Github user NicoK commented on a diff in the pull request:

    https://github.com/apache/flink/pull/4509#discussion_r152859069
  
    --- Diff: 
flink-runtime/src/test/java/org/apache/flink/runtime/io/network/partition/consumer/RemoteInputChannelTest.java
 ---
    @@ -301,81 +306,388 @@ public void testProducerFailedException() throws 
Exception {
        }
     
        /**
    -    * Tests {@link RemoteInputChannel#recycle(MemorySegment)}, verifying 
the exclusive segment is
    -    * recycled to available buffers directly and it triggers notify of 
announced credit.
    +    * Tests to verify that the input channel requests floating buffers 
from buffer pool
    +    * in order to maintain backlog + initialCredit buffers available once 
receiving the
    +    * sender's backlog, and registers as listener if no floating buffers 
available.
         */
        @Test
    -   public void testRecycleExclusiveBufferBeforeReleased() throws Exception 
{
    -           final SingleInputGate inputGate = mock(SingleInputGate.class);
    -           final RemoteInputChannel inputChannel = 
spy(createRemoteInputChannel(inputGate));
    +   public void testRequestFloatingBufferOnSenderBacklog() throws Exception 
{
    +           // Setup
    +           final NetworkBufferPool networkBufferPool = new 
NetworkBufferPool(12, 32, MemoryType.HEAP);
    +           final SingleInputGate inputGate = createSingleInputGate();
    +           final RemoteInputChannel inputChannel = 
createRemoteInputChannel(inputGate);
    +           try {
    +                   final int numFloatingBuffers = 10;
    +                   final BufferPool bufferPool = 
spy(networkBufferPool.createBufferPool(numFloatingBuffers, numFloatingBuffers));
    +                   inputGate.setBufferPool(bufferPool);
    +
    +                   // Assign exclusive segments to the channel
    +                   final int numExclusiveBuffers = 2;
    +                   
inputGate.setInputChannel(inputChannel.partitionId.getPartitionId(), 
inputChannel);
    +                   inputGate.assignExclusiveSegments(networkBufferPool, 
numExclusiveBuffers);
    +
    +                   assertEquals("There should be " + numExclusiveBuffers + 
" buffers available in the channel",
    +                           numExclusiveBuffers, 
inputChannel.getNumberOfAvailableBuffers());
     
    -           // Recycle exclusive segment
    -           
inputChannel.recycle(MemorySegmentFactory.allocateUnpooledSegment(1024, 
inputChannel));
    +                   // Receive the producer's backlog
    +                   inputChannel.onSenderBacklog(8);
     
    -           assertEquals("There should be one buffer available after 
recycle.",
    -                   1, inputChannel.getNumberOfAvailableBuffers());
    -           verify(inputChannel, times(1)).notifyCreditAvailable();
    +                   // Request the number of floating buffers by the 
formula of backlog + initialCredit - availableBuffers
    +                   verify(bufferPool, times(8)).requestBuffer();
    +                   verify(bufferPool, 
times(0)).addBufferListener(inputChannel);
    +                   assertEquals("There should be 10 buffers available in 
the channel",
    +                           10, inputChannel.getNumberOfAvailableBuffers());
     
    -           
inputChannel.recycle(MemorySegmentFactory.allocateUnpooledSegment(1024, 
inputChannel));
    +                   inputChannel.onSenderBacklog(11);
     
    -           assertEquals("There should be two buffers available after 
recycle.",
    -                   2, inputChannel.getNumberOfAvailableBuffers());
    -           // It should be called only once when increased from zero.
    -           verify(inputChannel, times(1)).notifyCreditAvailable();
    +                   // Need extra three floating buffers, but only two 
buffers available in buffer pool, register as listener as a result
    +                   verify(bufferPool, times(11)).requestBuffer();
    +                   verify(bufferPool, 
times(1)).addBufferListener(inputChannel);
    +                   assertEquals("There should be 12 buffers available in 
the channel",
    +                           12, inputChannel.getNumberOfAvailableBuffers());
    +
    +                   inputChannel.onSenderBacklog(12);
    +
    +                   // Already in the status of waiting for buffers and 
will not request any more
    +                   verify(bufferPool, times(11)).requestBuffer();
    +                   verify(bufferPool, 
times(1)).addBufferListener(inputChannel);
    +
    --- End diff --
    
    Thanks, the extensions you implemented for 
`testRequestFloatingBufferOnSenderBacklog()` were exactly what I missed 
previously and was not covered by `testFairDistributionFloatingBuffers()`. 
Based on your additions, I came up with some more (corner) cases that should be 
covered (see above)


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