server/tailsql: swap database handles outside the server lock

The server needs to lock access to the list of handles, but the swap into an
existing handle does not depend on server state. Ensure we do the swap outside
the server lock so a slow query (which delays the handle swap) does not block
the UI while it runs.

server/tailsql/tailsql.go

@@ -184,13 +184,17 @@ func (s *Server) SetDB(source string, db *sql.DB, opts *DBOptions) bool {
 		panic("new database is nil")
 	}
 	s.mu.Lock()
-	defer s.mu.Unlock()
 
 	for _, src := range s.dbs {
 		if src.Source() == source {
+			s.mu.Unlock()
+
+			// Perform the swap outside the service lock, since it may wait if a
+			// query is in-flight and we don't need or want to block the rest of
+			// the UI while that's happening.
 			old := src.swap(db, opts)
 			old.Close()
-			return true
+			return false
 		}
 	}
 	s.dbs = append(s.dbs, &dbHandle{
@@ -199,6 +203,7 @@ func (s *Server) SetDB(source string, db *sql.DB, opts *DBOptions) bool {
 		label: opts.label(),
 		named: opts.namedQueries(),
 	})
+	s.mu.Unlock()
 	return false
 }