JAVA-6071
This ticket involves two separate bugs in a test case testBulkWriteHandlesWriteErrorsAcrossBatches for reactive driver , specifically when ordered field is false

IllegalState exception on Mono Timeout

The first part of the failed test is

[2026/01/21 16:07:57.619] FAILURE: org.opentest4j.AssertionFailedError: Unexpected exception type thrown, expected: <com.mongodb.ClientBulkWriteException> but was: <java.lang.IllegalStateException> (org.opentest4j.AssertionFailedError)

Here is the link to the build
Here is the log that caught my attention

[2026/01/21 16:03:37.770]     00:03:37.750 [cluster-ClusterId{value='697169596f43721a0d1b8470', description='null'}-localhost:27017] INFO  org.mongodb.driver.cluster - Monitor thread successfully connected to server with description ServerDescription{address=localhost:27017, type=STANDALONE, cryptd=false, state=CONNECTED, ok=true, minWireVersion=0, maxWireVersion=25, maxDocumentSize=16777216, logicalSessionTimeoutMinutes=30, roundTripTimeNanos=3744986, minRoundTripTimeNanos=0}
[2026/01/21 16:05:39.870]   6. MongoClient.bulkWrite handles individual WriteErrors across batches

As you can see the time diff between two last logs is exactly two minutes , and according to our settings, the sync driver blocks mono by two minutes

So timeout happened and Mono.block threw IllegalStateException
This PR does not solve this issue, the IllegalState is expected, unless we want to override timeout for this test case only as it deals with a huge number of documents per batch

Connection leak

Second asserting that failed on this test case is connection leak

[2026/01/21 16:07:57.619] The connection pool listener reports '1' open connections.
[2026/01/21 16:07:57.619] 		at com.mongodb.assertions.Assertions.assertTrue(Assertions.java:190)
[2026/01/21 16:07:57.619] 		at com.mongodb.reactivestreams.client.syncadapter.SyncMongoClient$ConnectionPoolCounter.assertConnectionsClosed(SyncMongoClient.java:373)
[2026/01/21 16:07:57.619] 		at com.mongodb.reactivestreams.client.syncadapter.SyncMongoClient.close(SyncMongoClient.java:305)
[2026/01/21 16:07:57.619] 		at com.mongodb.client.CrudProseTest.testBulkWriteHandlesWriteErrorsAcrossBatches(CrudProseTest.java:246)
[2026/01/21 16:07:57.619] 		... 39 more

Before looking at at the leak, I started checking what the test case is actually doing I noticed that it tries to insert documents in batches of 10K invalid(duplicate ids) docs per batch
The assertion that happens for connection leak happens here
The code is running a loop which fails after 2 seconds if not all connections are closed, in my local env I noticed that 2 seconds is not enough for such a huge number of documents to be processed so I introduced a new method that can propagate and override this timeout, after running this test case 100 times locally I didn't notice the connection leak anymore

To test it locally

1. change the Mono timeout from 2 min to 2 seconds so that you can simulate IllegalStateException
2. add RepeatedTest to this test case, in my case , I hardcoded ordered=false instead of relying on parametrized test so that I can use RepeatedTest

NullPointerException

The last part of this failed test is NullPointer here

[2026/01/21 16:07:57.619] Exception in thread "Thread-29" java.lang.NullPointerException: Cannot invoke "java.nio.ByteBuffer.hasRemaining()" because "this.buf" is null
[2026/01/21 16:07:57.619] 	at org.bson.ByteBufNIO.hasRemaining(ByteBufNIO.java:91)

The exception makes sense; hasRemaining() throws an NPE because the underlying buffer becomes null once released. While I couldn't reproduce this after 100 local test runs, I added a guard for ByteBufNIO. By using asNio() first (which acts as a direct getter for ByteBufNIO but avoids the buffer copying found in Netty or other implementations), we can safely check if the underlying buffer is null before checking hasRemaining , for other implementation we still rely on hasRemaining

I don't like this approach because of abstraction leak and open for any suggestions