poetry not trying secondary source if default source is offline

• I am on the latest Poetry version.
• I have searched the issues of this repo and believe that this is not a duplicate.
• If an exception occurs when executing a command, I executed it again in debug mode (-vvv option).

• OS version and name: macOS version 12.4 (Monterey)
• Poetry version: 1.1.14
• Link of a Gist with the contents of your pyproject.toml file: https://gist.github.com/jamesrobb/15b51866bab62fade3ade8668cd2a343

Issue

I've been trying to setup poetry such that it tries to retrieve packages from a private server first, and should that fail, it uses the public PyPi server as a backup. This is how my sources are configured:

[[tool.poetry.source]]
name = "foo"
url = "https://foo.bar/simple/"
default = true
secondary = false

[[tool.poetry.source]]
name = 'pypi-public'
url = 'https://pypi.python.org/simple'
default = false
secondary = true

To test this I tried to install a common python package (pandas), but I get an error that a connection cannot be established to foo.bar:

$ poetry add pandas                                                                                                                                     

  ConnectionError

  HTTPSConnectionPool(host='foo.bar', port=443): Max retries exceeded with url: /simple/pandas/ (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x10266dfc0>: Failed to establish a new connection: [Errno 8] nodename nor servname provided, or not known'))

  at /opt/homebrew/Cellar/poetry/1.1.14/libexec/lib/python3.10/site-packages/requests/adapters.py:565 in send
      561│             if isinstance(e.reason, _SSLError):
      562│                 # This branch is for urllib3 v1.22 and later.
      563│                 raise SSLError(e, request=request)
      564│
    → 565│             raise ConnectionError(e, request=request)
      566│
      567│         except ClosedPoolError as e:
      568│             raise ConnectionError(e, request=request)

The fact that it can't establish a connection to foo.bar of course makes sense, but what I don't understand why it isn't even trying to use the public PyPi source I specified as secondary.

Is this an actual bug or am I misunderstanding the intended behavior of poetry? I would think (intuitively) that poetry would try use a secondary source if the default is unavailable (.e.g, it's down for maintenance).

How can I go about configuring poetry to behave in the way I had hoped it to? (trying my private source first, and then falling back to the public PyPi source).

Reproduce the issue using the provided source configuration and the offline foo.bar endpoint, then inspect Poetry's package source selection and connection-error handling. Done means a failed default source allows resolution from the configured secondary PyPI source, with tests covering the fallback behavior.