ensurepip uses cache directory

[kkonopko]

BPO	40448
Nosy	@pfmoore, @ncoghlan, @ned-deily, @dstufft, @pradyunsg, @miss-islington, @kkonopko
PRs	bpo-40448: ensurepip: Do not use cache #19812 [3.9] bpo-40448: ensurepip: Do not use cache (GH-19812) #20893 [3.8] bpo-40448: ensurepip: Do not use cache (GH-19812) #20894 [3.7] bpo-40448: ensurepip: Do not use cache (GH-19812) #20895

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-06-15.19:24:55.289>
created_at = <Date 2020-04-30.09:39:11.935>
labels = ['type-bug', 'expert-installation', '3.9', '3.10', '3.8', 'build', '3.7']
title = 'ensurepip uses cache directory'
updated_at = <Date 2020-06-15.19:24:55.281>
user = 'https://github.com/kkonopko'

bugs.python.org fields:

activity = <Date 2020-06-15.19:24:55.281>
actor = 'ned.deily'
assignee = 'none'
closed = True
closed_date = <Date 2020-06-15.19:24:55.289>
closer = 'ned.deily'
components = ['Build', 'Installation']
creation = <Date 2020-04-30.09:39:11.935>
creator = 'kkonopko'
dependencies = []
files = []
hgrepos = []
issue_num = 40448
keywords = ['patch']
message_count = 7.0
messages = ['367751', '369436', '369437', '371579', '371580', '371581', '371582']
nosy_count = 8.0
nosy_names = ['paul.moore', 'ncoghlan', 'ned.deily', 'dstufft', 'pradyunsg', 'Marcus.Smith', 'miss-islington', 'kkonopko']
pr_nums = ['19812', '20893', '20894', '20895']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue40448'
versions = ['Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[kkonopko]

ensurepip optionally installs or upgrades 'pip' and 'setuptools' using the version of those modules bundled with Python. The internal PIP
installation routine by default temporarily uses its cache, if it
exists. This is undesirable as Python builds and installations may be
independent of the user running the build, whilst PIP cache location
is dependent on the user's environment and outside of the build
environment.

At the same time, there's no value in using the cache while installing
bundled modules. By design ensurepip does not access network so there's no point in checking or in any way accessing the cache.

This causes a problem in somewhat less usual build environments where Python is built into a designated DESTDIR with --prefix specified etc. If it does not have write permission to the cache directory (eg. $HOME/.cache/pip on Linux), it issues a warning but it installs bundled 'pip' and 'setuptools' fine. But if it does not have execute access (to read directories), it fails hard.

strace also shows a lot of checks and even temporary use of the cache directory while processing whl files.

[ned-deily]

Thanks for the PR. We should get a review from at least one of the pip developers. @oradyunsg, @dstufft, @paul.moore, opinions?

[pfmoore]

PR looks good to me. There's a test failure that needs fixing and the PR needs a news entry, but otherwise looks fine.

[ned-deily]

New changeset 4a3a682 by Krzysztof Konopko in branch 'master':
bpo-40448: ensurepip: Do not use cache (GH-19812)
4a3a682

[miss-islington]

New changeset b46beb2 by Miss Islington (bot) in branch '3.7':
bpo-40448: ensurepip: Do not use cache (GH-19812)
b46beb2

[miss-islington]

New changeset a1d3be4 by Miss Islington (bot) in branch '3.8':
bpo-40448: ensurepip: Do not use cache (GH-19812)
a1d3be4

[miss-islington]

New changeset 9a0624a by Miss Islington (bot) in branch '3.9':
bpo-40448: ensurepip: Do not use cache (GH-19812)
9a0624a