The prefix will now be correct in case of Nix env.
Note, however, that creating a venv from a Nix env still does not function. This does not seem to be possible
with the current approach either, because venv will copy or symlink our Python wrapper. In case it symlinks
(the default) it won't see a pyvenv.cfg. If it is copied I think it should function but it does not...
This is needed in case of `python.buildEnv` to make sure site.PREFIXES
does not only point to the unwrapped executable prefix.
--------------------------------------------------------------------------------
This PR is a story where your valiant hero sets out on a very simple adventure but ends up having to slay dragons, starts questioning his own sanity and finally manages to gain enough knowledge to slay the evil dragon and finally win the proverbial price.
It all started out on sunny spring day with trying to tackle the Nixops plugin infrastructure and make that nice enough to work with.
Our story begins in the shanty town of [NixOps-AWS](https://github.com/nixos/nixops-aws) where [mypy](http://mypy-lang.org/) type checking has not yet been seen.
As our deuteragonist (@grahamc) has made great strides in the capital city of [NixOps](https://github.com/nixos/nixops) our hero wanted to bring this out into the land and let the people rejoice in reliability and a wonderful development experience.
The plugin work itself was straight forward and our hero quickly slayed the first small dragon, at this point things felt good and our hero thought he was going to reach the town of NixOps-AWS very quickly.
But alas! Mypy did not want to go, it said:
`Cannot find implementation or library stub for module named 'nixops'`
Our hero felt a small sliver of life escape from his body. Things were not going to be so easy.
After some frustration our hero discovered there was a [rule of the land of Python](https://www.python.org/dev/peps/pep-0561/) that governed the import of types into the kingdom, more specificaly a very special document (file) called `py.typed`.
Things were looking good.
But no, what the law said did not seem to match reality. How could things be so?
After some frustrating debugging our valiant hero thought to himself "Hmm, I wonder if this is simply a Nix idiosyncrasy", and it turns out indeed it was.
Things that were working in the blessed way of the land of Python (inside a `virtualenv`) were not working the way they were from his home town of Nix (`nix-shell` + `python.withPackages`).
After even more frustrating attempts at reading the mypy documentation and trying to understand how things were supposed to work our hero started questioning his sanity.
This is where things started to get truly interesting.
Our hero started to use a number of powerful weapons, both forged in the land of Python (pdb) & by the mages of UNIX (printf-style-debugging & strace).
After first trying to slay the dragon simply by `strace` and a keen eye our hero did not spot any weak points.
Time to break out a more powerful sword (`pdb`) which also did not divulge any secrets about what was wrong.
Our hero went back to the `strace` output and after a fair bit of thought and analysis a pattern started to emerge. Mypy was looking in the wrong place (i.e. not in in the environment created by `python.withPackages` but in the interpreter store path) and our princess was in another castle!
Our hero went to the pub full of old grumpy men giving out the inner workings of the open source universe (Github) and acquired a copy of Mypy.
He littered the code with print statements & break points.
After a fierce battle full of blood, sweat & tears he ended up in 20f7f2dd71/mypy/sitepkgs.py and realised that everything came down to the Python `site` module and more specifically https://docs.python.org/3.7/library/site.html#site.getsitepackages which in turn relies on https://docs.python.org/3.7/library/site.html#site.PREFIXES .
Our hero created a copy of the environment created by `python.withPackages` and manually modified it to confirm his findings, and it turned out it was indeed the case.
Our hero had damaged the dragon and it was time for a celebration.
He went out and acquired some mead which he ingested while he typed up his story and waited for the dragon to finally die (the commit caused a mass-rebuild, I had to wait for my repro).
In the end all was good in [NixOps-AWS](https://github.com/nixos/nixops-aws)-town and type checks could run. (PR for that incoming tomorrow).
- Replaced python override from the final stdenv, instead we
propagate our bootstrap python to stage4 and override both
CF and xnu to use it.
- Removed CF argument from python interpreters, this is redundant
since it's not overidden anymore.
- Inherit CF from stage4, making it the same as the stdenv.
When `makeWrapperArgs` variable is not set, `declare -p makeWrapperArgs`
will return with 1 and print an error message to stderr.
I did not handle the non-existence case in b0633406cb
because I thought `mk-python-derivation` will always define `makeWrapperArgs`
but `wrapProgram` can be called independently. And even with `mk-python-derivation`,
`makeWrappers` will not be set unless explicitly declared in the derivation
because of https://github.com/NixOS/nix/issues/1461.
I was lead to believe that because the builds were succeeding and I confirmed
that the mechanism fails when the variable is not defined and `-o nounset` is enabled.
It appears that `wrapPython` setup hook is not running under `-o nounset`, though,
invaldating the assumption.
Now we are checking that the variable exists before checking its type, which
will get rid of the warning and also prevent future error when `-o nounset`
is enabled in the setup hook.
For more information, see the discussion at
https://github.com/NixOS/nixpkgs/commit/a6bb2ede232940a96150da7207a3ecd15eb6328
Bash takes an assignment of a string to an array variable:
local -a user_args
user_args="(foo bar)"
to mean appending the string to the array, not parsing the string into
an array as is the case when on the same line as the declaration:
local -a user_args="(foo bar)"
b0633406cb extracted the declaration before
the newly branched code block, causing string makeWrapperArgs being added
to the array verbatim.
Since local is function scoped, it does not matter if we move it inside
each of the branches so we fix it this way.
When `makeWrapperArgs` is a Bash array, we only passed the first
item to `wrapProgram`. We need to use `"${makeWrapperArgs[@]}"`
to extract all the items. But that breaks the common string case so
we need to handle that case separately.
This will turn manylinux support back on by default.
PIP will now do runtime checks against the compatible glibc version to
determine if the current interpreter is compatible with a given
manylinux specification. However it will not check if any of the
required libraries are present.
The motivation here is that we want to support building python packages
with wheels that require manylinux support. There is no real change for
users of source builds as they are still buildings packages from source.
The real noticeable(?) change is that impure usages (e.g. running `pip
install package`) will install manylinux packages that previously
refused to install.
Previously we did claim that we were not compatible with manylinux and
thus they wouldn't be installed at all.
Now impure users will have basically the same situation as before: If
you require some wheel only package it didn't work before and will not
properly work now. Now the program will fail during runtime vs during
installation time.
I think it is a reasonable trade-off since it allows us to install
manylinux packages with nix expressions and enables tools like
poetry2nix.
This should be a net win for users as it allows wheels, that we
previously couldn't really support, to be used.
