nix-env can read priorities from a derivations meta attributes, but this
only works when installing a nix expression.
nix-env can also install bare store paths, however meta attributes are
not readable in that case. This means that a store path can not be
installed with a specific priority.
Some cases where it is advantageous to install a store path: a remote
host following a `nix copy`, or any time you want to save some
evaluation time and happen to already know the store path.
This PR addresses this shortcoming by adding a --priority flag to
nix-env --install.
pararameterisation is not actually needed the way things are currently
set up, and it confused me when trying to understand what the code does.
all but one test sources vars-and-functions.sh, which nominally only
defines variables, but in practice is always coupled with the actual
initialisation. while the cleaner way of making this more legible would
be to source variables and initialisation separately, this would produce
a huge diff.
the change requires a few small fixes to keep the tests working:
- only create test home directory during initialisation
that vars-and-functions.sh wrote to the file system seems not write
- fix creation of the test directory
due to statefulness, the test home directory was implicitly creating
the test root, too. decoupling that made it apparent that this was
probably not intentional, and certainly confusing.
- only source vars-and-functions.sh if init.sh is not needed
there is one test case that only needs a helper function but no
initialisation side effects
- remove some unnecessary cleanups and split parts of re-used test code
there were confusing bits in how initialisation code was repurposed,
which break if trying to refactor the outer layers naively...
