[Bug] QueryFileMonitor source is always measured as busy

[thswlsqls]

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Paimon version
master @ 4af1470

Compute Engine
Flink

Minimal reproduce step

1. Start the query service on any table: CALL sys.query_service('default.t', 1).
2. Watch the FileMonitor-<table> source task in the Flink Web UI while no new files are committed.

QueryFileMonitor.Reader.pollNext() (paimon-flink/paimon-flink-common/src/main/java/org/apache/paimon/flink/service/QueryFileMonitor.java, line 97-105) calls Thread.sleep(monitorInterval) on the mailbox thread and then returns InputStatus.MORE_AVAILABLE. It does not override isAvailable(), so AbstractNonCoordinatedSourceReader.isAvailable() keeps returning an already completed future.

The sibling MonitorSource.Reader in the same module was changed in #6396 to await the interval asynchronously, override isAvailable() and return NOTHING_AVAILABLE. QueryFileMonitor was not covered by that change.

What doesn't meet your expectations?
Expected: the source is measured as idle while waiting for the next discovery interval.

Actual: busyTimeMsPerSecond stays at 1000, so the Web UI reports this source as a permanent bottleneck. Checkpoint triggering is also delayed by up to continuous.discovery-interval (default 10s) while the mailbox thread is blocked.

Anything else?
N/A

Are you willing to submit a PR?

• I'm willing to submit a PR!