MySQL 隔离与 MVCC：事务读到的是哪一版数据

import java.util.*;
public class Main {
    record Version(long transaction,String value,boolean deleted) {}
    record View(long reader,long next,Set<Long> active) {
        View {
            if (reader<=0||next<=0) throw new IllegalArgumentException("view");
            active=Set.copyOf(active);
            for (long id:active)
                if (id<=0||id>=next) throw new IllegalArgumentException("active id");
        }
        boolean visible(long transaction) {
            if (transaction<=0) throw new IllegalArgumentException("transaction");
            // 自身修改单独可见，不被普通快照历史限制掩盖。
            if (transaction==reader) return true;
            // 未来事务与快照时仍活跃的事务均不可见。
            return transaction<next&&!active.contains(transaction);
        }
    }
    static Optional<String> read(List<Version> chain,View view) {
        for (Version version:chain) {
            if (!view.visible(version.transaction())) continue;
            // 首个可见删除即不存在，不继续寻找旧值。
            return version.deleted()?Optional.empty():Optional.of(version.value());
        }
        return Optional.empty();
    }
    static Version value(long tx,String text) { return new Version(tx,text,false); }
    static void eq(Object expected,Object actual) {
        if (!Objects.equals(expected,actual)) throw new AssertionError(actual);
    }
    public static void main(String[] args) {
        View rr=new View(50,60,Set.of(20L,55L));
        eq(Optional.of("old"),read(List.of(value(55,"new"),value(40,"old")),rr));
        eq(Optional.of("old"),read(List.of(value(65,"future"),value(40,"old")),rr));
        eq(Optional.of("mine"),read(List.of(value(50,"mine"),value(40,"old")),rr));
        eq(Optional.empty(),read(List.of(new Version(40,null,true),value(10,"past")),rr));
        eq(Optional.of("early"),read(List.of(value(20,"active"),value(10,"early")),rr));
        View fresh=new View(50,70,Set.of());
        eq(Optional.of("future"),read(List.of(value(65,"future"),value(40,"old")),fresh));
        eq(Optional.empty(),read(List.of(),rr));
        System.out.println("PASS 7 MVCC visibility tests");
    }
}