Add command to check if a path in qubesdb exists

[DemiMarie]

qubesdb-exists will be used in the firewall code to check if a given network device is going to be part of a bridge or not.

[marmarek]

Why not simply qubesdb-read and look at its exit code? Separate tool for this seems to be an overkill.

[DemiMarie]

This approach is less error-prone. On success, one gets either true (exists) or false (missing) and an exit status of 0. On failure, one gets a nonzero exit status.

[marmarek]

qubesdb-exists will be used in the firewall code to check if a given network device is going to be part of a bridge or not.

Can you be a bit more specific? Don't you need to read a value anyway?

[DemiMarie]

qubesdb-exists will be used in the firewall code to check if a given network device is going to be part of a bridge or not.

Can you be a bit more specific? Don't you need to read a value anyway?

I need to check if an interface is managed by @fepitre’s bridge extension. My current plan is to have that extension use a specific qubesdb entry (currently /net-config/$MAC/custom) to indicate that it is managed by something other than the normal logic. The value doesn’t matter.