If ‘--link’ is given, nix-push will create hard links to the NAR files
in the store, rather than copying them. This is faster and requires
less disk space. However, it doesn't work if the store is on a
different file system.
I.e. do what git does. I'm too lazy to keep the builtin help text up
to date :-)
Also add ‘--help’ to various commands that lacked it
(e.g. nix-collect-garbage).
With this flag, if any valid derivation output is missing or corrupt,
it will be recreated by using a substitute if available, or by
rebuilding the derivation. The latter may use hash rewriting if
chroots are not available.
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.
In Nixpkgs, the attribute in all-packages.nix corresponding to a
package is usually equal to the package name. However, this doesn't
work if the package contains a dash, which is fairly common. The
convention is to replace the dash with an underscore (e.g. "dbus-lib"
becomes "dbus_glib"), but that's annoying. So now dashes are valid in
variable / attribute names, allowing you to write:
dbus-glib = callPackage ../development/libraries/dbus-glib { };
and
buildInputs = [ dbus-glib ];
Since we don't have a negation or subtraction operation in Nix, this
is unambiguous.
Using the immutable bit is problematic, especially in conjunction with
store optimisation. For instance, if the garbage collector deletes a
file, it has to clear its immutable bit, but if the file has
additional hard links, we can't set the bit afterwards because we
don't know the remaining paths.
So now that we support having the entire Nix store as a read-only
mount, we may as well drop the immutable bit. Unfortunately, we have
to keep the code to clear the immutable bit for backwards
compatibility.
It turns out that the immutable bit doesn't work all that well. A
better way is to make the entire Nix store a read-only bind mount,
i.e. by doing
$ mount --bind /nix/store /nix/store
$ mount -o remount,ro,bind /nix/store
(This would typically done in an early boot script, before anything
from /nix/store is used.)
Since Nix needs to be able to write to the Nix store, it now detects
if /nix/store is a read-only bind mount and then makes it writable in
a private mount namespace.
