In Darcs, any command can have a post hook (and a pre hook), and the hook
command can be set using a command-line option to the darcs command that you
run. So, in the Vervis SSH server, if we add a --posthook option when running
`darcs apply` to apply remotely received patches, we get a chance to process
the patch data much like in the git post-receive hook.
The setup this patch creates is similar to the git one: It writes a
_darcs/prefs/defaults file to all Darcs repos, and that defaults file sets the
posthook line for `darcs apply`. The posthook line simply executes the actual
hook program written in Haskell.
The current hook program is a one-liner that prints a line to stdout, so every
time you `darcs push` you can tell the hook got executed. The next step is to
implement the actual hook logic, by reading patch data from the environment
variable in which Darcs puts it.
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