fix(http): avoid typed-nil pool panic on Serve failure; clarify attr error

NewPool returns a concrete *static_pool.Pool, so assigning its result
directly to the api.Pool interface field on the error path boxed a typed
nil into p.pool. The simplified Stop guard (`if p.pool != nil`) then sees
a non-nil interface and calls Destroy on the nil receiver, which panics
(Destroy dereferences sp.log immediately). Assign to p.pool only when the
returned pointer is non-nil so the interface never holds a typed nil and
the existing guard stays correct.

Also distinguish the wrong-type case in attributes.Set from the
missing-key case with its own error message for easier diagnosis.

Author: rustatian

attributes/attributes.go

@@ -89,7 +89,7 @@ func Set(r *http.Request, key string, value string) error {
 
 	a, ok := v.(attrs)
 	if !ok {
-		return errors.New("unable to find `psr:attributes` context key")
+		return errors.New("unexpected type stored under `psr:attributes` context key")
 	}
 	a.set(key, value)
 	return nil

plugin.go

@@ -127,12 +127,18 @@ func (p *Plugin) Serve() chan error {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 
-	var err error
-	p.pool, err = p.server.NewPool(context.Background(), p.cfg.Pool, map[string]string{RrMode: RrModeHTTP}, p.log)
+	// NewPool returns a concrete *static_pool.Pool; assign it to the api.Pool
+	// interface field only when non-nil so p.pool never holds a typed nil
+	// (which would make the p.pool != nil guard in Stop spuriously true and
+	// panic inside Destroy on a nil receiver).
+	np, err := p.server.NewPool(context.Background(), p.cfg.Pool, map[string]string{RrMode: RrModeHTTP}, p.log)
 	if err != nil {
 		errCh <- err
 		return errCh
 	}
+	if np != nil {
+		p.pool = np
+	}
 
 	// request queue + worker-facing ConnectRPC server
 	p.queue = proxy.NewQueue(p.cfg.Proxy.InboxSize)