settings: use delegation instead of inheritance for NMSettingsConnection and NMConnection
NMConnection is an interface, which is implemented by the types NMSimpleConnection (libnm-core), NMSettingsConnection (src) and NMRemoteConnection (libnm). NMSettingsConnection does a lot of things already: 1) it "is-a" NMDBusObject and exports the API of a connection profile on D-Bus 2) it interacts with NMSettings and contains functionality for tracking the profiles. 3) it is the base-class of types like NMSKeyfileConnection and NMIfcfgConnection. These handle how the profile is persisted on disk. 4) it implements NMConnection interface, to itself track the settings of the profile. 3) and 4) would be better implemented via delegation than inheritance. Address 4) and don't let NMSettingsConnection implemente the NMConnection interface. Instead, a settings-connection references now a NMSimpleConnection instance, to which it delegates for keeping the actual profiles. Advantages: - by delegating, there is a clearer separation of what NMSettingsConnection does. For example, in C we often required casts from NMSettingsConnection to NMConnection. NMConnection is a very trivial object with very little logic. When we have a NMConnection instance at hand, it's good to know that it is *only* that simple instead of also being an entire NMSettingsConnection instance. The main purpose of this patch is to simplify the code by separating the NMConnection from the NMSettingsConnection. We should generally be aware whether we handle a NMSettingsConnection or a trivial NMConnection instance. Now, because NMSettingsConnection no longer "is-a" NMConnection, this distinction is apparent. - NMConnection is implemented as an interface and we create NMSimpleConnection instances whenever we need a real instance. In GLib, interfaces have a performance overhead, that we needlessly pay all the time. With this change, we no longer require NMConnection to be an interface. Thus, in the future we could compile a version of libnm-core for the daemon, where NMConnection is not an interface but a GObject implementation akin to NMSimpleConnection. - In the previous implementation, we cannot treat NMConnection immutable and copy-on-write. For example, when NMDevice needs a snapshot of the activated profile as applied-connection, all it can do is clone the entire NMSettingsConnection as a NMSimpleConnection. Likewise, when we get a NMConnection instance and want to keep a reference to it, we cannot do that, because we never know who also references and modifies the instance. By separating NMSettingsConnection we could in the future have NMConnection immutable and copy-on-write, to avoid all unnecessary clones.
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