[opam-devel] OPAM 1.1 backwards-compatibility
louis.gesbert at ocamlpro.com
Mon Jan 19 04:41:10 GMT 2015
A note on the continued compatibility of the main package repository with OPAM 1.1, following the github issue at . Sorry for the length; there's a tl;dr at the start of each paragraph.
## Current situation of OPAM 1.1 support
[TL;DR] it works for existing installations, but newcomers are greeted by lots of scary warnings and
required to `opam update`.
* The base idea was to redirect OPAM 1.1 clients to a repo at https://opam.ocaml.org/1.1/ , which is automatically rewritten from the main (1.2) repo still hosted at https://opam.ocaml.org/ . This parts works quite well.
* Sadly, a bug in OPAM 1.1 prevents redirection on `opam init`. So OPAM 1.1 will try to load the default repository, in 1.2 format, until you run `opam update` the first time.
* The current consequence of this is frightening screens full of errors at `opam init` with OPAM 1.1, but mostly harmless. This is documented in the FAQ , but is likely to turn away new 1.1 users, e.g. on Ubuntu.
* Another OPAM 1.1 bug causes a _fatal_ parsing error on the `!=` comparator in package version constraints. The consequence is that, were we to start using it in the (1.2) repo, OPAM 1.1 will fail to `init` before getting the chance to be redirected, unless you explicitely direct it to the new repo with `opam init https://opam.ocaml.org/1.1`, as documented in the FAQ.
* A completely different issue, the OPAM package in Ubuntu "utopic" has a broken solver interface , it'll seem to work at first, until you try to upgrade.
## Where we want to go, and consequences
[TL;DR] some 1.2 enhancements to the repo are blocked unless we accept to turn the scary warnings
into a fatal error. Specific init command (workaround) and existing setups would be
* The `!=` limitation is annoying because it forces ugly workarounds in the current, main repository (detailed reasons in )
* It doesn't look like much but it actually blocks widespread usage of the new OPAM 1.2 features (new fields like `dev-repo`, etc.) that go through PR #3143  for some sanitization prerequisites. In short, I could change #3143 not to use `!=` but that would become an issue on the longer term.
* If we accept `!=` in the repo, OPAM 1.1 will _fail_ to initialize rather than output lots of errors that get fixed by `opam update`. Existing installations, or `opam init https://opam.ocaml.org/1.1` will still work without problem.
* At some point we'll want to freeze 1.1 support anyway ; but stopping updates to opam.ocaml.org/1.1 won't solve this specific issue
[TL;DR] some other solutions possible, but mostly: how important is OPAM 1.1 support at the moment ?
* An unpleasant, but reliable workaround could be to redirect the main repository to opam.ocaml.org/1.2 and serve a compatible repository at opam.ocaml.org/ . This may get misleading for users, and on the website.
* On a plus note, the "scary warnings" do mention upgrading OPAM.
* This raises the main question: how important is OPAM 1.1 support, how many users will encounter these issues, and when can we safely drop it ? I haven't heard many complaints, but the users hit by this aren't the most likely to give feedback (but maybe to turn around).
Sorry for this sad state of affairs, remarks welcome !
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