This operation allows fixing corrupted or accidentally deleted store
paths by redownloading them using substituters, if available.
Since the corrupted path cannot be replaced atomically, there is a
very small time window (one system call) during which neither the old
(corrupted) nor the new (repaired) contents are available. So
repairing should be used with some care on critical packages like
Glibc.
Since SubstitutionGoal::finished() in build.cc computes the hash
anyway, we can prevent the inefficiency of computing the hash twice by
letting the substituter tell Nix about the expected hash, which can
then verify it.
SQLite manifest cache. The DBI AutoCommit feature caused every
process to have an active transaction at all times, which could
indefinitely block processes wanting to update the manifest cache.
* Disable fsync() in the manifest cache because we don't need
integrity (the cache can always be recreated if it gets corrupted).
This significantly speeds up the download-using-manifests
substituter, especially if manifests are very large. For instance,
one "nix-build -A geeqie" operation that updated four packages using
binary patches went from 18.5s to 1.6s. It also significantly
reduces memory use.
The cache is kept in /nix/var/nix/manifests/cache.sqlite. It's
updated automatically when manifests are added to or removed from
/nix/var/nix/manifests. It might be interesting to have nix-pull
store manifests directly in the DB, rather than storing them as
separate flat files, but then we would need a command line interface
to delete manifests from the DB.
`nix-store -q --hash' to get the hash of the base path rather than
`nix-hash'. However, only do this for estimating the size of a
download, not for the actual substitution, because sometimes the
contents of store paths are modified (which they shouldn't, of
course).
downloaded files; rather, check the hash of the unpacked store
path.
When the server produces bzipped NAR archives on demand (like Hydra
does), the hash of the file is not known in advance; it's streamed
from the server. Thus the manifest doesn't contain a hash for the
bzipped NAR archive. However, the server does know the hash of the
*uncompressed* NAR archive (the "NarHash" field), since it's stored
in the Nix database (nix-store -q --hash /nix/store/bla). So we use
that instead for checking the integrity of the download.
again. (After the previous substituter mechanism refactoring I
didn't update the code that obtains the references of substitutable
paths.) This required some refactoring: the substituter programs
are now kept running and receive/respond to info requests via
stdin/stdout.
need any info on substitutable paths, we just call the substituters
(such as download-using-manifests.pl) directly. This means that
it's no longer necessary for nix-pull to register substitutes or for
nix-channel to clear them, which makes those operations much faster
(NIX-95). Also, we don't have to worry about keeping nix-pull
manifests (in /nix/var/nix/manifests) and the database in sync with
each other.
The downside is that there is some overhead in calling an external
program to get the substitutes info. For instance, "nix-env -qas"
takes a bit longer.
Abolishing the substitutes table also makes the logic in
local-store.cc simpler, as we don't need to store info for invalid
paths. On the downside, you cannot do things like "nix-store -qR"
on a substitutable but invalid path (but nobody did that anyway).
* Never catch interrupts (the Interrupted exception).
