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Unverified Commit 24b892e8 authored by Ed Morley's avatar Ed Morley Committed by GitHub
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Perform editable package .pth and .egg-link path rewriting at runtime (#1252)

Currently the build system performs builds in a different directory
(`/tmp/build_<hash>`) to which the app will be run at runtime (`/app`).
This means that any hardcoded paths in the slug must be rewritten by the
buildpack, so the app still works after being moved.

One such case of hardcoded paths, are the `.pth` and `.egg-link` files
that are created in the `site-packages` directory by Pip/setuptools for
editable package installs (aka develop mode). The most common way
someone might use editable mode is via `-e <package specifier>` entries
in their `requirements.txt` file.

Until now, the Python buildpack rewrote paths inside these files during
the compile itself, which meant the build-time paths were no longer
present when subsequent buildpacks ran. This happened to work due to
an interaction of legacy setuptools behaviour and a buildpack bug, but
stops working in setuptools 47.2.0 or later - as described in #1006.

Longer term we would like to stop building in one location and running
the app from another, so that the path rewriting isn't required at all.
However that change breaks some other buildpacks so requires a long-term
transition plan.

In the meantime, this change moves path rewriting to a `.profile.d/`
script, so that it occurs at runtime, and so after all other buildpacks
have run.

Additional test coverage of editable packages was added previously in #1251
and #1253, and has confirmed that this new `profile.d/` script approach will
prevent the issues in #1006 when setuptools is updated in a future PR.

There is one subtle implication of moving this path rewriting, in that
subsequent cached builds will no longer see the existing package as
being already installed, so won't uninstall if before reinstalling it (as
seen in the test log output change). However this is not believed to
have any significant impact.

Fixes #1006. (And so unblocks updating to the newer setuptools required for #1248.)
GUS-W-7828034.
parent e315f1f0
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