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From "ASF GitHub Bot (JIRA)" <>
Subject [jira] [Commented] (CXF-7109) ClientCallback may be invoked twice when Async HTTP Transport is used
Date Wed, 26 Oct 2016 00:54:58 GMT


ASF GitHub Bot commented on CXF-7109:

GitHub user tadayosi opened a pull request:

    [CXF-7109] ClientCallback may be invoked twice when Async HTTP Transport is used
    I think the best way to make a client callback invoked once and only once is to remove
from the exchange immediately after its `handle***()` method is invoked once. It should prevent
a client callback from being called more than once not only for the [CXF-7109](
case but also for any other potential cases where the callback may be invoked more than once
    I added `remove(Class<T>)` to `Exchange` (`StringMap`) for convenience and symmetry.
It is necessary to remove the callback right before calling its `handle***()`, as otherwise
some other thread may grab it concurrently before the current thread invokes `handle***()`.
    I also added a test case `AsyncHTTPConduitTest#testCallAsyncCallbackInvokedOnlyOnce()`,
but please note this test fails only on RHEL 6.8 even without this fix. In order to check
if this fix is really effective, you need to run the test on RHEL 6.8 to compare the results.

You can merge this pull request into a Git repository by running:

    $ git pull CXF-7109

Alternatively you can review and apply these changes as the patch at:

To close this pull request, make a commit to your master/trunk branch
with (at least) the following in the commit message:

    This closes #187
commit 4eb81d3044c0f663d580cdbd3b611d5e3b1b4ac5
Author: Tadayoshi Sato <>
Date:   2016-10-25T09:45:42Z

    [CXF-7109] ClientCallback may be invoked twice when Async HTTP Transport is used


> ClientCallback may be invoked twice when Async HTTP Transport is used
> ---------------------------------------------------------------------
>                 Key: CXF-7109
>                 URL:
>             Project: CXF
>          Issue Type: Bug
>          Components: Core, Transports
>    Affects Versions: 3.1.8
>         Environment: RHEL 6.8
>            Reporter: Tadayoshi Sato
> With Async HTTP Transport ({{cxf-rt-transports-http-hc}}) enabled, the {{ClientCallback}}
object passed for the invocation of the client may be called twice when a failure occurs during
the invocation:
> {code:java}
> Client client = ...
> client.invoke(new ClientCallback { ... }, opName, params);
> {code}
> At this moment, this issue is reproduced only on RHEL 6.8, but code analysis shows that
it's a general multithreading issue with Async HTTP Transport and in theory it can happen
on any platform.
> Here is what I've found with the code analysis:
> If for some reason the target endpoint is not reachable, this issue can happen when the
callback for Apache HC {{HttpAsyncClient}} \[1] is invoked and thus shuts down {{SharedOutputBuffer}}
earlier than {{HTTPConduit}} tries to write it before closing the conduit itself \[2].
> \[1]
> \[2]
> When the above issue happens, the interceptor chain's fault observer is eventually invoked
twice, once from the response workqueue thread \[3] that is spawn by the Apache HC {{HttpAsyncClient}}
callback and the second time from the requesting thread at the interceptor chain \[4]; Thus
the client callback is also invoked twice \[5]!
> \[3]
> \[4]
> \[5]
> Why it happens so reliably only on RHEL 6.8 is still unclear (probably due to some thread
scheduling logic specific to the kernel for RHEL 6.8), but since it's essentially an ordering
issue for two distinct threads, in theory it can happen on any platforms. In general, CXF
should really enforce that every client callback be invoked once and only once.

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