node: update default branch for canonical refs

After a fetch is performed, `set_head` is called, which essentially updates the
default branch based on the canonical reference rule for that branch.

Later, `set_canonical_refs` is called, which originally could be the empty set
of rules. This would mean that the event for the default branch would never be
emitted.

The approach taken here is to always use the default branch rule if there are no
rules set in the identity document. Unfortunately, this ends up in computing the
default branch twice, due to `set_head`.

Another approach is to use the result of `set_head` to update the set of
`UpdatedCanonicalRefs`, to prevent the need for defaulting to the default branch
rule. However, this does not prevent the double counting as soon as the rules
are added to the identity document, since the default branch rule will then be
synthesised into the rule set.

crates/radicle-node/src/worker/fetch.rs

@@ -1,3 +1,5 @@
+use radicle::identity::doc::CanonicalRefsError;
+use radicle::identity::CanonicalRefs;
 pub(crate) use radicle_protocol::worker::fetch::error;
 
 use std::collections::BTreeSet;
@@ -367,14 +369,16 @@ fn set_canonical_refs(
     applied: &Applied,
 ) -> Result<Option<UpdatedCanonicalRefs>, error::Canonical> {
     let identity = repo.identity()?;
-    let rules = match identity
-        .canonical_refs()?
-        .map(|crefs| crefs.rules().clone())
-        .filter(|rules| !rules.is_empty())
-    {
-        None => return Ok(None),
-        Some(rules) => rules,
-    };
+    // TODO(finto): it's unfortunate that we may end up computing the default
+    // branch again after `set_head` is called after the fetch. This is due to
+    // the storage capabilities being leaked to this part of the code base.
+    let rules = identity
+        .canonical_refs_or_default(|| {
+            let rule = identity.doc().default_branch_rule()?;
+            Ok::<_, CanonicalRefsError>(CanonicalRefs::from_iter([rule]))
+        })?
+        .rules()
+        .clone();
 
     let mut updated_refs = UpdatedCanonicalRefs::default();
     let refnames = applied
@@ -389,6 +393,7 @@ fn set_canonical_refs(
             _ => None,
         })
         .collect::<BTreeSet<_>>();
+
     for name in refnames {
         let canonical = match rules.canonical(name.clone(), repo) {
             Some(canonical) => canonical,