https://issues.apache.org/bugzilla/show_bug.cgi?id=56518
Bug ID: 56518
Summary: NIO async servlet limit latch leak
Product: Tomcat 7
Version: 7.0.53
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P2
Component: Connectors
Assignee: [email protected]
Reporter: [email protected]
Created attachment 31613
--> https://issues.apache.org/bugzilla/attachment.cgi?id=31613&action=edit
the sample webapp to reproduce the bug
we have encouter this bug in a real product webapp.
I have tested this in linux x86, oracle jdk jdk1.7.0_55, tomcat 7.0.53 and
tomcat 8.0.5.
CONFIG:
we change HTTP Connector to NIO in "server.xml",
e.g. protocol="org.apache.coyote.http11.Http11NioProtocol"
WEBAPP LOGIC:
the simplified situation:
1. call "req.startAsync()" to start async serlvet, then execute the async logic
in our user thread.
2. sometimes the user thread be interrupted (by some timeout logic of our
code).
3. some user code call "resp.flushBuffer()" to send response to client
PROBLEM:
in the situation descibed above, the "LimitLatch.countDown()" is not called.
when the connections limit latch count up to max ( default "10000" ),
tomcat DO not accept any connection, all incoming client hangs.
REPRODUCER:
in a clean tomcat-7.0.53 installation:
1. change the default "server.xml" Connector config.
(1) change protocol="org.apache.coyote.http11.Http11NioProtocol"
(2) Optional, add maxConnections="100" to reproduce the bug faster.
2. copy the sample webapp in the attachment to "webapps/ROOT.war"
3. start tomcat.
4. make plenty request to "/async.html",
for (( i = 0; i < 15000; ++i )) ; do echo $i; curl localhost:8080/async.html;
done
each request is likely cause a limit latch leak.
when the requests reaches maxConnections (100 as we set above) or some more,
the client ( curl ) hangs.
TECHNIC-DETAILS:
after some debug, wo found these:
1. when the thread was interrupted, when the user code call
"resp.flushBuffer()",
the NioChannel was Closed by jdk NIO code, and a ClosedByInterruptException is
thrown.
2. when the channel closed, the SelectionKey was removed by Poller thread,
stack trace:
Daemon Thread [http-nio-8080-ClientPoller-0] (Suspended)
owns: Object (id=3346)
owns: HashSet<E> (id=3354)
owns: EPollSelectorImpl (id=82)
owns: Collections$UnmodifiableSet<E> (id=3355)
owns: Util$2 (id=3356)
SocketChannelImpl(AbstractSelectableChannel).removeKey(SelectionKey) line:
114
EPollSelectorImpl(AbstractSelector).deregister(AbstractSelectionKey) line:
168
EPollSelectorImpl.implDereg(SelectionKeyImpl) line: 162
EPollSelectorImpl(SelectorImpl).processDeregisterQueue() line: 131
EPollSelectorImpl.doSelect(long) line: 69
EPollSelectorImpl(SelectorImpl).lockAndDoSelect(long) line: 69
EPollSelectorImpl(SelectorImpl).select(long) line: 80
NioEndpoint$Poller.run() line: 1163
Thread.run() line: 662
3. when we call "ctx.complete()", it run to
"org.apache.tomcat.util.net.NioEndpoint.processSocket(NioChannel, SocketStatus,
boolean)", code is below:
public boolean processSocket(NioChannel socket, SocketStatus status,
boolean dispatch) {
try {
KeyAttachment attachment =
(KeyAttachment)socket.getAttachment(false);
if (attachment == null) {
return false;
}
since the SelectionKey was removed, the "attachment" returns null.
the logic is break, "AbstractEndpoint.countDownConnection()" is not called, a
limit latch leak happens.
WORK-AROUND:
some work-around:
1. switch to the stable BIO connector.
2. avoid call "resp.flushBuffer()" in the user thread.
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