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SOLR-18551: Prevent mmap leak in IndexFetcher - #5173
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gerlowskija wants to merge 2 commits into
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gerlowskija wants to merge 2 commits into
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Prior to this commit, IndexFetcher's "slowFileExists" method checked existence by attempting to open and then close the file. When MMapDirectoryFactory was in use, this "open" creates an mmap that remains somewhat "leaked" until the DirectoryReader is closed, due to idiosyncracies in how Lucene pools mmaps into "arenas" based on their segment. This caused a pretty significant mmap leak in later Solr 9.x releases. It's less likely to impact Solr 10.x or later versions due to changes in Lucene, but it's still possible. This commit fixes the issue by reworking IndexFetcher's "exists" check. Now instead of opening the file, IndexFetcher does a size-check which can be used to determine existence with only an OS 'stat' call and without creating an mmap at all.
jaykay12
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Oct 16, 2026
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+1 on using fileLength - definitely better than READONCE for just existence check.
Please fix the PR title to SOLR-18551
also, will this be backported in 9.x branch?
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Lets refresh this javadoc as well?
dsmiley
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the PR template 1st line includes a JIRA URL with that is to be edited. Please retain that. Otherwise, as a reviewer I have extra steps to go look in JIRA beyond clicking a link.
gerlowskija
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https://issues.apache.org/jira/browse/SOLR-18551
Description
Prior to this commit, IndexFetcher's "slowFileExists" method checked existence by attempting to open and then close the file. When MMapDirectoryFactory was in use, this "open" creates an mmap that remains somewhat "leaked" until the DirectoryReader is closed, due to idiosyncracies in how Lucene pools mmaps into "arenas" based on their segment.
This caused a pretty significant mmap leak in later Solr 9.x releases. It's less likely to impact Solr 10.x or later versions due to changes in Lucene, but it's still possible.
Solution
This commit fixes the issue by reworking IndexFetcher's "exists" check. Now instead of opening the file, IndexFetcher does a size-check which can be used to determine existence with only an OS 'stat' call and without creating an mmap at all.
Tests
None. I'd like to write tests here, but I can't think of a good way to write an automated test for this. If anyone has ideas I'm very open to suggestions!
Checklist
Please review the following and check all that apply:
mainbranch../gradlew check.