Rework the `Store` hierarchy so that there's now one hierarchy for the
store configs and one for the implementations (where each implementation
extends the corresponding config). So a class hierarchy like
```
StoreConfig-------->Store
| |
v v
SubStoreConfig----->SubStore
| |
v v
SubSubStoreConfig-->SubSubStore
```
(with virtual inheritance to prevent DDD).
The advantage of this architecture is that we can now introspect the configuration of a store without having to instantiate the store itself
Add a new `init()` method to the `Store` class that is supposed to
handle all the effectful initialisation needed to set-up the store.
The constructor should remain side-effect free and just initialize the
c++ data structure.
The goal behind that is that we can create “dummy” instances of each
store to query static properties about it (the parameters it accepts for
example)
Directly register the store classes rather than a function to build an
instance of them.
This gives the possibility to introspect static members of the class or
choose different ways of instantiating them.
Add a fallback path in `queryPartialDerivationOutputMap` for daemons
that don't support it.
Also upstreams a couple methods from `SSHStore` to `RemoteStore` as this
is needed to handle the fallback path.
When deploying a Hydra instance with current Nix master, most builds
would not run because of errors like this:
queue monitor: error: --- Error --- hydra-queue-runner
error: --- UsageError --- nix-daemon
not a content address because it is not in the form '<prefix>:<rest>': /nix/store/...-somedrv
The last error message is from parseContentAddress, which expects a
colon-separated string, however what we got here is a store path.
Looking at the worker protocol, the following message sent to the Nix
daemon caused the error above:
0x1E -> wopQuerySubstitutablePathInfos
0x01 -> Number of paths
0x16 -> Length of string
"/nix/store/...-somedrv"
0x00 -> Length of string
""
Looking at writeStorePathCAMap, the store path is indeed the first field
that's transmitted. However, readStorePathCAMap expects it to be the
*second* field *on my machine*, since expression evaluation order is a
classic form of unspecified behaviour[1] in C++.
This has been introduced in https://github.com/NixOS/nix/pull/3689,
specifically in commit 66a62b3189.
[1]: https://en.wikipedia.org/wiki/Unspecified_behavior#Order_of_evaluation_of_subexpressions
Signed-off-by: aszlig <aszlig@nix.build>
This means profiles outside of /nix/var/nix/profiles don't get
garbage-collected. It also means we don't need to scan
/nix/var/nix/profiles for GC roots anymore, except for compatibility
with previously existing generations.
