opam var XXX fails when no switch is selected

[Khady]

When no switch is selected, for example because I don't have a global switch and I'm not in a folder with a local switch, the command opam var and all the sub commands are returning an error code 50.

~$ opam init --bare
~$ cd ~/project-with-local-switch
~/project-with-local-switch$ opam sw create . 4.12.1 --no-install
~$ cd
~$ opam var; echo $?

<><> Global opam variables ><><><><><><><><><><><><><><><><><><><><><><><><><><>
arch              x86_64                  # Inferred from system
exe                                       # Suffix needed for executable filenames (Windows)
jobs              255                     # The number of parallel jobs set up in opam configuration
make              make                    # The 'make' command to use
opam-version      2.1.2                   # The currently running opam version
os                linux                   # Inferred from system
os-distribution   debian                  # Inferred from system
os-family         debian                  # Inferred from system
os-version        10                      # Inferred from system
root              /home/louis.roche/.opam # The current opam root directory
switch                                    # The identifier of the current switch
sys-ocaml-arch                            # Target architecture of the OCaml compiler present on your system
sys-ocaml-cc                              # Host C Compiler type of the OCaml compiler present on your system
sys-ocaml-libc                            # Host C Runtime Library type of the OCaml compiler present on your system
sys-ocaml-version                         # OCaml version present on your system independently of opam, if any

<><> Configuration variables from the current switch ><><><><><><><><><><><><><>
[ERROR] No switch is currently set. Please use 'opam switch' to set or install a switch
50
~$ opam var root; echo $?
[ERROR] No switch is currently set. Please use 'opam switch' to set or install a switch
50
~$ cd ~/project-with-local-switch
~/project-with-local-switch$ opam var root; echo $?
/home/louis.roche/.opam
0

That's with opam version 2.1.2. With 2.0.8 the behavior was different from what I could reproduce and opam config var root wouldn't return an error.

ocamllabs/vscode-ocaml-platform#701

[kit-ty-kate]

As a workaround opam var --global root works without a switch

[rjbou]

Digging & fixing that, i found another bug : setting an option on a non-existent switch don't fail and write the switch config file! It leads to opam internals to be inconsistent 😨