Thanks for reporting @npordash! Are you in a situation where `listener.onDataRead` is called multiple times in `DelegatingDecompressorFrameListener`, and you are trying to call `consumeBytes(..)` for data which was not returned by previous calls to `Http2FrameListener.onDataRead`?
 > I tested this with both 4.1.0.Final and 4.1.1.Final.

Do you have a unit test which you would be willing to contribute? I have a PR but it would be nice to verify against a unit test if you already have one.
 Thanks for the quick response!

I'm not in a situation where `onDataRead` is being called multiple times, at least I have yet to observe that happening. The exception gets thrown on the first data frame received.

My use-case is that I'm exposing inbound data frames as an `rx.Observable<ByteBuf>` and I only want to report the processing of the bytes when a `ByteBuf` has been sent to a subscriber of the `Observable`. It's possible that the `ByteBuf` is sent to a subscriber inside the call to `onDataRead` if the processing is synchronous which means that the call to `consumeBytes` might happen before `onDataRead` returns as well.

Unfortunately, I do not have an existing unit test for this yet. I stumbled across this while I was prototyping which involved manual testing with `nghttp` to send data to a netty http2 server 😦  
 > which means that the call to consumeBytes might happen before onDataRead returns as well.

As long as you are not double releasing the bytes you should be OK. I think I found the issue and I'll ask you to verify with the PR when its ready.
 Awesome, thanks again!

nit: spelling on the method name ... should be `...Bytes`
 Is this condition ever possible? I think it only would be if padding were `< 0`, which will never happen.  Suggest either getting rid of this condition or moving it somewhere more clear.
 Same comment as below.
 Don't we also  have to remove the property from the stream?
 Don't we have to mark these bytes as consumed?
 +1
 I guess this is no longer needed now that it is being used strictly to increment ... overflow should be guarded by flow control.
 removed
 There was an ordering issue related to the stream being removed if an error occurred and attempting to return bytes to flow control. So if we removed the property we would not be able to return bytes to flow control in this condition. Ideally we would remove it, but I don't think it hurts anything to leave it in the property map.
 Good point ... we should consume immediately instead of returning the bytes. I'll add a comment on the return statement too.
 Shouldn't this be a call to `decompressor.consumeBytes()`?
 The `decompressor` just keeps state to be able to convert bytes the users sees (decompressed) to bytes the flow controller knows about (compressed). `ConsumedBytesConverter` wraps the flow controller so that when ever bytes are returned it uses the `decompressor` to translate from what the user knows (decompressed) to what the flow controller knows (compressed). So I think we actually want to return bytes to the flow controller here ... this is effectively what is done when bytes are returned by this method (done by `DefaultHttp2ConnectionDecoder`) however bcz there may be multiple frames decompressed it makes sense to return the bytes immediately instead of waiting for the method to return.
 @nmittler - WDYT?
 @Scottmitch Ah I had forgotten that the flow controller is actually the `ConsumedBytesConverter`.  Maybe add a comment?
 done

DelegatingDecompressorFrameListener doesn't support deferral of processed bytes
`Http2FrameListener.onDataRead` states that reporting of processed bytes to the local flow controller can be deferred by returning something less than `data + padding` and then later doing something equivalent to `connection().local().flowController().consumeBytes(Http2Stream, int)`.

This works fine until you start using a `DelegatingDecompressorFrameListener` because once you call `consumeBytes` you end up with the following exception:

```
io.netty.handler.codec.http2.Http2Exception: Error while returning bytes to flow control window
    at io.netty.handler.codec.http2.DelegatingDecompressorFrameListener$ConsumedBytesConverter.consumeBytes(DelegatingDecompressorFrameListener.java:361)
...
Caused by: java.lang.IllegalArgumentException: processed bytes cannot be negative
    at io.netty.handler.codec.http2.DelegatingDecompressorFrameListener$Http2Decompressor.incrementProcessedBytes(DelegatingDecompressorFrameListener.java:409)
    at io.netty.handler.codec.http2.DelegatingDecompressorFrameListener$Http2Decompressor.consumeProcessedBytes(DelegatingDecompressorFrameListener.java:445)
    at io.netty.handler.codec.http2.DelegatingDecompressorFrameListener$ConsumedBytesConverter.consumeBytes(DelegatingDecompressorFrameListener.java:349)
```

As far as I can tell the reason for this is because `Http2Decompressor` is expecting `incrementProcessedBytes` to be called prior to `consumeProcessedBytes` and you can't consume more than it has been incremented, which makes sense, but `DelegatingDecompressorFrameListener.onDataRead` only calls `incrementProcessedBytes` _after_ all calls to `onDataRead` returns and only increments the processed bytes based on what was accumulated from the `onDataRead` calls which effectively means you need to fully consume the buffer in `onDataRead`.

I tested this with both 4.1.0.Final and 4.1.1.Final.

codec-http2/src/main/java/io/netty/handler/codec/http2/DelegatingDecompressorFrameListener.java
codec-http2/src/main/java/io/netty/handler/codec/http2/DelegatingDecompressorFrameListener.java
diff --git a/codec-http2/src/main/java/io/netty/handler/codec/http2/DelegatingDecompressorFrameListener.java b/codec-http2/src/main/java/io/netty/handler/codec/http2/DelegatingDecompressorFrameListener.java
index 50439c9fe7a..78ef230c62f 100644
--- a/codec-http2/src/main/java/io/netty/handler/codec/http2/DelegatingDecompressorFrameListener.java
+++ b/codec-http2/src/main/java/io/netty/handler/codec/http2/DelegatingDecompressorFrameListener.java
@@ -44,7 +44,7 @@ public class DelegatingDecompressorFrameListener extends Http2FrameListenerDecor
     private final Http2Connection connection;
     private final boolean strict;
     private boolean flowControllerInitialized;
-    final Http2Connection.PropertyKey propertyKey;
+    private final Http2Connection.PropertyKey propertyKey;
 
     public DelegatingDecompressorFrameListener(Http2Connection connection, Http2FrameListener listener) {
         this(connection, listener, true);
@@ -62,7 +62,7 @@ public DelegatingDecompressorFrameListener(Http2Connection connection, Http2Fram
             public void onStreamRemoved(Http2Stream stream) {
                 final Http2Decompressor decompressor = decompressor(stream);
                 if (decompressor != null) {
-                    cleanup(stream, decompressor);
+                    cleanup(decompressor);
                 }
             }
         });
@@ -80,7 +80,6 @@ public int onDataRead(ChannelHandlerContext ctx, int streamId, ByteBuf data, int
 
         final EmbeddedChannel channel = decompressor.decompressor();
         final int compressedBytes = data.readableBytes() + padding;
-        int processedBytes = 0;
         decompressor.incrementCompressedBytes(compressedBytes);
         try {
             // call retain here as it will call release after its written to the channel
@@ -97,43 +96,44 @@ public int onDataRead(ChannelHandlerContext ctx, int streamId, ByteBuf data, int
                 // not be provided with data and thus could not return how many bytes were processed. We will assume
                 // there is more data coming which will complete the decompression block. To allow for more data we
                 // return all bytes to the flow control window (so the peer can send more data).
-                decompressor.incrementDecompressedByes(compressedBytes);
-                processedBytes = compressedBytes;
-            } else {
-                try {
-                    decompressor.incrementDecompressedByes(padding);
-                    for (;;) {
-                        ByteBuf nextBuf = nextReadableBuf(channel);
-                        boolean decompressedEndOfStream = nextBuf == null && endOfStream;
-                        if (decompressedEndOfStream && channel.finish()) {
-                            nextBuf = nextReadableBuf(channel);
-                            decompressedEndOfStream = nextBuf == null;
-                        }
-
-                        decompressor.incrementDecompressedByes(buf.readableBytes());
-                        processedBytes += listener.onDataRead(ctx, streamId, buf, padding, decompressedEndOfStream);
-                        if (nextBuf == null) {
-                            break;
-                        }
-
-                        padding = 0; // Padding is only communicated once on the first iteration
-                        buf.release();
-                        buf = nextBuf;
+                decompressor.incrementDecompressedBytes(compressedBytes);
+                return compressedBytes;
+            }
+            try {
+                Http2LocalFlowController flowController = connection.local().flowController();
+                decompressor.incrementDecompressedBytes(padding);
+                for (;;) {
+                    ByteBuf nextBuf = nextReadableBuf(channel);
+                    boolean decompressedEndOfStream = nextBuf == null && endOfStream;
+                    if (decompressedEndOfStream && channel.finish()) {
+                        nextBuf = nextReadableBuf(channel);
+                        decompressedEndOfStream = nextBuf == null;
+                    }
+
+                    decompressor.incrementDecompressedBytes(buf.readableBytes());
+                    // Immediately return the bytes back to the flow controller. ConsumedBytesConverter will convert
+                    // from the decompressed amount which the user knows about to the compressed amount which flow
+                    // control knows about.
+                    flowController.consumeBytes(stream,
+                            listener.onDataRead(ctx, streamId, buf, padding, decompressedEndOfStream));
+                    if (nextBuf == null) {
+                        break;
                     }
-                } finally {
+
+                    padding = 0; // Padding is only communicated once on the first iteration.
                     buf.release();
+                    buf = nextBuf;
                 }
+                // We consume bytes each time we call the listener to ensure if multiple frames are decompressed
+                // that the bytes are accounted for immediately. Otherwise the user may see an inconsistent state of
+                // flow control.
+                return 0;
+            } finally {
+                buf.release();
             }
-            decompressor.incrementProcessedBytes(processedBytes);
-            // The processed bytes will be translated to pre-decompressed byte amounts by DecompressorGarbageCollector
-            return processedBytes;
         } catch (Http2Exception e) {
-            // Consider all the bytes consumed because there was an error
-            decompressor.incrementProcessedBytes(compressedBytes);
             throw e;
         } catch (Throwable t) {
-            // Consider all the bytes consumed because there was an error
-            decompressor.incrementProcessedBytes(compressedBytes);
             throw streamError(stream.id(), INTERNAL_ERROR, t,
                     "Decompressor error detected while delegating data read on streamId %d", stream.id());
         }
@@ -250,24 +250,12 @@ Http2Decompressor decompressor(Http2Stream stream) {
     }
 
     /**
-     * Release remaining content from the {@link EmbeddedChannel} and remove the decompressor
-     * from the {@link Http2Stream}.
+     * Release remaining content from the {@link EmbeddedChannel}.
      *
-     * @param stream The stream for which {@code decompressor} is the decompressor for
      * @param decompressor The decompressor for {@code stream}
      */
-    private void cleanup(Http2Stream stream, Http2Decompressor decompressor) {
-        final EmbeddedChannel channel = decompressor.decompressor();
-        if (channel.finish()) {
-            for (;;) {
-                final ByteBuf buf = channel.readInbound();
-                if (buf == null) {
-                    break;
-                }
-                buf.release();
-            }
-        }
-        decompressor = stream.removeProperty(propertyKey);
+    private static void cleanup(Http2Decompressor decompressor) {
+        decompressor.decompressor().finishAndReleaseAll();
     }
 
     /**
@@ -340,26 +328,18 @@ public void receiveFlowControlledFrame(Http2Stream stream, ByteBuf data, int pad
         @Override
         public boolean consumeBytes(Http2Stream stream, int numBytes) throws Http2Exception {
             Http2Decompressor decompressor = decompressor(stream);
-            Http2Decompressor copy = null;
+            if (decompressor != null) {
+                // Convert the decompressed bytes to compressed (on the wire) bytes.
+                numBytes = decompressor.consumeBytes(stream.id(), numBytes);
+            }
             try {
-                if (decompressor != null) {
-                    // Make a copy before hand in case any exceptions occur we will roll back the state
-                    copy = new Http2Decompressor(decompressor);
-                    // Convert the uncompressed consumed bytes to compressed (on the wire) bytes.
-                    numBytes = decompressor.consumeProcessedBytes(numBytes);
-                }
                 return flowController.consumeBytes(stream, numBytes);
             } catch (Http2Exception e) {
-                if (copy != null) {
-                    stream.setProperty(propertyKey, copy);
-                }
                 throw e;
             } catch (Throwable t) {
-                if (copy != null) {
-                    stream.setProperty(propertyKey, copy);
-                }
-                throw new Http2Exception(INTERNAL_ERROR,
-                        "Error while returning bytes to flow control window", t);
+                // The stream should be closed at this point. We have already changed our state tracking the compressed
+                // bytes, and there is no guarantee we can recover if the underlying flow controller throws.
+                throw streamError(stream.id(), INTERNAL_ERROR, t, "Error while returning bytes to flow control window");
             }
         }
 
@@ -379,17 +359,9 @@ public int initialWindowSize(Http2Stream stream) {
      */
     private static final class Http2Decompressor {
         private final EmbeddedChannel decompressor;
-        private int processed;
         private int compressed;
         private int decompressed;
 
-        Http2Decompressor(Http2Decompressor rhs) {
-            this(rhs.decompressor);
-            processed = rhs.processed;
-            compressed = rhs.compressed;
-            decompressed = rhs.decompressed;
-        }
-
         Http2Decompressor(EmbeddedChannel decompressor) {
             this.decompressor = decompressor;
         }
@@ -401,53 +373,49 @@ EmbeddedChannel decompressor() {
             return decompressor;
         }
 
-        /**
-         * Increment the number of decompressed bytes processed by the application.
-         */
-        void incrementProcessedBytes(int delta) {
-            if (processed + delta < 0) {
-                throw new IllegalArgumentException("processed bytes cannot be negative");
-            }
-            processed += delta;
-        }
-
         /**
          * Increment the number of bytes received prior to doing any decompression.
          */
         void incrementCompressedBytes(int delta) {
-            if (compressed + delta < 0) {
-                throw new IllegalArgumentException("compressed bytes cannot be negative");
-            }
+            assert delta >= 0;
             compressed += delta;
         }
 
         /**
-         * Increment the number of bytes after the decompression process. Under normal circumstances this
-         * delta should not exceed {@link Http2Decompressor#processed)}.
+         * Increment the number of bytes after the decompression process.
          */
-        void incrementDecompressedByes(int delta) {
-            if (decompressed + delta < 0) {
-                throw new IllegalArgumentException("decompressed bytes cannot be negative");
-            }
+        void incrementDecompressedBytes(int delta) {
+            assert delta >= 0;
             decompressed += delta;
         }
 
         /**
-         * Decrements {@link Http2Decompressor#processed} by {@code processedBytes} and determines the ratio
-         * between {@code processedBytes} and {@link Http2Decompressor#decompressed}.
+         * Determines the ratio between {@code numBytes} and {@link Http2Decompressor#decompressed}.
          * This ratio is used to decrement {@link Http2Decompressor#decompressed} and
          * {@link Http2Decompressor#compressed}.
-         * @param processedBytes The number of post-decompressed bytes that have been processed.
+         * @param streamId the stream ID
+         * @param decompressedBytes The number of post-decompressed bytes to return to flow control
          * @return The number of pre-decompressed bytes that have been consumed.
          */
-        int consumeProcessedBytes(int processedBytes) {
-            // Consume the processed bytes first to verify that is is a valid amount
-            incrementProcessedBytes(-processedBytes);
-
-            double consumedRatio = processedBytes / (double) decompressed;
+        int consumeBytes(int streamId, int decompressedBytes) throws Http2Exception {
+            if (decompressedBytes < 0) {
+                throw new IllegalArgumentException("decompressedBytes must not be negative: " + decompressedBytes);
+            }
+            if (decompressed - decompressedBytes < 0) {
+                throw streamError(streamId, INTERNAL_ERROR,
+                        "Attempting to return too many bytes for stream %d. decompressed: %d " +
+                                "decompressedBytes: %d", streamId, decompressed, decompressedBytes);
+            }
+            double consumedRatio = decompressedBytes / (double) decompressed;
             int consumedCompressed = Math.min(compressed, (int) Math.ceil(compressed * consumedRatio));
-            incrementDecompressedByes(-Math.min(decompressed, (int) Math.ceil(decompressed * consumedRatio)));
-            incrementCompressedBytes(-consumedCompressed);
+            if (compressed - consumedCompressed < 0) {
+                throw streamError(streamId, INTERNAL_ERROR,
+                        "overflow when converting decompressed bytes to compressed bytes for stream %d." +
+                                "decompressedBytes: %d decompressed: %d compressed: %d consumedCompressed: %d",
+                        streamId, decompressedBytes, decompressed, compressed, consumedCompressed);
+            }
+            decompressed -= decompressedBytes;
+            compressed -= consumedCompressed;
 
             return consumedCompressed;
         }
