Keep stale virtual member copy when the remote lookup fails (#171)

A transient remote lookup error dropped the serve_stale copy of a virtual member and forced a blocking fetch, so a short DB blip stalled every merge.

- serve the existing copy on a lookup error and refresh it in the background
- fetch directly without storing anything on a lookup error with no copy
- create copies only after a successful lookup with serve_stale on

Reviewed-on: #171
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/virtual/engine.go

@@ -548,8 +548,8 @@ func (e *Engine) fetchAlpineMember(ctx context.Context, name, dir string) (*Alpi
 // the background, one refresh per member at a time; the next merge picks it up.
 func staleMember[T any](ctx context.Context, e *Engine, key, name string, fetch func(context.Context) (*T, error)) (*T, error) {
 	sk := key + "\x00" + name
-	remote, err := e.getRemote(ctx, name)
-	if err != nil || !remote.ServeStale {
+	remote, lookupErr := e.getRemote(ctx, name)
+	if lookupErr == nil && !remote.ServeStale {
 		e.mu.Lock()
 		delete(e.stale, sk)
 		e.mu.Unlock()
@@ -560,6 +560,11 @@ func staleMember[T any](ctx context.Context, e *Engine, key, name string, fetch
 		e.stale = map[string]*staleEntry{}
 	}
 	prev := e.stale[sk]
+	// A failed lookup serves an existing copy but never creates one.
+	if lookupErr != nil && (prev == nil || prev.m == nil) {
+		e.mu.Unlock()
+		return fetch(ctx)
+	}
 	// A cold fetch holds a placeholder so eviction or toggle-off during it is seen.
 	token := prev
 	if prev == nil || prev.m == nil {