Presto自定义函数@SqlNullable血泪史

看到标题我们会想到是由于@SqlNullable注解引发的问题，我们先看一段代码，正是这段有意思的代码，让我纠结了2个多小时，引发了Presto的问题。

@Description("user_id")
@ScalarFunction("user_id")
@SqlType(StandardTypes.VARCHAR)
public static Slice userId(@SqlType(StandardTypes.VARCHAR) Slice value) {
    String _value = value.toStringUtf8();
    if (StringUtils.containsWhitespace(_value)) {
        _value = StringUtils.replace(_value, " ", "+");
    } 
    return Slices.utf8Slice(makeErrorMsgBase64(_value));
}

这段代码很简单，就是我们将传递进来的base64的字符串解码成实际的字符串，单单从代码上看是不会有什么问题的。当我们实际运行这个函数的时候问题就出现了，以下是一个使用示例：

select user_id(str) from temp.users limit 100;

看执行的SQL似乎也没有什么异常，但就是这么简简单单的一个函数引发了Presto的问题，那就是java.lang.NullPointerException: undefined错误，这个错误的具体内容如下

java.lang.NullPointerException: undefined
    at io.prestosql.type.VarcharOperators.equal(VarcharOperators.java:53)
    at io.prestosql.$gen.CursorProcessor_20200927_063218_2398.filter(Unknown Source)
    at io.prestosql.$gen.CursorProcessor_20200927_063218_2398.process(Unknown Source)
    at io.prestosql.operator.ScanFilterAndProjectOperator$RecordCursorToPages.process(ScanFilterAndProjectOperator.java:323)
    at io.prestosql.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
    at io.prestosql.operator.WorkProcessorUtils.getNextState(WorkProcessorUtils.java:221)
    at io.prestosql.operator.WorkProcessorUtils$YieldingProcess.process(WorkProcessorUtils.java:181)
    at io.prestosql.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
    at io.prestosql.operator.WorkProcessorUtils.getNextState(WorkProcessorUtils.java:221)
    at io.prestosql.operator.WorkProcessorUtils.lambda$processStateMonitor$2(WorkProcessorUtils.java:200)
    at io.prestosql.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
    at io.prestosql.operator.WorkProcessorUtils.lambda$flatten$6(WorkProcessorUtils.java:277)
    at io.prestosql.operator.WorkProcessorUtils$3.process(WorkProcessorUtils.java:319)
    at io.prestosql.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
    at io.prestosql.operator.WorkProcessorUtils$3.process(WorkProcessorUtils.java:306)
    at io.prestosql.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
    at io.prestosql.operator.WorkProcessorUtils.getNextState(WorkProcessorUtils.java:221)
    at io.prestosql.operator.WorkProcessorUtils.lambda$processStateMonitor$2(WorkProcessorUtils.java:200)
    at io.prestosql.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
    at io.prestosql.operator.WorkProcessorUtils.getNextState(WorkProcessorUtils.java:221)
    at io.prestosql.operator.WorkProcessorUtils.lambda$finishWhen$3(WorkProcessorUtils.java:215)
    at io.prestosql.operator.WorkProcessorUtils$ProcessWorkProcessor.process(WorkProcessorUtils.java:372)
    at io.prestosql.operator.WorkProcessorSourceOperatorAdapter.getOutput(WorkProcessorSourceOperatorAdapter.java:149)
    at io.prestosql.operator.Driver.processInternal(Driver.java:379)
    at io.prestosql.operator.Driver.lambda$processFor$8(Driver.java:283)
    at io.prestosql.operator.Driver.tryWithLock(Driver.java:675)
    at io.prestosql.operator.Driver.processFor(Driver.java:276)
    at io.prestosql.execution.SqlTaskExecution$DriverSplitRunner.processFor(SqlTaskExecution.java:1076)
    at io.prestosql.execution.executor.PrioritizedSplitRunner.process(PrioritizedSplitRunner.java:[e-mail])
    at io.prestosql.execution.executor.TaskExecutor$TaskRunner.run(TaskExecutor.java:484)
    at io.prestosql.$gen.Presto_341____20200925_110330_2.run(Unknown Source)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    at java.base/java.lang.Thread.run(Thread.java:834)

通过上面错误我们推算出应该是由于数据导致空指针异常，那么问题出在哪里呢？问题就出在查询str这个字段中，这个字段我们经过实际的查询发现是有''的数据，在做转换的时候出现了空指针问题，于是我们修改UDF的源码为

@Description("user_id")
@ScalarFunction("user_id")
@SqlType(StandardTypes.VARCHAR)
@SqlNullable
public static Slice userId(@SqlNullable @SqlType(StandardTypes.VARCHAR) Slice value) {
    String _value = value.toStringUtf8();
    if (StringUtils.containsWhitespace(_value)) {
        _value = StringUtils.replace(_value, " ", "+");
    } 
    return Slices.utf8Slice(makeErrorMsgBase64(_value));
}

我们在方法和参数上添加了@SqlNullable注解用于标记此函数可以接收空的数据，这似乎看着也没有问题，我们将该函数重新发布，再次执行SQL发现还存在相同的问题，于是又将代码修改为以下内容

@Description("user_id")
@ScalarFunction("user_id")
@SqlType(StandardTypes.VARCHAR)
public static Slice userId(@SqlNullable @SqlType(StandardTypes.VARCHAR) Slice value) {
    String _value = value.toStringUtf8();
    if (StringUtils.containsWhitespace(_value)) {
        _value = StringUtils.replace(_value, " ", "+");
    } 
    return Slices.utf8Slice(makeErrorMsgBase64(_value));
}

删除了方法上的@SqlNullable注解，再次运行发现不会再出现这个错误，但是Presto服务中不断的报出空指针错误，只是不在反馈给查询客户端，原本以为此问题已经解决，然而更有意思的事情发生了，我们使用了343版本测试成功后，线上版本是341，升级线上后发现此问题再次复现，如果再次在方法上加上@SqlNullable注解在341版本上又会修复这个问题，目前已将这个问题反馈给官方，推荐大家使用343版本！