to detect hits even with unsupported servers, which in
turn means we benefit from all the new hit behavior also here.
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If we have the expected hashes we can check with them if the file we
have in partial we got a 416 for is the expected file. We detected this
with same-size before, but not every server sends a good Content-Range
header with a 416 response.
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If we get a IMSHit for the Transaction-Manager (= the InRelease file or
as its still supported fallback Release + Release.gpg combo) we can
assume that every file we would queue based on this manager, but already
have locally is current and hence would get an IMSHit, too. We therefore
save us and the server the trouble and skip the queuing in this case.
Beside speeding up repetative executions of 'apt-get update' this way we
also avoid hitting hashsum errors if the indexes are in fact already
updated, but the Release file isn't yet as it is the case on well
behaving mirrors as Release files is updated last.
The implementation is a bit harder than the theory makes it sound as we
still have to keep reverifying the Release files (e.g. to detect now expired
once to avoid an attacker being able to silently stale us) and have to
handle cases in which the Release file hits, but some indexes aren't
present (e.g. user added a new foreign architecture).
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We autocreate for a while now the last two directories in /var/lib/apt/lists
(similar for /var/cache/apt/archives) which is very nice for systems having
any of those on tmpfs or other non-persistent storage. This also means
though that this creation is effected by the default umask, so for
people with aggressive umasks like 027 the directories will be created
with 750, which means all non-root users are left out, which is usually
exactly what we want then this umask is set, but the cache and lib
directories contain public knowledge. There isn't any need to protect
them from viewers and they render apt completely useless if not
readable.
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We create our own directories here and work without root in them, so we
can also test the locking with them as it is how we usually operate.
Git-Dch: Ignore
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Git-Dch: Ignore
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Adds some infrastructure to run tests automatically for certain
commands. The first command being 'apt-get update' (and 'apt update')
which check for correct permission and owner of the files in lists/.
Git-Dch: Ignore
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If we get an IMS hit for an InRelease file we use the file we already
have and pass it into reverification, but this changes the permissions
and on abort of the transaction they weren't switched back.
This is now done, additionally, every file in partial which hasn't
failed gets permission and owner changed for root access as well, as it
is very well possible that the next invocation will (re)use these files.
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The old way of handling this was that pkgAcqMetaIndex was responsible
to check/move both Release and Release.gpg in place. This breaks
the assumption of the transaction that each pkgAcquire::Item has
a single File that its responsible for.
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