SourceCoordinator error splitRequest check cause HybridSource loss of data and hang

If we use hybrid source, for example, filesystem source A read a.csv, filesystem B read b.csv. It's a very simple case, but it will hang in second source with:

10802 [SourceCoordinator-Source: hybrid_source[1]] INFO  org.apache.flink.runtime.source.coordinator.SourceCoordinator [] - Source Source: hybrid_source[1] received split request from parallel task 0 (#0)
10802 [SourceCoordinator-Source: hybrid_source[1]] INFO  org.apache.flink.connector.file.src.impl.StaticFileSplitEnumerator [] - Subtask 0 (on host '') is requesting a file source split
10803 [SourceCoordinator-Source: hybrid_source[1]] INFO  org.apache.flink.connector.file.src.assigners.LocalityAwareSplitAssigner [] - Assigning split to non-localized request: Optional[FileSourceSplit: file:/Users/xxx/a.csv [0, 49) (no host info) ID=0000000001 position=null]
10808 [SourceCoordinator-Source: hybrid_source[1]] INFO  org.apache.flink.connector.file.src.impl.StaticFileSplitEnumerator [] - Assigned split to subtask 0 : FileSourceSplit: file:/Users/xxx/a.csv [0, 49) (no host info) ID=0000000001 position=null
10816 [Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.hybrid.HybridSourceReader [] - Adding splits subtask=0 sourceIndex=0 currentReader=org.apache.flink.connector.file.src.impl.FileSourceReader@1e8e1971 [HybridSourceSplit

]
10817 [Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.reader.SourceReaderBase [] - Adding split(s) to reader: [FileSourceSplit: file:/Users/xxx/a.csv [0, 49) (no host info) ID=0000000001 position=null]
10822 [Source Data Fetcher for Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.reader.fetcher.SplitFetcher [] - Starting split fetcher 0
10864 [Source Data Fetcher for Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.reader.fetcher.SplitFetcher [] - Finished reading from splits [0000000001]
+I[hello_a, flink, 1]
+I[hello_a, hadoop, 2]
+I[hello_a, world, 3]
10866 [Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.reader.SourceReaderBase [] - Finished reading split(s) [0000000001]
10868 [Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.reader.fetcher.SplitFetcherManager [] - Closing splitFetcher 0 because it is idle.
10868 [Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.reader.fetcher.SplitFetcher [] - Shutting down split fetcher 0
10868 [Source Data Fetcher for Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.reader.fetcher.SplitFetcher [] - Split fetcher 0 exited.
10869 [SourceCoordinator-Source: hybrid_source[1]] INFO  org.apache.flink.runtime.source.coordinator.SourceCoordinator [] - Source Source: hybrid_source[1] received split request from parallel task 0 (#0)
10870 [SourceCoordinator-Source: hybrid_source[1]] INFO  org.apache.flink.connector.file.src.impl.StaticFileSplitEnumerator [] - Subtask 0 (on host '') is requesting a file source split
10872 [SourceCoordinator-Source: hybrid_source[1]] INFO  org.apache.flink.connector.file.src.impl.StaticFileSplitEnumerator [] - No more splits available for subtask 0
10872 [Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.reader.SourceReaderBase [] - Reader received NoMoreSplits event.
10872 [Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.hybrid.HybridSourceReader [] - End of input subtask=0 sourceIndex=0 org.apache.flink.connector.file.src.impl.FileSourceReader@1e8e1971
StaticFileSplitEnumerator:org.apache.flink.connector.file.src.impl.StaticFileSplitEnumerator@69906bb9
10874 [SourceCoordinator-Source: hybrid_source[1]] INFO  org.apache.flink.connector.base.source.hybrid.HybridSourceSplitEnumerator [] - Starting enumerator for sourceIndex=1
10879 [Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.hybrid.HybridSourceReader [] - Switch source event: subtask=0 sourceIndex=1 source=org.apache.flink.connector.file.src.FileSource@12ef574f
10879 [Source: hybrid_source[1] -> Sink: print_out[2] (1/1)#0] INFO  org.apache.flink.connector.base.source.reader.SourceReaderBase [] - Closing Source Reader.
10882 [SourceCoordinator-Source: hybrid_source[1]] INFO  org.apache.flink.runtime.source.coordinator.SourceCoordinator [] - Source Source: hybrid_source[1] received split request from parallel task 0 (#0)

do not read next source data and hang at 'received split request from parallel task'

The reason is that in the 1.16 & master, latest code add context.hasNoMoreSplits check then call enumerator.handleSplitRequest.  We do understand the comments for reducing the call splits. But it not consider the the situation about HybridSource. When a subtask hasNoMoreSplits, it will switch to next source. But here just set a check without this situation. When first source read finish, the context just let this subtask with noMoreSplit Status. And the later check can't assign splits with next sources. However , the flink 1.15 is correct.

SourceCoordinator

private void handleRequestSplitEvent(int subtask, int attemptNumber, RequestSplitEvent event) {
    LOG.info(
            "Source {} received split request from parallel task {} (#{})",
            operatorName,
            subtask,
            attemptNumber);
    // request splits from the enumerator only if the enumerator has un-assigned splits
    // this helps to reduce unnecessary split requests to the enumerator
    if (!context.hasNoMoreSplits(subtask)) {
        enumerator.handleSplitRequest(subtask, event.hostName());
    }
} 

SourceCoordinator call `context.hasNoMoreSplits` check cause the subtask not read the other child sources in hybrid source.

SourceCoordinatorContext

boolean hasNoMoreSplits(int subtaskIndex) { return subtaskHasNoMoreSplits[subtaskIndex]; }


@Override
public void signalNoMoreSplits(int subtask) {
    checkSubtaskIndex(subtask);

    // Ensure the split assignment is done by the coordinator executor.
    callInCoordinatorThread(
            () -> {
                subtaskHasNoMoreSplits[subtask] = true;
                signalNoMoreSplitsToAttempts(subtask);
                return null; // void return value
            },
            "Failed to send 'NoMoreSplits' to reader " + subtask);
}

context set subtask noMoreSplit is true if source is done (without considering the hybrid situation).

1.15

public void handleEventFromOperator(int subtask, OperatorEvent event) {
    runInEventLoop(
            () -> {
                if (event instanceof RequestSplitEvent) {
                    LOG.info(
                            "Source {} received split request from parallel task {}",
                            operatorName,
                            subtask);
                    enumerator.handleSplitRequest(
                            subtask, ((RequestSplitEvent) event).hostName());
                }