Uwe Schindler created LUCENE-6169:
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Summary: Recent Java 9 commit breaks fsync on directory
Key: LUCENE-6169
URL: https://issues.apache.org/jira/browse/LUCENE-6169
Project: Lucene - Core
Issue Type: Bug
Components: core/store
Reporter: Uwe Schindler
I open this issue to keep track of the communication with Oracle and OpenJDK
about this:
Basically, what happens: In LUCENE-5588 we added support to FSDirectory to be
able to sync on directory metadata changes (means the contents of the directory
itsself). This is very important on Unix system (maybe also on Windows),
because fsyncing a single file does not necessarily writes the directory's
contents to disk. Lucene uses this for commits. We first do an atomic rename of
the segments file (to make the commit public), but we have to be sure that the
rename operation is written to disk. Because of that we must fsync the
directory.
To enforce this, you open a directory for read and then call fsync. In java
this can be done by opening a FileChannel on the direczory(for read) and call
fc.force() on it.
Unfortunately the commit
http://hg.openjdk.java.net/jdk9/jdk9/jdk/rev/e5b66323ae45 in OpenJDK 9 break
this. The corresponding issue is
https://bugs.openjdk.java.net/browse/JDK-8066915. The JDK now explicitly checks
if a file is a directory and disallows opening a FileChannel on it. This breaks
our commit safety.
Because this behaviour is undocumented (not even POSIX has explicit semantics
for syncing directories), we know that it worked at least on MacOSX and Linux.
The code in IOUtils is currently written in a way that it tries to sync the
diretory, but swallows any Exception. So this change does not break Liucene,
but it breaks our commit safety. During testing we assert that the fsync
actually works on Linux and MacOSX, in production code the user will notice
nothing.
We should take action and contact Alan Bateman about his commit and this issue
on the mailing list, possibly through Rory O'Donnell.
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