MySQL 锁与死锁：为什么等待，失败后怎样重试

import java.util.*;
public class Main {
    static class WaitGraph {
        final Map<String,Set<String>> edges=new HashMap<>();
        List<String> add(String waiter,String holder) {
            Objects.requireNonNull(waiter); Objects.requireNonNull(holder);
            Map<String,String> parent=new HashMap<>();
            Deque<String> queue=new ArrayDeque<>();
            parent.put(holder,null); queue.add(holder);
            while (!queue.isEmpty()&&!parent.containsKey(waiter)) {
                String node=queue.remove();
                for (String next:edges.getOrDefault(node,Set.of())) {
                    if (parent.containsKey(next)) continue;
                    // 首次访问前驱构成最短路径树。
                    parent.put(next,node); queue.add(next);
                }
            }
            List<String> cycle=new ArrayList<>();
            if (parent.containsKey(waiter)) {
                List<String> path=new ArrayList<>();
                for (String n=waiter;n!=null;n=parent.get(n)) path.add(n);
                Collections.reverse(path);
                cycle.add(waiter); cycle.addAll(path); // 新增边接上已有路径。
            }
            edges.computeIfAbsent(waiter,k->new LinkedHashSet<>()).add(holder);
            edges.computeIfAbsent(holder,k->new LinkedHashSet<>());
            return List.copyOf(cycle);
        }
        void finish(String transaction) {
            edges.remove(transaction);
            // 释放后还要清除别人指向它的等待关系。
            for (Set<String> holders:edges.values()) holders.remove(transaction);
        }
    }
    static void eq(Object expected,Object actual) {
        if (!Objects.equals(expected,actual)) throw new AssertionError(actual);
    }
    public static void main(String[] args) {
        WaitGraph g=new WaitGraph();
        eq(List.of(),g.add("A","B"));
        eq(List.of(),g.add("B","C"));
        eq(List.of("C","A","B","C"),g.add("C","A"));
        g.finish("B");
        eq(false,g.edges.get("A").contains("B"));
        eq(List.of(),g.add("A","D"));
        eq(List.of("D","C","A","D"),g.add("D","C"));
        WaitGraph self=new WaitGraph();
        eq(List.of("X","X"),self.add("X","X"));
        WaitGraph independent=new WaitGraph();
        independent.add("P","Q"); eq(List.of(),independent.add("R","S"));
        System.out.println("PASS 8 wait graph tests");
    }
}