aef431fbd1
## Motivation Nix currently hard fails if a substituter is inaccessible, even when they are other substituters available, unless `fallback = true`. This breaks nix build, run, shell et al entirely. This would modify the default behaviour so that nix would actually use the other available substituters and not hard error. Here is an example before vs after when using dotenv where I have manually stopped my own cache to trigger this issue, before and after the patch. The initial error is really frustrating because there is other caches available.   ## Context https://github.com/NixOS/nix/issues/3514#issuecomment-2905056198 is the earliest issue I could find, but there are many duplicates. There is an initial PR at https://github.com/NixOS/nix/pull/7188, but this appears to have been abandoned - over 2 years with no activity, then a no comment review in jan. There was a subsequent PR at https://github.com/NixOS/nix/pull/8983 but this was closed without merge - over a year without activity. <!-- Non-trivial change: Briefly outline the implementation strategy. --> I have visualised the current and proposed flows. I believe my logic flows line up with what is suggested in https://github.com/NixOS/nix/pull/7188#issuecomment-1375652870 but correct me if I am wrong. Current behaviour:  Proposed behaviour:  [Charts in lucid](https://lucid.app/lucidchart/1b51b08d-6c4f-40e0-bf54-480df322cccf/view) <!-- Invasive change: Discuss alternative designs or approaches you considered. --> Possible issues to think about: - I could not figure out where the curl error is created... I can't figure out how to swallow it and turn it into a warn or better yet, a debug log. - Unfortunately, in contrast with the previous point, I'm not sure how verbose we want to warns/traces to be - personally I think that the warn that a substituter has been disabled (when it happens) is sufficient, and that the next one is being used, but this is personal preference.
Nix
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