[HTTPASYNC-86] Race condition in async client when a keep alive connection is reused in POST request method

• Type: Bug
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 4.0.2, 4.1-beta1
• Fix Version/s: 4.1
• Environment: Ubuntu 12.04 64 bits Kernel 3.2.0-74
• Assignee: Unassigned
• Reporter: Jose Dillet
• Created: 12/Dec/14 17:21
• Updated: 04/Dec/17 10:41
• Resolved: 10/Feb/15 12:26

Description

I have found a race condition in the async client when a POST request reuses a keep alive connection. We have around 2%-4% of our runs in a Continuous Integration environment failing because of this issue.

I have found a consistent way of forcing the failure by using a debugger, so I can force the events to happen in the order which uncovers the problem.

I have tested it on httpasyncclient 4.0.2 / httpcore 4.3.2 and httpasyncclient 4.1-beta1 / httpcore 4.4-beta1 and it happens in both versions (the lines provided in the steps to reproduce are from version 4.0.2).

How to reproduce:
1) Add these breakpoints:
a - DefaultClientExchangeHandlerImpl:304
b - DefaultClientExchangeHandlerImpl:168

2) Run AsyncClientRaceConditionSample sample program
Processing first request:

• Program hits breakpoint a -> Resume
• Program hits breakpoint b -> Resume

Processing second request:

• Program hits breakpoints a and b (in different threads)
• Resume a)
• Resume b)

(Probably adding a small sleep after DefaultClientExchangeHandlerImpl:303 only for the testing would also uncover the issue).

ConnectionClosedException is thrown with this stack trace:

Exception in thread "main" java.util.concurrent.ExecutionException: org.apache.http.ConnectionClosedException: Connection closed
at org.apache.http.concurrent.BasicFuture.getResult(BasicFuture.java:68)
at org.apache.http.concurrent.BasicFuture.get(BasicFuture.java:77)
at AsyncClientRaceConditionSample.process(AsyncClientRaceConditionSample.java:81)
at AsyncClientRaceConditionSample.main(AsyncClientRaceConditionSample.java:45)
Caused by: org.apache.http.ConnectionClosedException: Connection closed
at org.apache.http.impl.nio.client.DefaultClientExchangeHandlerImpl.connectionAllocated(DefaultClientExchangeHandlerImpl.java:305)
at org.apache.http.impl.nio.client.DefaultClientExchangeHandlerImpl.access$000(DefaultClientExchangeHandlerImpl.java:63)
at org.apache.http.impl.nio.client.DefaultClientExchangeHandlerImpl$1.completed(DefaultClientExchangeHandlerImpl.java:360)
at org.apache.http.impl.nio.client.DefaultClientExchangeHandlerImpl$1.completed(DefaultClientExchangeHandlerImpl.java:357)
at org.apache.http.concurrent.BasicFuture.completed(BasicFuture.java:115)
at org.apache.http.impl.nio.conn.PoolingNHttpClientConnectionManager$InternalPoolEntryCallback.completed(PoolingNHttpClientConnectionManager.java:465)
at org.apache.http.impl.nio.conn.PoolingNHttpClientConnectionManager$InternalPoolEntryCallback.completed(PoolingNHttpClientConnectionManager.java:449)
at org.apache.http.concurrent.BasicFuture.completed(BasicFuture.java:115)
at org.apache.http.nio.pool.AbstractNIOConnPool.fireCallbacks(AbstractNIOConnPool.java:443)
at org.apache.http.nio.pool.AbstractNIOConnPool.lease(AbstractNIOConnPool.java:276)
at org.apache.http.impl.nio.conn.PoolingNHttpClientConnectionManager.requestConnection(PoolingNHttpClientConnectionManager.java:261)
at org.apache.http.impl.nio.client.DefaultClientExchangeHandlerImpl.requestConnection(DefaultClientExchangeHandlerImpl.java:351)
at org.apache.http.impl.nio.client.DefaultClientExchangeHandlerImpl.start(DefaultClientExchangeHandlerImpl.java:126)
at org.apache.http.impl.nio.client.InternalHttpAsyncClient.execute(InternalHttpAsyncClient.java:129)
at org.apache.http.impl.nio.client.CloseableHttpAsyncClient.execute(CloseableHttpAsyncClient.java:72)
at org.apache.http.impl.nio.client.CloseableHttpAsyncClient.execute(CloseableHttpAsyncClient.java:102)
at org.apache.http.impl.nio.client.CloseableHttpAsyncClient.execute(CloseableHttpAsyncClient.java:87)
at AsyncClientRaceConditionSample.process(AsyncClientRaceConditionSample.java:80)
... 1 more

Some analysis from my side:
1 - When the first request is processed, this is the execution order of relevant lines (and the thread where it is processed):
a - (I/O dispatcher 1) DefaultClientExchangeHandlerImpl<T>.connectionAllocated(NHttpClientConnection) line: 303
b - (I/O dispatcher 1) DefaultClientExchangeHandlerImpl<T>.connectionAllocated(NHttpClientConnection) line: 304
c - (I/O dispatcher 1) CPoolProxy.detach() line: 60
d - (I/O dispatcher 1) DefaultClientExchangeHandlerImpl<T>.responseCompleted() line: 168

2 - When the second request is processed c) might happen before b) which leads to the problem:
a - (main) DefaultClientExchangeHandlerImpl<T>.connectionAllocated(NHttpClientConnection) line: 303
c - (I/O dispatcher 1) CPoolProxy.detach() line: 60
b - (main) DefaultClientExchangeHandlerImpl<T>.connectionAllocated(NHttpClientConnection) line: 304
e - (main) BasicFuture<T>.failed(Exception) line: 122
d - (I/O dispatcher 1) DefaultClientExchangeHandlerImpl<T>.responseCompleted() line: 168

The problem is that in case 2 the connection can be closed before request is completed because a) is processed in main thread and c) is processed in I/O dispatcher thread, so the ordering b < c is not guaranteed.
However in case 1 because all the events are processed in I/O dispatcher thread, the ordering b < c is guaranteed (this seems to be true even if there are several dispatcher threads).