Try harder to prevent SIGSEGV on cloned MMapIndexInputs
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Key: LUCENE-3588
URL: https://issues.apache.org/jira/browse/LUCENE-3588
Project: Lucene - Java
Issue Type: Improvement
Components: core/store
Affects Versions: 3.4, 3.5
Reporter: Uwe Schindler
Assignee: Uwe Schindler
Fix For: 3.6, 4.0
We are unmapping mmapped byte buffers which is disallowed by the JDK, because
it has the risk of SIGSEGV when you access the mapped byte buffer after
unmapping.
We currently prevent this for the main IndexInput by setting its buffer to
null, so we NPE if somebody tries to access the underlying buffer. I recently
fixed also the stupid curBuf (LUCENE-3200) by setting to null.
The big problem are cloned IndexInputs which are generally not closed. Those
still contain references to the unmapped ByteBuffer, which lead to SIGSEGV
easily. The patch from Mike in LUCENE-3439 prevents most of this in Lucene 3.5,
but its still not 100% safe (as it uses non-volatiles).
This patch will fix the remaining issues by also setting the buffers of clones
to null when the original is closed. The trick is to record weak references of
all clones created and close them together with the original. This uses a
ConcurrentHashMap<WeakReference<MMapIndexInput>,?> as store with the logic
borrowed from WeakHashMap to cleanup the GCed references (using ReferenceQueue).
If we respin 3.5, we should maybe also get this in.
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