So all these years I was totally deluded about the meaning of "set
-e". You might think that it causes statements like "false && true" or
"! true" to fail, but it doesn't...
the hook every time we want to ask whether we can run a remote build
(which can be very often), we now reuse a hook process for answering
those queries until it accepts a build. So if there are N
derivations to be built, at most N hooks will be started.
The /bin/sh interpreter on Solaris doesn't understand $(...) syntax for running
sub-shells. Consequently, this test fails on Solaris. To remedy the situation,
the script either needs to be run by /bin/bash -- which is non-standard --, or
it needs to use the ancient but portable `...` syntax.
in parallel. Hooks are more efficient: locks on output paths are
only acquired when the hook says that it is willing to accept a
build job. Hooks now work in two phases. First, they should first
tell Nix whether they are willing to accept a job. Nix guarantuees
that no two hooks will ever be in the first phase at the same time
(this simplifies the implementation of hooks, since they don't have
to perform locking (?)). Second, if they accept a job, they are
then responsible for building it (on the remote system), and copying
the result back. These can be run in parallel with other hooks and
locally executed jobs.
The implementation is a bit messy right now, though.
* The directory `distributed' shows a (hacky) example of a hook that
distributes build jobs over a set of machines listed in a
configuration file.
