Windows uninstallation should not remove launcher if other versions remain

[chrullrich]

BPO	25165
Nosy	@pfmoore, @tjguk, @zware, @zooba, @chrullrich

^{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 = 'https://github.com/zooba'
closed_at = <Date 2015-10-01.22:21:01.841>
created_at = <Date 2015-09-18.09:03:04.029>
labels = ['type-bug', 'expert-installation', 'OS-windows']
title = 'Windows uninstallation should not remove launcher if other versions remain'
updated_at = <Date 2015-10-01.22:21:01.840>
user = 'https://github.com/chrullrich'

bugs.python.org fields:

activity = <Date 2015-10-01.22:21:01.840>
actor = 'steve.dower'
assignee = 'steve.dower'
closed = True
closed_date = <Date 2015-10-01.22:21:01.841>
closer = 'steve.dower'
components = ['Installation', 'Windows']
creation = <Date 2015-09-18.09:03:04.029>
creator = 'chrullrich'
dependencies = []
files = []
hgrepos = []
issue_num = 25165
keywords = []
message_count = 3.0
messages = ['250957', '251377', '252068']
nosy_count = 6.0
nosy_names = ['paul.moore', 'tim.golden', 'python-dev', 'zach.ware', 'steve.dower', 'chrullrich']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue25165'
versions = ['Python 3.5']

[chrullrich]

Uninstalling 3.5 on Windows removes the py.exe launcher. If it was installed in the system-wide path (%SYSTEMROOT%), it may have replaced an existing copy from an earlier version. In this case, removing the launcher will break this remaining installation, requiring a repair.

Instead, the launcher should not be deleted. It uses the static CRT, hence has no DLL dependencies that would be removed by the uninstallation, and it will -- as PEP-397 demands -- select the Python version it starts based on the command line and the available installations.

Ceterum censeo: This bug could have been avoided by using MSI as the distribution package format and using the same component code for the launcher file in all supporting versions. Even if the file itself is different, the identical component code would have ensured through reference counting that it would only have been removed with the last referencing installation. Upward compatibility of older launchers could have become a problem, though.

[zooba]

I'm going to look into having the launcher's MSI stay behind when the bundle is removed, so it will have to be uninstalled separately. It can already be installed separately, though there isn't a really easy way to get the installer so you can do that.

(Because prior versions of Python did not always use the same component ID for the launcher, we have to have a break here. However, we now have a single MSI that is identical for all future versions, so the problem should not arise again.)

[python-dev]

New changeset a2d30dfa46a7 by Steve Dower in branch '3.5':
Issue bpo-25165: Windows uninstallation should not remove launcher if other versions remain
https://hg.python.org/cpython/rev/a2d30dfa46a7