feat(tvix/castore/directory): add ClosureValidator

This can be used to validate a Directory closure (connected DAG of
Directories), and their insertion order.

Directories need to be inserted (via `add`), in an order from the leaves
to the root. During insertion, we validate as much as we can at that
time:

 - individual validation of Directory messages
 - validation of insertion order (no upload of not-yet-known Directories)
 - validation of size fields of referred Directories

Internally it keeps all received Directories (and their sizes) in a HashMap,
keyed by digest.

Once all Directories have been inserted, a drain() function can be
called to get a (deduplicated and) validated list of directories, in
from-leaves-to-root order (to be stored somewhere).

While assembling that list, a check for graph connectivity is performed
too, to ensure there's no separate components being sent (and only one
root).

It adds a test suite for these cases, which is much nicer to test than
where we previously had these checks (only in the gRPC server wrapper).

Followup CLs will move the existing putters to use this.

Change-Id: Ie88c832924c170a24626e9e3e91d868497b5d7a4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/11220
Tested-by: BuildkiteCI
Reviewed-by: raitobezarius <tvl@lahfa.xyz>
Autosubmit: flokli <flokli@flokli.de>

tvix/castore/src/directoryservice/mod.rs

@@ -2,6 +2,7 @@ use crate::{proto, B3Digest, Error};
 use futures::stream::BoxStream;
 use tonic::async_trait;
 
+mod closure_validator;
 mod from_addr;
 mod grpc;
 mod memory;
@@ -9,6 +10,7 @@ mod sled;
 mod traverse;
 mod utils;
 
+pub use self::closure_validator::ClosureValidator;
 pub use self::from_addr::from_addr;
 pub use self::grpc::GRPCDirectoryService;
 pub use self::memory::MemoryDirectoryService;