It seems like `9.0.1-2.3~ubuntu1.18.04.1` breaks the `--extra-index-url`
functionality of PIP.

In my understanding, the idea is for `--extra-index-url` to provide a
PyPI-compliant repository that offers a few additional packages, allowing PIP
to fall back onto the instance configured as `--index-url` (defaults to
upstream PyPI). This has been the case with earlier versions, allowing us to
host an internal PyPI repository containing a subset of packages, as well as a
generic local caching PyPI mirror.

In `9.0.1-2.3~ubuntu1.18.04.1`, it seems like the fallback mechanisms do not
work properly, instead failing if _either_ the `--index-url` _or_
`--extra-index-url` instances lack the package.

---

With `~/.config/pip/pip.conf` as:

    [global]
    index-url = https://<user>:<api-token>@artifactory.internal/artifactory/api/pypi/PyPI_mirror/simple
    extra-index-url = https://<user>:<api-token>@artifactory.internal/artifactory/api/pypi/PyPI_extra/simple

With `python3-pip` version `9.0.1-2.3~ubuntu1.18.04.1`, I get a session like:

    ~$ python3 -m venv env/pip-latest
    ~$ . env/pip-latest/bin/activate
    (pip-latest) ~$ pip --version
    pip 9.0.1 from /home/dandersson/env/pip-latest/lib/python3.6/site-packages (python 3.6)
    (pip-latest) ~$ pip install hpt
    Collecting hpt
    Exception:
    Traceback (most recent call last):
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/basecommand.py", line 215, in main
        status = self.run(options, args)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/commands/install.py", line 353, in run
        wb.build(autobuilding=True)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/wheel.py", line 749, in build
        self.requirement_set.prepare_files(self.finder)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/req/req_set.py", line 380, in prepare_files
        ignore_dependencies=self.ignore_dependencies))
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/req/req_set.py", line 554, in _prepare_file
        require_hashes
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/req/req_install.py", line 278, in populate_link
        self.link = finder.find_requirement(self, upgrade)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 465, in find_requirement
        all_candidates = self.find_all_candidates(req.name)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 423, in find_all_candidates
        for page in self._get_pages(url_locations, project_name):
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 568, in _get_pages
        page = self._get_page(location)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 683, in _get_page
        return HTMLPage.get_page(link, session=self.session)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 795, in get_page
        resp.raise_for_status()
      File "/home/dandersson/env/pip-latest/share/python-wheels/requests-2.18.4-py2.py3-none-any.whl/requests/models.py", line 935, in raise_for_status
        raise HTTPError(http_error_msg, response=self)
    requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/simple/hpt/

PIP fails with a traceback due to not finding `hpt` on the `--index-url`
instance -- but the point of giving `--extra-index-url` is that `hpt` resides
on that instance.

Trying to install a package that should be present on the `--index-url`
instance:

    (pip-latest) ~$ pip install requests
    Collecting requests
    Exception:
    Traceback (most recent call last):
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/basecommand.py", line 215, in main
        status = self.run(options, args)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/commands/install.py", line 353, in run
        wb.build(autobuilding=True)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/wheel.py", line 749, in build
        self.requirement_set.prepare_files(self.finder)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/req/req_set.py", line 380, in prepare_files
        ignore_dependencies=self.ignore_dependencies))
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/req/req_set.py", line 554, in _prepare_file
        require_hashes
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/req/req_install.py", line 278, in populate_link
        self.link = finder.find_requirement(self, upgrade)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 465, in find_requirement
        all_candidates = self.find_all_candidates(req.name)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 423, in find_all_candidates
        for page in self._get_pages(url_locations, project_name):
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 568, in _get_pages
        page = self._get_page(location)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 683, in _get_page
        return HTMLPage.get_page(link, session=self.session)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 795, in get_page
        resp.raise_for_status()
      File "/home/dandersson/env/pip-latest/share/python-wheels/requests-2.18.4-py2.py3-none-any.whl/requests/models.py", line 935, in raise_for_status
        raise HTTPError(http_error_msg, response=self)
    requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://artifactory.internal/artifactory/api/pypi/PyPI_extra/simple/requests/

Now it fails with a traceback since `requests` does not exist on the
`--extra-index-url` instance, but the purpose of that instance is to provide a
small subset of extra packages, not to be a full mirror.

Changing `~/.config/pip/pip.conf` to:

    [global]
    index-url = https://<user>:<api-token>@artifactory.internal/artifactory/api/pypi/PyPI_mirror/simple

removing the `--extra-index-url` instance altogether, it behaves as expected,
not finding the `hpt` package, but being able to install `requests`:

    (pip-latest) ~$ pip install hpt
    Collecting hpt
    Exception:
    Traceback (most recent call last):
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/basecommand.py", line 215, in main
        status = self.run(options, args)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/commands/install.py", line 353, in run
        wb.build(autobuilding=True)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/wheel.py", line 749, in build
        self.requirement_set.prepare_files(self.finder)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/req/req_set.py", line 380, in prepare_files
        ignore_dependencies=self.ignore_dependencies))
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/req/req_set.py", line 554, in _prepare_file
        require_hashes
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/req/req_install.py", line 278, in populate_link
        self.link = finder.find_requirement(self, upgrade)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 465, in find_requirement
        all_candidates = self.find_all_candidates(req.name)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 423, in find_all_candidates
        for page in self._get_pages(url_locations, project_name):
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 568, in _get_pages
        page = self._get_page(location)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 683, in _get_page
        return HTMLPage.get_page(link, session=self.session)
      File "/home/dandersson/env/pip-latest/lib/python3.6/site-packages/pip/index.py", line 795, in get_page
        resp.raise_for_status()
      File "/home/dandersson/env/pip-latest/share/python-wheels/requests-2.18.4-py2.py3-none-any.whl/requests/models.py", line 935, in raise_for_status
        raise HTTPError(http_error_msg, response=self)
    requests.exceptions.HTTPError: 404 Client Error: Not Found for url: https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/simple/hpt/

It is expected to not be able to install `hpt`, but PIP should not fail with a
traceback, but just with a diagnostic message like `No matching distribution
found for hpt`.

Installing a common package will correctly use the `--index-url` instance:

    (pip-latest) ~$ pip install requests
    Collecting requests
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/51/bd/23c926cd341ea6b7dd0b2a00aba99ae0f828be89d72b2190f27c11d4b7fb/requests-2.22.0-py2.py3-none-any.whl
    Collecting idna<2.9,>=2.5 (from requests)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/14/2c/cd551d81dbe15200be1cf41cd03869a46fe7226e7450af7a6545bfc474c9/idna-2.8-py2.py3-none-any.whl
    Collecting certifi>=2017.4.17 (from requests)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/69/1b/b853c7a9d4f6a6d00749e94eb6f3a041e342a885b87340b79c1ef73e3a78/certifi-2019.6.16-py2.py3-none-any.whl
    Collecting urllib3!=1.25.0,!=1.25.1,<1.26,>=1.21.1 (from requests)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/e6/60/247f23a7121ae632d62811ba7f273d0e58972d75e58a94d329d51550a47d/urllib3-1.25.3-py2.py3-none-any.whl
    Collecting chardet<3.1.0,>=3.0.2 (from requests)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/bc/a9/01ffebfb562e4274b6487b4bb1ddec7ca55ec7510b22e4c51f14098443b8/chardet-3.0.4-py2.py3-none-any.whl
    Installing collected packages: idna, certifi, urllib3, chardet, requests
    Successfully installed certifi-2019.6.16 chardet-3.0.4 idna-2.8 requests-2.22.0 urllib3-1.25.3

---

Downgrading `python3-pip` to `9.0.1-2`:

    ~$ sudo apt install python3-pip=9.0.1-2 python-pip-whl=9.0.1-2
    Reading package lists... Done
    Building dependency tree
    Reading state information... Done
    The following packages will be DOWNGRADED:
      python-pip-whl python3-pip
    0 upgraded, 0 newly installed, 2 downgraded, 0 to remove and 5 not upgraded.
    Need to get 1 493 kB of archives.
    After this operation, 226 kB disk space will be freed.
    Do you want to continue? [Y/n]
    Get:1 http://se.archive.ubuntu.com/ubuntu bionic/universe amd64 python3-pip all 9.0.1-2 [114 kB]
    Get:2 http://se.archive.ubuntu.com/ubuntu bionic/universe amd64 python-pip-whl all 9.0.1-2 [1 379 kB]
    Fetched 1 493 kB in 0s (4 177 kB/s)
    dpkg: warning: downgrading python3-pip from 9.0.1-2.3~ubuntu1.18.04.1 to 9.0.1-2
    (Reading database ... 196581 files and directories currently installed.)
    Preparing to unpack .../python3-pip_9.0.1-2_all.deb ...
    Unpacking python3-pip (9.0.1-2) over (9.0.1-2.3~ubuntu1.18.04.1) ...
    dpkg: warning: downgrading python-pip-whl from 9.0.1-2.3~ubuntu1.18.04.1 to 9.0.1-2
    Preparing to unpack .../python-pip-whl_9.0.1-2_all.deb ...
    Unpacking python-pip-whl (9.0.1-2) over (9.0.1-2.3~ubuntu1.18.04.1) ...
    Setting up python-pip-whl (9.0.1-2) ...
    Setting up python3-pip (9.0.1-2) ...
    Processing triggers for man-db (2.8.3-2ubuntu0.1) ...

and retrying the first attempt with `~/.config/pip/pip.conf` as:

    [global]
    index-url = https://<user>:<api-token>@artifactory.internal/artifactory/api/pypi/PyPI_mirror/simple
    extra-index-url = https://<user>:<api-token>@artifactory.internal/artifactory/api/pypi/PyPI_extra/simple

yields a session like:

    ~$ python3 -m venv env/pip-previous
    ~$ . env/pip-previous/bin/activate
    (pip-previous) ~$ pip --version
    pip 9.0.1 from /home/dandersson/env/pip-previous/lib/python3.6/site-packages (python 3.6)
    (pip-previous) ~$ pip install hpt
    Collecting hpt
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_extra/packages/hpt/0.1.397/hpt-0.1.397.tar.gz
    Collecting argh (from hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/06/1c/e667a7126f0b84aaa1c56844337bf0ac12445d1beb9c8a6199a7314944bf/argh-0.26.2-py2.py3-none-any.whl
    Collecting pyyaml==5.1 (from hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/9f/2c/9417b5c774792634834e730932745bc09a7d36754ca00acf1ccd1ac2594d/PyYAML-5.1.tar.gz
    Collecting argcomplete (from hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/4d/82/f44c9661e479207348a979b1f6f063625d11dc4ca6256af053719bbb0124/argcomplete-1.10.0-py2.py3-none-any.whl
    Collecting distro (from hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/ea/35/82f79b92fa4d937146c660a6482cee4f3dfa1f97ff3d2a6f3ecba33e712e/distro-1.4.0-py2.py3-none-any.whl
    Collecting requests (from hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/51/bd/23c926cd341ea6b7dd0b2a00aba99ae0f828be89d72b2190f27c11d4b7fb/requests-2.22.0-py2.py3-none-any.whl
    Collecting termcolor (from hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/8a/48/a76be51647d0eb9f10e2a4511bf3ffb8cc1e6b14e9e4fab46173aa79f981/termcolor-1.1.0.tar.gz
    Collecting cilib==v0.1.397 (from hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_extra/packages/cilib/0.1.397/cilib-0.1.397.tar.gz
    Collecting idna<2.9,>=2.5 (from requests->hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/14/2c/cd551d81dbe15200be1cf41cd03869a46fe7226e7450af7a6545bfc474c9/idna-2.8-py2.py3-none-any.whl
    Collecting chardet<3.1.0,>=3.0.2 (from requests->hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/bc/a9/01ffebfb562e4274b6487b4bb1ddec7ca55ec7510b22e4c51f14098443b8/chardet-3.0.4-py2.py3-none-any.whl
    Collecting certifi>=2017.4.17 (from requests->hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/69/1b/b853c7a9d4f6a6d00749e94eb6f3a041e342a885b87340b79c1ef73e3a78/certifi-2019.6.16-py2.py3-none-any.whl
    Collecting urllib3!=1.25.0,!=1.25.1,<1.26,>=1.21.1 (from requests->hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/e6/60/247f23a7121ae632d62811ba7f273d0e58972d75e58a94d329d51550a47d/urllib3-1.25.3-py2.py3-none-any.whl
    Collecting hvac (from cilib==v0.1.397->hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/7b/2c/b95b551769459406ca440788c32d699820f4c875020ff95c6e3844eee6d3/hvac-0.9.2-py2.py3-none-any.whl
    Collecting psycopg2-binary (from cilib==v0.1.397->hpt)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/b1/35/75c9c2d9cfc073ab6c42b2d8e91ff58c9b99f4ed7ed56b36647642e6080e/psycopg2_binary-2.8.3-cp36-cp36m-manylinux1_x86_64.whl
    [...SNIP...]
    Successfully installed argcomplete-1.10.0 argh-0.26.2 certifi-2019.6.16 chardet-3.0.4 cilib-0.1.397 distro-1.4.0 hpt-0.1.397 hvac-0.9.2 idna-2.8 psycopg2-binary-2.8.3 pyyaml-5.1 requests-2.22.0 termcolor-1.1.0 urllib3-1.25.3

PIP now correctly resolves the respective packages to their respective
instances, using `--extra-index-url` for the packages that are _only_ present
there, and falling back to `--index-url` for everything else.

Changing `~/.config/pip/pip.conf` to:

    [global]
    index-url = https://<user>:<api-token>@artifactory.internal/artifactory/api/pypi/PyPI_mirror/simple

cleaning out the virtual environment and trying again:

    (pip-previous) ~$ deactivate
    ~$ rm -rf env/pip-previous/
    ~$ python3 -m venv env/pip-previous
    ~$ . env/pip-previous/bin/activate
    (pip-previous) ~$ pip --version
    pip 9.0.1 from /home/dandersson/env/pip-previous/lib/python3.6/site-packages (python 3.6)
    (pip-previous) ~$ pip install hpt
    Collecting hpt
      Could not find a version that satisfies the requirement hpt (from versions: )
    No matching distribution found for hpt
    (pip-previous) ~$ pip install requests
    Collecting requests
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/51/bd/23c926cd341ea6b7dd0b2a00aba99ae0f828be89d72b2190f27c11d4b7fb/requests-2.22.0-py2.py3-none-any.whl
    Collecting chardet<3.1.0,>=3.0.2 (from requests)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/bc/a9/01ffebfb562e4274b6487b4bb1ddec7ca55ec7510b22e4c51f14098443b8/chardet-3.0.4-py2.py3-none-any.whl
    Collecting urllib3!=1.25.0,!=1.25.1,<1.26,>=1.21.1 (from requests)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/e6/60/247f23a7121ae632d62811ba7f273d0e58972d75e58a94d329d51550a47d/urllib3-1.25.3-py2.py3-none-any.whl
    Collecting certifi>=2017.4.17 (from requests)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/69/1b/b853c7a9d4f6a6d00749e94eb6f3a041e342a885b87340b79c1ef73e3a78/certifi-2019.6.16-py2.py3-none-any.whl
    Collecting idna<2.9,>=2.5 (from requests)
      Using cached https://artifactory.internal/artifactory/api/pypi/PyPI_mirror/packages/14/2c/cd551d81dbe15200be1cf41cd03869a46fe7226e7450af7a6545bfc474c9/idna-2.8-py2.py3-none-any.whl
    Installing collected packages: chardet, urllib3, certifi, idna, requests
    Successfully installed certifi-2019.6.16 chardet-3.0.4 idna-2.8 requests-2.22.0 urllib3-1.25.3

---

In summary, from my point-of-view, the `9.0.1-2.3~ubuntu1.18.04.1` version
breaks all functionality related to `--extra-index-url`, as well as regresses
the behaviour when a package is not found when using `--index-url`.

I will be happy to provide any additional information that can help.