markrmiller opened a new pull request, #3843:
URL: https://github.com/apache/solr/pull/3843
ParallelHttpShardHandler incorrectly composes FutureTask with
CompletableFuture, resulting in lost exceptions and hanging distributed
requests.
During shard submission, ParallelHttpShardHandler wraps each
makeShardRequest invocation inside a FutureTask, and then runs that task via
CompletableFuture.runAsync. The intent is to submit shard requests
asynchronously and record their completion or failure using a whenComplete
handler.
However, FutureTask.run() captures exceptions internally rather than
throwing them. When it’s executed inside CompletableFuture.runAsync, the
enclosing CompletableFuture cannot detect that a failure occurred, because
runAsync only completes exceptionally when the submitted Runnable throws.
As a result, if super.makeShardRequest(...) throws an exception:
The FutureTask records the exception internally.
The CompletableFuture completes successfully (no exception propagated).
The whenComplete callback receives a null Throwable, so the error handler
(recordShardSubmitError) is never called.
The shard response is never recorded.
The coordinating request waits indefinitely for a shard response that will
never arrive.
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