The download command using legacy-resolver fails for --constraint where the constraints are disjoint / a superset.

[jsirois]

Environment

• pip version: 20.3.2
• Python version: CPython 3.9
• OS: Linux

Description
When --constraint is used and the constraints contain requirements not in the transitive closure of a download, the download command fails to filter out InstallRequirements that are constraint-only and thus have have no link resulting in an AssertionError.

Expected behavior
No error should occur.

How to Reproduce
Perform a download of a pinned requirement using a constraints file that only contains pinned requirements, one of which is disjoint with the set of transitive requirements of the original download subject requirement. The output demonstrates all this.

Output

$ pip --version
pip 20.3.2 from /home/jsirois/.local/lib/python3.9/site-packages/pip (python 3.9)
$ cat constraints.txt 
ansicolors==1.1.8
$ rm -rf wheels/ && pip download --dest wheels/ --use-deprecated legacy-resolver ansicolors==1.1.8 --constraint constraints.txt
Collecting ansicolors==1.1.8
  Using cached ansicolors-1.1.8-py2.py3-none-any.whl (13 kB)
Saved ./wheels/ansicolors-1.1.8-py2.py3-none-any.whl
Successfully downloaded ansicolors
$ echo pex==2.1.23 >> constraints.txt
$ cat constraints.txt 
ansicolors==1.1.8
pex==2.1.23
$ rm -rf wheels/ && pip download --dest wheels/ --use-deprecated legacy-resolver ansicolors==1.1.8 --constraint constraints.txt
Collecting ansicolors==1.1.8
  Using cached ansicolors-1.1.8-py2.py3-none-any.whl (13 kB)
Saved ./wheels/ansicolors-1.1.8-py2.py3-none-any.whl
ERROR: Exception:
Traceback (most recent call last):
  File "/home/jsirois/.local/lib/python3.9/site-packages/pip/_internal/cli/base_command.py", line 224, in _main
    status = self.run(options, args)
  File "/home/jsirois/.local/lib/python3.9/site-packages/pip/_internal/cli/req_command.py", line 180, in wrapper
    return func(self, options, args)
  File "/home/jsirois/.local/lib/python3.9/site-packages/pip/_internal/commands/download.py", line 138, in run
    preparer.save_linked_requirement(req)
  File "/home/jsirois/.local/lib/python3.9/site-packages/pip/_internal/operations/prepare.py", line 531, in save_linked_requirement
    assert req.link is not None
AssertionError
$ rm -rf wheels/ && pip download --dest wheels/ ansicolors==1.1.8 --constraint constraints.txt
Collecting ansicolors==1.1.8
  Using cached ansicolors-1.1.8-py2.py3-none-any.whl (13 kB)
Saved ./wheels/ansicolors-1.1.8-py2.py3-none-any.whl
Successfully downloaded ansicolors

[uranusjr]

Did this ever work prior to 20.3? There is a reason we implemented a new resolver from scratch and try to remove the legacy resolver. This is one of the known issues with it that’s not reallly viable to handle. We are not going to fix issues in the legacy resolver for all the same reasons we implemented a new one.

[jsirois]

Yes, it has worked since day one of the introduction of the download command in 417f79d. A git bisect turns up b28e2c4 as the commit that introduced the bug. So this is a regression in the legacy-resolver introduced by work bringing the 2020-resolver to life.

[jsirois]

I'll optimistically post a fix PR and link here in case you all are willing to accept it. If not, our project will just fork and apply the same PR in order to provide our users a smooth transition to migrate from the legacy-resolver to the 2020-resolver. As it stands, many of those users use --constraint and so otherwise cannot smoothly transition.

[uranusjr]

Given this and #9271, I feel we should revert the patch and discuss what download and wheel should do in the new resoler’s context.