Don't error if package index is missing (fix #10504)

[ulysses4ever]

Cherry picked MercuryTechnologies@ec242b1

Would be great to have a test...

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[gbaz]

As per my comments in #10504 I've come around to seeing this as the correct fix. If the missing package index causes problems down the line we'll error anyway. If not, then this is intended behavior and warning is fine.

The semantics of active-repositories is that it picks which of the repositories cabal pulls from are active and in what order -- it was not designed to choose which repositories cabal connects to entirely.

[ulysses4ever]

The test failure is interesting:

-Error: [Cabal-7160]
-The package list for 'repo.invalid' does not exist. Run 'cabal update' to download it.
+Warning: The package list for 'repo.invalid' does not exist. Run 'cabal update' to download it.
+Error: [Cabal-7100]
+There is no package named 'a-b-s-e-n-t'. 
+You may need to run 'cabal update' to get the latest list of available packages.

Arguably, failing earlier in this case is preferable because the reason we failed is not that something is inherently wrong with the a-b-s-e-n-t package but because our repository wasn't configured correctly in the first place, and no matter what package we'll try to get, we'll fail.

So, in this case, I'd argue that reverting the error mode makes the error message worse.

[ulysses4ever]

For posterity: on today's Cabal meeting people agreed that we want to revert it, if only temporarily, until a better solution is found, in the interest of releasing 3.14 without this overly strict behavior.

[ulysses4ever]

@mergify backport 3.14

[mergify]

backport 3.14

✅ Backports have been created

• #10542 Backport #10506: Don't error if package index is missing (fix #10504) has been created for branch 3.14