`opam switch create` should check whether cached sources are up to date

[wang-d]

opam switch create catenable ocaml-base-compiler.4.07.1 

failed with output (trimmed) like this:

<><> Gathering sources ><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
[ocaml-base-compiler.4.07.1] found in cache
...
[ERROR] The compilation of ocaml-base-compiler failed at
        "/home/???/.opam/opam-init/hooks/sandbox.sh build make world".
...
# /usr/bin/ld: backtrace.pic.o:/home/???/.opam/catenable/.opam-switch/build/ocaml-base-compile
r.4.07.1/byterun/backtrace.c:31: multiple definition of `caml_debug_info'; backtrace_prim.pic.o:/hom
e/???/.opam/catenable/.opam-switch/build/ocaml-base-compiler.4.07.1/byterun/backtrace_prim.c:4
7: first defined here                                                                              
# collect2: error: ld returned 1 exit status
# make[1]: *** [Makefile:208: libcamlrun_shared.so] Error 1
# make[1]: Leaving directory '/home/???/.opam/catenable/.opam-switch/build/ocaml-base-compiler
.4.07.1/byterun'                                                                                   
# make: *** [Makefile:389: coldstart] Error 2

I was confused because it looked exactly like this bug, which had been fixed months ago.

The problem went away after I did this:

opam update -a
opam upgrade

I'm guessing that opam switch create used the sources it found in my cache without checking whether they were up to date with the repo. It only picked up the patch after I instructed the update and upgrade. Does that sound plausible?

If so, would it be possible to make opam check whether its cached sources are up to date before using them?

# opam config report
# opam-version      2.0.0 
# self-upgrade      no
# system            arch=x86_64 os=linux os-distribution=debian os-version=10.0
[ERROR] No switch is currently set. Please use 'opam switch' to set or install a switch
# read-state        OpamStd.OpamSys.Exit(50)

As an aside (I know this is not your fault), I encountered this error while following this quick start guide for my first dune project. It was pretty discouraging to start my first dune project with two hours investigating a link error in the compiler, followed by an hour setting up automatic formatting, followed by an hour investigating and then working around a type error in code generated by a ppx.

If you're wondering why I started my first project with old compiler sources in my opam cache, I think they were left over from the last time I tried to set up OCaml, which resulted in my filing issues with the ocaml and opam repos. :-(

Luckily, I have had some positive experiences with OCaml before, so I am not so easily discouraged. :-)

[AltGr]

Indeed that seems like a lot of bumps you encountered on the road... Thanks for taking the time to report them so we can improve!

We do have a warning for such cases (code here), but indeed it seems like it's only printed on opam install, and not on switch create or upgrade. This should be fixed!

Various efforts to make getting started on a project easier are underway, for example you may want to check out drom. Be aware it's still in its alpha stage though.

[wang-d]

Thanks for the quick response! drom looks like it will solve many of my getting-started problems.

[rjbou]

It is already called for opam install and opam upgrade. I'll add it for opam switch create.