bug#60106: [Shepherd 0.9.3] ‘system*’ replacement cannot be passed environment variables

[Ludovic Courtès]

As we found out while debugging <https://issues.guix.gnu.org/60010>, the
Shepherd 0.9.3, with its ‘system*’ replacement (aka. ‘spawn-command’),
makes it very hard to spawn a command with different environment
variables.

The following options don’t work:

  • Changing shepherd’s own environment variables with ‘setenv’ for
    instance: ‘spawn-command’ calls ‘fork+exec-command’, whose default
    #:environment-variables is provided by the
    ‘default-environment-variables’ parameter, which gets its default
    value at when shepherd starts.  There’s no environment variable
    inheritance, contrary to the real ‘system*’.

  • Parameterizing ‘default-environment-variables’:

       (parameterize ((default-environment-variables …))
         (system* …))

    That won’t work because ‘spawn-command’ delegates to the process
    monitoring fiber, which has a different dynamic state and thus
    doesn’t see this change.

  • Even a plain (set! default-environment-variables …) won’t work,
    probably due to inlining within (shepherd services).

I think we’ll have to add a parameter to ‘spawn-command’ to specify
environment variables.

Ludo’.