Clearer message on error extracting sources

[dra27]

Noting https://github.com/ocaml/opam-repository/issues/23954#issuecomment-1598512594:

[ocamlbuild.0.14.2+win] downloaded from https://github.com/ocaml/ocamlbuild/archive/refs/tags/0.14.2.tar.gz
...
[ERROR] The sources of the following couldn't be obtained, aborting:
          - ocamlbuild.0.14.2+win

The underlying problem here is that tar was failing owing to a (now-reported) Cygwin packaging error. This can also happen even more confusingly:

[ocamlbuild.0.14.2+win] found in cache
...
[ERROR] The sources of the following couldn't be obtained, aborting:
          - ocamlbuild.0.14.2+win

We should update the error message to be clearer that the sources were obtained, but could not be extracted for some reason.

[dra27]

Naturally, this is also more weight for the already-planned switch to using ocaml-tar!

[Simn]

I came across this today and also got distracted by the error message, although it now at least prints some more information:

#=== ERROR while fetching sources for ocamlbuild.0.14.2 =======================#
OpamSolution.Fetch_fail("C:\\git\\opam\\opam.exe: \"lstat\" failed on C:\\Users\\simn\\AppData\\Local\\Temp\\opam-9612-d84f5a\\ocamlbuild-0.14.2\\examples\\07-dependent-projects\\libdemo: No such file or directory")

It took me forever to realize that this was related to symlinks after having many "but the file is clearly there..." moments, so I concur that a clearer error message would be good!

(Also, how do I actually resolve this?)

[kit-ty-kate]

@Simn this is surprising, ocamlbuild.0.14.2 should not be available on Windows. Only ocamlbuild.0.14.2+win is. Could you show the output of opam config report, opam repo and opam pin ?

[Simn]

I broke my setup after encountering this, but I think I accidentally built a mscv version when I meant to use MinGW. At least I'm assuming that that's what "on Windows" would actually mean. I was also surprised to find my opam root in the Windows Users directory, as opposed to Cygwin's ~/.opam like before. I'll try to reproduce the mess I made and report back!

[Simn]

Retracing my steps, something goes wrong during opam init already:

$ ./opam init git+https://github.com/ocaml-opam/opam-repository-mingw
No configuration file found, using built-in defaults.
[NOTE] Configured with Cygwin at C:\cygwin64 for depexts
Checking for available remotes: rsync and local, git.
  - you won't be able to use mercurial repositories unless you install the hg command on your system.
  - you won't be able to use darcs repositories unless you install the darcs command on your system.


<><> Fetching repository information ><><><><><><><><><><><><><><><><><><><><><>
[default] Initialised

<><> Required setup - please read <><><><><><><><><><><><><><><><><><><><><><><>

  In normal operation, opam only alters files within C:\Users\simn\AppData\Local\opam.

  However, to best integrate with your system, some environment variables
  should be set. If you allow it to, this initialisation step will update
  your bash configuration by adding the following line to ~\.bash_profile:

    test -r C:\Users\simn\AppData\Local\opam\opam-init\init.sh && . C:\Users\simn\AppData\Local\opam\opam-init\init.sh > /dev/null 2> /dev/null || true

  Otherwise, every time you want to access your opam installation, you will
  need to run:

    eval $(opam env)

  You can always re-run this setup with 'opam init' later.

Do you want opam to configure bash?
> 1. Yes, update ~\.bash_profile
  2. Yes, but don't setup any hooks. You'll have to run eval $(opam env) whenever you change your current 'opam switch'
  3. Select a different shell
  4. Specify another config file to update instead
  5. No, I'll remember to run eval $(opam env) when I need opam

[1/2/3/4/5] 1

User configuration:
  ~\.bash_profile is already up-to-date.
[NOTE] Make sure that ~\.bash_profile is well sourced in your ~/.bashrc.


<><> Creating initial switch 'default' (invariant ["ocaml" {>= "4.05.0"}]) ><><>

<><> Installing new switch packages <><><><><><><><><><><><><><><><><><><><><><>
Switch invariant: ["ocaml" {>= "4.05.0"}]

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
✶ installed base-bigarray.base
✶ installed base-threads.base
✶ installed base-unix.base
▼ retrieved ocaml-variants.4.14.1+msvc64c  (https://github.com/ocaml/ocaml/archive/refs/tags/4.14.1.tar.gz)
[ERROR] The compilation of ocaml-variants.4.14.1+msvc64c failed at "./configure
        --prefix=C:\\Users\\simn\\AppData\\Local\\opam\\default --with-flexdll --host=x86_64-pc-windows".

#=== ERROR while compiling ocaml-variants.4.14.1+msvc64c ======================#
# context     2.2.0~alpha4~dev | win32/x86_64 |  | git+https://github.com/ocaml-opam/opam-repository-mingw
# path        C:\Users\simn\AppData\Local\opam\default\.opam-switch\build\ocaml-variants.4.14.1+msvc64c
# command     C:\Users\simn\AppData\Local\opam\default\.opam-switch\build\ocaml-variants.4.14.1+msvc64c\./configure --prefix=C:\Users\simn\AppData\Local\opam\default --with-flexdll --host=x86_64-pc-windows
# exit-code   77
# env-file    C:\cygwin64\tmp\opam-simn-23812\ocaml-variants-23812-c79406.env
# output-file C:\cygwin64\tmp\opam-simn-23812\ocaml-variants-23812-c79406.out
### output ###
# [...]
# checking host system type... x86_64-pc-windows
# checking target system type... x86_64-pc-windows
# checking for x86_64-pc-windows-ld... no
# checking for x86_64-pc-windows-link... no
# checking for ld... no
# checking for link... link
# checking how to print strings... printf
# checking for x86_64-pc-windows-gcc... cl
# checking whether the C compiler works... no
# configure: error: in `/cygdrive/c/Users/simn/AppData/Local/opam/default/.opam-switch/build/ocaml-variants.4.14.1+msvc64c':
# configure: error: C compiler cannot create executables
# See `config.log' for more details



<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>
┌─ The following actions failed
│ λ build ocaml-variants 4.14.1+msvc64c
└─
┌─ The following changes have been performed (the rest was aborted)
│ ✶ install base-bigarray base
│ ✶ install base-threads  base
│ ✶ install base-unix     base
└─
# Run eval $(opam env --switch=default) to update the current shell environment
Switch initialisation failed: clean up? ('n' will leave the switch partially installed) [y/n] y
[NOTE] Opam has been initialised, but the initial switch creation failed.
       Use 'opam switch create <compiler>' to get started.

simn@SIMNPHONY20 /cygdrive/c/git/opam
$ opam config report
bash: opam: command not found

simn@SIMNPHONY20 /cygdrive/c/git/opam
$ .opam config report
bash: .opam: command not found

simn@SIMNPHONY20 /cygdrive/c/git/opam
$ ./opam config report
# opam config report
# opam-version         2.2.0~alpha4~dev (265fbcbfa68aef11c4ea4e905f614eac34688c13)
# self-upgrade         no
# system               arch=x86_64 os=win32 os-distribution=cygwin os-version=10.0.19041
# solver               builtin-mccs+glpk
# install-criteria     -removed,-count[avoid-version,changed],-count[version-lag,request],-count[version-lag,changed],-count[missing-depexts,changed],-changed
# upgrade-criteria     -removed,-count[avoid-version,changed],-count[version-lag,solution],-count[missing-depexts,changed],-new
# jobs                 15
# current-switch       none set

I don't know why it's going for 4.14.1+msvc64c automatically, I expected it to use mingw here, even more so because I'm (now) running this from within the cygwin shell.

Edit: An explicit opam switch create 4.14.1+mingw64c works fine, but the eval $(opam env --switch=4.14.1+mingw64c) then fails with bash: OPAM_LAST_ENV='C:\Users\simn\AppData\Local\opam\4.14.1+mingw64c\.opam-switch\last-env\env-f4346fb8d6031e4d9fc5f882b2d67e34-0';: command not found

And a subsequent opam install ocamlbuild then gives the error which originally brought me here.