proposal: remove shared connections

[amenzhinsky]

I'd propose remove shared connections to make the library less confusing choosing between shared and private connection and generally simplify it.

I've found myself misusing shared connections multiple times, such as closing a connection when it's shared with another part of the code base, subscribing to signals makes receive them other subscribers that don't expect to receive them at all, etc.
It's not possible to pass ConnOption to SystemBus and SessionBus because if it's called later by another piece of code with different options it'll return an inconsistent result.

As for me it's user's responsibility to implement connections sharing and in most situations connection is established once in main or a close to main function that passes it down to other clients.

conn, _ := dbus.SystemBus()
bluez, _ := bluez.Dial(bluez.WithConn(conn))
nm, _ := networkmanager.Dial(networkmanager.WithConn(conn))
...

For now we can add DialSystemBus(opts...) and DialSessionBus(opts...) and mark current functions as deprecated.

Also I'd make calling Auth and Hello implicit in Dial function as well, because they are implementation details, and explicit calling them should produce a warning, for low-level we have NewConn.

[jsouthworth]

Adding the additional convenience functions is a good idea. I'm not sure I'd support deprecating the older ones, I'd just say they are just special case and document the differences. Deprecation implies that they will be removed in the future, but I wouldn't want to see that as it would cause considerable updates when switching major versions, which we'd like to avoid. In general I'd like to see us avoiding any breaking changes in the future and make this library's external API additive only, avoiding the need to incrementing the module number again.

[probonopd]

This whole shared vs. private thing also cost me a lot of time, I was never able to use more than one connection (e.g., listen to UDisks2 and send desktop notifications at the same time).

The following works for me, I wonder whether the library could provide a convenience function for it:

	conn, err := dbus.SessionBusPrivate() // When using SessionBusPrivate(), need to follow with Auth(nil) and Hello()
	defer conn.Close()
	if err != nil {
		helpers.PrintError("SessionBusPrivate", err)
		return
	}
	if conn == nil {
		helpers.PrintError("No conn", err)
		return
	}

	if err = conn.Auth(nil); err != nil {
		helpers.PrintError("Auth", err)
		return
	}

	if err = conn.Hello(); err != nil {
		conn.Close()
		helpers.PrintError("Hello", err)
		return
	}

At least all examples could be updated with this.

[jsouthworth]

Feel free to open pull requests if you would like to contribute these enhancements.

[guelfey]

I guess this is mostly closed now with #185. FYI: the shared connections mirror the C API, where they address the case that there are multiple shared libraries that use the bus independently, but should still use the same connection (since multiple connections to the same bus from the same process are possible, but rather wasteful). This also applies to Go bindings in general, but maybe just isn't that relevant unless there actually are multiple libraries in the same process trying to use the same bus. Having easier access to a private connection makes sense, but I'd still keep the shared connections as a concept - maybe we should just make clearer what the implications of using one are.