Using in-development features in snapcraft.yaml

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If you need to use a new feature of snapd as soon as it is available, but before it is supported by the validation mechanisms in Snapcraft, you have the option of using the passthrough feature of snapcraft.yaml to specify the in-development feature you need.

The passthrough keyword

Snapcraft can accept the passthrough keyword in two places in the snapcraft.yaml file. It can be used at the top-level scope to access new top-level features, and also at the application scope to access new application-level features. Where either or these locations are used, they are referred to as the target scope.

The passthrough keyword defines a map whose keywords and values are copied directly into the corresponding scope in the snap.yaml description that snapd uses.

Although Snapcraft does not know about in-development features, it performs some checks to prevent accidental conflicts or unexpected behaviour as a result of using passthroughs.

  • It forbids the definition of an option inside a passthrough if that option is also directly present in the target scope.
  • It warns the developer about the options used in each passthrough, to remind the publisher that these options are not yet supported.
  • It may also report options that might already be supported, but it should not prevent these options from being passed through to snap.yaml since the option may be defined in a way that is not yet understood by Snapcraft.

Files using in-development features will need to be updated when those features become supported by Snapcraft.

Examples

In the following example, the passthrough keyword is used at the top-level scope to access two new, hypothetical, top-level features:

   name: foo
   passthrough:
       confinement: next-generation
       planet: saturn

This example enables the usage of a new value for the existing snapcraft-yaml-confinement keyword. It also enables an entirely new planet keyword to be used, along with a new value.

The following example shows the use of the keyword to access a new application-level feature:

   apps:
       foo:
           command: foo
           passthrough:
               daemon: complex

In this case, a passthrough is used within the scope of the app foo to set the existing daemon keyword to a value of complex.

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I would like to see a reference to the currently existing passthrough keys and accepting values, which seems to be scattered throughout the forum: