refstore/chain: Add chained refstore implementation

1 files changed, 130 insertions, 0 deletions

diff --git a/refstore/chain/chain.go b/refstore/chain/chain.go
new file mode 100644
index 00000000..bebe183f
--- /dev/null
+++ b/refstore/chain/chain.go
@@ -0,0 +1,130 @@
+// Package chain provides an ordered reference store chain implementation.
+package chain
+
+import (
+	"errors"
+	"fmt"
+
+	"codeberg.org/lindenii/furgit/ref"
+	"codeberg.org/lindenii/furgit/refstore"
+)
+
+// Chain queries multiple reference stores in order.
+type Chain struct {
+	backends []refstore.Store
+}
+
+// New creates an ordered reference store chain.
+func New(backends ...refstore.Store) *Chain {
+	return &Chain{
+		backends: append([]refstore.Store(nil), backends...),
+	}
+}
+
+// Resolve resolves a reference from the first backend that has it.
+func (chain *Chain) Resolve(name string) (ref.Ref, error) {
+	for i, backend := range chain.backends {
+		if backend == nil {
+			continue
+		}
+		resolved, err := backend.Resolve(name)
+		if err == nil {
+			return resolved, nil
+		}
+		if errors.Is(err, refstore.ErrReferenceNotFound) {
+			continue
+		}
+		return nil, fmt.Errorf("refstore: backend %d resolve: %w", i, err)
+	}
+	return nil, refstore.ErrReferenceNotFound
+}
+
+// ResolveFully resolves symbolic references through Resolve until detached.
+//
+// It intentionally does not call backend ResolveFully. This allows symbolic
+// references to cross backends in the chain.
+func (chain *Chain) ResolveFully(name string) (ref.Detached, error) {
+	cur := name
+	seen := map[string]struct{}{}
+	for {
+		if _, ok := seen[cur]; ok {
+			return ref.Detached{}, fmt.Errorf("refstore: symbolic reference cycle at %q", cur)
+		}
+		seen[cur] = struct{}{}
+
+		resolved, err := chain.Resolve(cur)
+		if err != nil {
+			return ref.Detached{}, err
+		}
+
+		switch resolved := resolved.(type) {
+		case ref.Detached:
+			return resolved, nil
+		case *ref.Detached:
+			if resolved == nil {
+				return ref.Detached{}, fmt.Errorf("refstore: backend returned nil detached reference")
+			}
+			return *resolved, nil
+		case ref.Symbolic:
+			if resolved.Target == "" {
+				return ref.Detached{}, fmt.Errorf("refstore: symbolic reference %q has empty target", resolved.Name())
+			}
+			cur = resolved.Target
+		case *ref.Symbolic:
+			if resolved == nil {
+				return ref.Detached{}, fmt.Errorf("refstore: backend returned nil symbolic reference")
+			}
+			if resolved.Target == "" {
+				return ref.Detached{}, fmt.Errorf("refstore: symbolic reference %q has empty target", resolved.Name())
+			}
+			cur = resolved.Target
+		default:
+			return ref.Detached{}, fmt.Errorf("refstore: unsupported reference type %T", resolved)
+		}
+	}
+}
+
+// List lists references from every backend and deduplicates by ref name.
+//
+// First-seen wins, so earlier backends have precedence.
+func (chain *Chain) List(pattern string) ([]ref.Ref, error) {
+	var refs []ref.Ref
+	seen := map[string]struct{}{}
+
+	for i, backend := range chain.backends {
+		if backend == nil {
+			continue
+		}
+		listed, err := backend.List(pattern)
+		if err != nil {
+			return nil, fmt.Errorf("refstore: backend %d list: %w", i, err)
+		}
+		for _, entry := range listed {
+			if entry == nil {
+				continue
+			}
+			name := entry.Name()
+			if _, ok := seen[name]; ok {
+				continue
+			}
+			seen[name] = struct{}{}
+			refs = append(refs, entry)
+		}
+	}
+
+	return refs, nil
+}
+
+// Close closes all backends and joins close errors.
+func (chain *Chain) Close() error {
+	var errs []error
+	for _, backend := range chain.backends {
+		if backend == nil {
+			continue
+		}
+		if err := backend.Close(); err != nil {
+			errs = append(errs, err)
+		}
+	}
+	return errors.Join(errs...)
+}