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.
This is a problem because one process may set the immutable bit before
the second process has created its link.
Addressed random Hydra failures such as:
error: cannot rename `/nix/store/.tmp-link-17397-1804289383' to
`/nix/store/rsvzm574rlfip3830ac7kmaa028bzl6h-nixos-0.1pre-git/upstart-interface-version':
Operation not permitted
optimiseStore() now creates persistent, content-addressed hard links
in /nix/store/.links. For instance, if it encounters a file P with
hash H, it will create a hard link
P' = /nix/store/.link/<H>
to P if P' doesn't already exist; if P' exist, then P is replaced by a
hard link to P'. This is better than the previous in-memory map,
because it had the tendency to unnecessarily replace hard links with a
hard link to whatever happened to be the first file with a given hash
it encountered. It also allows on-the-fly, incremental optimisation.
I was bitten one time too many by Python modifying the Nix store by
creating *.pyc files when run as root. On Linux, we can prevent this
by setting the immutable bit on files and directories (as in ‘chattr
+i’). This isn't supported by all filesystems, so it's not an error
if setting the bit fails. The immutable bit is cleared by the garbage
collector before deleting a path. The only tricky aspect is in
optimiseStore(), since it's forbidden to create hard links to an
immutable file. Thus optimiseStore() temporarily clears the immutable
bit before creating the link.
