Otherwise it will set the parent's stdin to non-blocking mode, causing
the subsequent read of the set of inputs/outputs to fail randomly.
That's insane.

Before selecting a machine, build-remote.pl will try to run the
command "nix-builds-inhibited" on the machine.  If this command exists
and returns a 0 exit code, then the machine won't be used.  It's up to
the user to provide this command, but it would typically be a script
that checks whether there is enough disk space and whether the load is
not too high.

This doesn't work if there is no output named "out".  Hydra didn't use
it anyway.

Don't pass --timeout / --max-silent-time to the remote builder.
Instead, let the local Nix process terminate the build if it exceeds a
timeout.  The remote builder will be killed as a side-effect.  This
gives better error reporting (since the timeout message from the
remote side wasn't properly propagated) and handles non-Nix problems
like SSH hangs.

This allows providing additional binary caches, useful in scripts like
Hydra's build reproduction scripts, in particular because untrusted
caches are ignored.

Problem noticed by niksnut.

Based on https://github.com/NixOS/nix/pull/6 from shlevy

For instance, this prevents paths from being deleted that are in use
by a "nix-build --run-env" session.

I'm not sure if it has ever worked correctly.  The line "lastWait =
after;" seems to mean that the timer was reset every time a build
produced log output.

Note that the timeout is now per build, as documented ("the maximum
number of seconds that a builder can run").

Reported by Peter Simons.

Reported by Johan Grande.

It is surprisingly impossible to check if a mountpoint is a bind mount
on Linux, and in my previous commit I forgot to check if /nix/store was
even a mountpoint at all. statvfs.f_flag is not populated with MS_BIND
(and even if it were, my check was wrong in the previous commit).

Luckily, the semantics of mount with MS_REMOUNT | MS_BIND make both
checks unnecessary: if /nix/store is not a mountpoint, then mount will
fail with EINVAL, and if /nix/store is not a bind-mount, then it will
not be made writable. Thus, if /nix/store is not a mountpoint, we fail
immediately (since we don't know how to make it writable), and if
/nix/store IS a mountpoint but not a bind-mount, we fail at first write
(see below for why we can't check and fail immediately).

Note that, due to what is IMO buggy behavior in Linux, calling mount
with MS_REMOUNT | MS_BIND on a non-bind readonly mount makes the
mountpoint appear writable in two places: In the sixth (but not the
10th!) column of mountinfo, and in the f_flags member of struct statfs.
All other syscalls behave as if the mount point were still readonly (at
least for Linux 3.9-rc1, but I don't think this has changed recently or
is expected to soon). My preferred semantics would be for MS_REMOUNT |
MS_BIND to fail on a non-bind mount, as it doesn't make sense to remount
a non bind-mount as a bind mount.

makeStoreWritable: Use statvfs instead of /proc/self/mountinfo to find out if /nix/store is a read-only bind mount

/nix/store could be a read-only bind mount even if it is / in its own filesystem, so checking the 4th field in mountinfo is insufficient.

Signed-off-by: Shea Levy <shea@shealevy.com>

Fixes Hydra bug #67.