On 20/06/2022 11:39, Thomas Hoffmann (Speed4Trade GmbH) wrote:
Hello Mark,
thanks for your reply!
-----Ursprüngliche Nachricht-----
Von: Mark Thomas <ma...@apache.org>
Gesendet: Montag, 20. Juni 2022 12:06
An: users@tomcat.apache.org
Betreff: Re: AW: AW: Filehandle left open when using sendfile
On 16/06/2022 19:58, Thomas Hoffmann (Speed4Trade GmbH) wrote:
<snip/>
In the meantime I stumbled upon this bug-Report:
https://bugs.java.com/bugdatabase/view_bug.do?bug_id=4715154
So maybe the problem lies even deeper.
Similar description here:
https://cemerick.com/blog/2006/08/30/memory-mapping-files-in-java-
caus
es-problems.html
Some ppl suggest to use java.lang.ref.Cleaner or don’t use Memory-
Mapped files under Windows.
I don’t know if there are other solutions.
Your research looks to be exhaustive. I can't find any better ideas.
Using the java.lang.ref.Cleaner looks to be a viable option. We know when
the mapped file is no longer being used. However, that requires Java 12
onwards.
This is only going to be required if the file locking is an issue. In read-only
scenarios or when using an OS other than Windows it won't be an issue.
So, what do we want to do?
1. Disable sendfile for HTTP/2 if running on Windows?
2. Document the potential issues with sendfile + HTTP/2 + Windows if
resources are not read-only?
3. Use the JreCompat mechanism to clear the references if possible:
- if running on Windows
- on all OSes
- if enabled via configuration
Something else?
Mark
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I did some further searching on this topic.
Several posts disregard using java.lang.ref.Cleaner because if the buffer is
used afterwards, it will crash the VM. But if used carefully it works.
If we use this option, it should be possible to use it appropriately
carefully.
About your suggestions:
2) Documenting would be helpful, if lock can't be prevented.
I also found documentation at e.g.
https://docs.oracle.com/javase/9/docs/api/java/nio/channels/FileChannel.html#map-java.nio.channels.FileChannel.MapMode-long-long-
" The buffer and the mapping that it represents will remain valid until the
buffer itself is garbage-collected."
Which is essentially the problem. Using the Cleaner would clean up the
reference sooner.
3) As JreCompat is a bit risky, enabling via config sounds safe to me.
JreCompat is perfectly safe. The jdk.internal.misc.Unsafe API is where
the risk is and this is primarily the risk of the crash mentioned above
that we should be able to avoid.
Some other (theoretical?) options:
4) In an older version of Tomcat native lib there seemed to be a native
Implementation of MMap:
https://tomcat.apache.org/tomcat-10.0-doc/api/org/apache/tomcat/jni/Mmap.html
I read that this was an alternative to the Java memory mapped file. But
it was removed in newer versions. Maybe it can be resurrected for this case and
used if native lib is available(?)
Sorry, no. We are moving away from the native library. Eventually we
will just use project Panama to wrap OpenSSL. Until then, we are
removing everything that isn't required to support the use of OPenSSl
with NIO and NIO2.
The primary reason for this is stability.
5) Instead of FileChannel.map maybe a normal ByteBuffer with
FileChannel.read(buffer) can be used (?)
That is worth considering. The other sendfile implementations don't use
a memory mapped file.
I'll start a discussion on the dev list.
One remaining question:
I didn’t find FileChannel.map in the other connectors. Is
Http2AsyncUpgradeHandler the only occurrence?
In the main code base, yes. There is another usage in the test code but
that is less of a concern.
Mark
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