charlesconnell opened a new pull request, #6168:
URL: https://github.com/apache/hbase/pull/6168
To call a coprocessor endpoint asynchronously, you start by calling
`AsyncTable#coprocessorService()`, which gives you a
`CoprocessorServiceBuilder`, and a few steps later you can talk to your
coprocessor over the network. One argument to `AsyncTable#coprocessorService()`
is a `CoprocessorCallback` object, which contains several methods that will be
called during the lifecycle of a coprocessor endpoint call. `AsyncTableImpl`'s
implementation of `AsyncTable#coprocessorService()` wraps your
`CoprocessorCallback` with its own that delegates the work to a thread pool. A
snippet of this:
```
@Override
public void onRegionComplete(RegionInfo region, R resp) {
pool.execute(context.wrap(() -> callback.onRegionComplete(region,
resp)));
}
...
@Override
public void onComplete() {
pool.execute(context.wrap(callback::onComplete));
}
```
The trouble with this is that your implementations of `onRegionComplete()`
and `onComplete()` will end up getting called in a random order, and/or at the
same time. The tasks of calling them are delegated to a thread pool, and the
completion of those tasks is not waited on, so the thread pool can choose any
ordering it wants to. Troublingly, onComplete() can be called before the final
`onRegionComplete()`, which is an violation of [the contract specified in the
`CoprocessorCallback#onComplete()`
javadoc](https://github.com/apache/hbase/blob/41dd87cd908d4d089d0b8cff6c88c01ed60622c5/hbase-client/src/main/java/org/apache/hadoop/hbase/client/AsyncTable.java#L671).
This PR eliminates the use a thread pool for calling the coprocessor
callback, and calls them directly on the thread that received the response from
the server.
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