Set PYTHONPYCACHEPREFIX to /tmp during build phase. #442
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ryanmoran
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joshuatcasey
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- This enables reproducible builds by discarding the `__pycache__` directories (instead of the current behavior of persisting them in the cpython layer). - The `__pycache__` directories are not reproducible and are effectively opaque to us. We cannot make them reproducible, so discarding them is the only option for reproducible builds. - The PYTHONPYCACHEPREFIX environment variable was chosen to allow users to override it if needed, whereas setting PYTHONDONTWRITEBYTECODE=true would have been harder, if not impossible, to allow users to override. - Setting this environment variable as a build environment on the layer (v.s. e.g. in process via os.SetEnv()) means all subsequent buildpacks (e.g. pip, poetry, etc) will have this environment variable set automatically by the lifecycle. - We only set this environment variable for the build phase (and not the launch phase) to allow users to continue to leverage `__pycache__` directories at launch time if they wish. The current launch-time behavior of defaulting to $HOME/.pycache is unchanged. - Also update the README to include this new feature and various other minor README improvements.
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Summary
This PR enables reproducible builds by discarding the
__pycache__directories at the end of the build phase for this buildpack and all subsequent buildpacks that depend on this.This is in contrast to the current behavior of persisting the
__pycache__directories in each buildpack's layer.It also updates the README to include this new configuration and various other minor README improvements.
We have not observed any performance issues with this configuration: discarding the
__pycache__directories at the end of each buildpack's build phase does not seem to impact the performance of any buildpack.Background
__pycache__directories are not reproducible and are effectively opaque to us. We cannot make them reproducible, so discarding them is the only option for reproducible builds.PYTHONPYCACHEPREFIXenvironment variable was chosen to allow users to override it if needed, whereas settingPYTHONDONTWRITEBYTECODE=truewould have been harder, if not impossible, to allow users to override.os.SetEnv()) means all subsequent buildpacks (e.g. pip, poetry, etc) will have this environment variable set automatically by the lifecycle, rather than having to implement this themselves in each buildpack.__pycache__directories at launch time if they wish. The current launch-time behavior of defaulting to$HOME/.pycacheis unchanged.Use Cases
Enables reproducible builds in this buildpack and some subsequent buildpacks.
Fixes #426
Checklist