This patch adds re-mapping functions for plugs and slots. The idea with
re-mapping is that snapd can provide the appearance of a particular plug
and slot placement as observed in the on-disk state and over-the-wire
communication while having a different actual placement in memory. In
practice this is designed to allow us to place implicit slots on the
snapd snap.

Re-mapping is implemented by a golang interface. Currently only a no-op
mapper is used. The mapper is global but could be moved to the interface
manager after changing a large number of functions to become methods on
the interface manager. This can be done in a follow-up, if desired.

Apart from the golang interface, which provides the low-level
operations, a number of helpers for other data types are provided. This
includes connection references and plug and slot information objects.
Those helpers are used by the daemon API layer in one of the subsequent
patches.

Two real mappers are implemented: a no-op mapper and a core<=>snapd
mapper. For testing a upper<=>lower case mapper is used, for extra
clarity. Everything is accompanied by unit tests.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

The helpers for re-mapping the aforementioned types are no longer
necessary due to re-factoring and cleanup of the daemon code.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

This patch changes getConns and setConns to re-map connection state just
after loading from the state and just before being saved to the state.
This is coupled with explicit tests for {get,set}Conns that we apparently
didn't have.

Note that at this stage the code is still dormant since re-mapping is
a no-op until the "snapd" snap becomes the host of the implicit slots.

This is the "back" side of the re-mapping. The "front" side applies to
re-mapping of the API layer. It is provided in the follow-up patch.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

This patch changes interface connection, disconnection and query APIs to
apply re-mapping on incoming requests and outgoing responses. This
allows code to issue explicit connect and disconnect API requests for
the "core" snap even though implicit slots are on the "snapd" snap.
Similarly the API for querying interface connections (both the legacy,
as used by "snap interfaces" and the per-interface as used by "snap
interface") are updated to take advantage of this feature.

Note that at this stage the code is still dormant since re-mapping is a
no-op until the "snapd" snap becomes the host of the implicit slots.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

We don't need a test that doesn't work. I added tests for this in the
previous patch anyway.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

The re-mapping helper functions were returning a boolean if a mapping
was actually applied (something changed) but this value is actually no
longer used anywhere. In the spirit of YAGNI let's remove it.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

The remaping functions were modifying their arguments. This patch makes
them return a modified copy instead.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

This patch changes a number of things, the main concept is that
the mapping system can now offer separate replacements for state
handling, where we want to stick to the word "core" for compatibility,
and the API layer where we want to start using the "system" word
as a nickname to whatever is providing implicit slots.

As such the mapping functions are duplicated, once for state and once
for request/response. The intermediate helpers for working with
connection references are dropped because this just multiplies trivial
functions that don't need to be used.

The existing NilMapper is expanded to cover the new API. The existing
CoreSnapdMapper is split into two new mappers: CoreSnapdSystemMapper
which uses core in the state, snapd in memory and system in the API layer
and a new CoreCoreSystemMapper which uses core in the state and in memory
but system in the API layer.

That last new mapper is a candidate for transition of spread tests
that could start to talk about "system" natively (without the hacks in
the client) and would then work without change once the core-snapd-system
mapper can be used, which will happen once snapd starts to host implicit
slots.

The daemon and state are obviously updated to switch to the appopriate
mapping helper.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

While testing the interface mapper I realized the nickname system is
still in use and actually clashes with the interface mapper. Looking at
it close it is obvious that the nickname system must be folded into the
interface mapper for consistency and that the interface mapper must be
able to map snap names more than than plug or slot references.

Without this change we can never have implicit slots on the "snapd" snap
because implicit gadget connections would resolve to "core" (which can
be absent) and because the nickname handling is would clash with the
idea of using "system" in the API layer but "snapd" in the memory
(because nicknames would effectively change "system" to "core".

To fix this I extended the interface mapper to also be a snap name
mapper and removed the nickname system entirely. This also allows us to
drop the ever-growing list of special cases in the client (for interface
abbreviation logic).

More precisely this patch:
 - drops the {Use,Drop}Nick functions from the snap package
 - changes the default mapper to CoreCoreSystem mapper
 - adds a SnapMapper that has request/response state save/load functions
 - makes InterfaceMapper a superset of SnapMapper
 - refactors CoreCoreSystemMapper and CoreSnapdSystemMapper
 - makes the daemon use RemapSnapFromRequest instead of the nick
 - makes gadget resolver use RemapSnapFromRequest to resolve "system"
 - makes the client abbreviate just "system" slots

This comes without a full array of tests but without breaking any
existing test (woot). It also passes spread which is promising.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

With the insight from the previous patch I have now simplified all of
the interface mapper to just a SnapMapper wich can be thought as
server-side nickname system.

This mostly cuts the amount of boilerplate code that doesn't do anything
we currently need by limiting the re-mapping to just snap names and to
not be able to differentiate plugs and slots anymore.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

This patch just restores original code where it doesn't make any
meaningful changes.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

…slot-remapping

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>