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https://issues.apache.org/jira/browse/LUCY-119?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Marvin Humphrey resolved LUCY-119.
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Resolution: Fixed
Committed as r957776.
> Lucy::Store::Lock
> -----------------
>
> Key: LUCY-119
> URL: https://issues.apache.org/jira/browse/LUCY-119
> Project: Lucy
> Issue Type: New Feature
> Components: Core - Store
> Reporter: Marvin Humphrey
> Assignee: Marvin Humphrey
> Priority: Blocker
> Attachments: lock.patch
>
>
> This patch provides four classes:
> * Lock - abstract base class
> * LockFileLock - implementation of Lock using lockfiles.
> * SharedLock - read locks.
> * LockErr - Subclass of Err allowing users to catch locking exceptions.
> The lockfiles are encoded as JSON and contain both a PID and a host. Lock's
> Clear_Stale() method will only zap lockfiles if "host" matches and the PID is
> inactive. However, the default for "host" is empty string, since working on a
> shared volume is not the common case and we don't want to force everyone to
> supply it all the time. Omitting host is perfectly safe for indexes on local
> volumes, but it's an unsafe default for applications designed to work with
> shared
> volumes; such apps must be careful that each machine is supplies a unique
> value for "host".
> This locking implementation of also provides support for read locks. Right
> now, each read lock is a separate file. In the future, we should change this
> to use hard linking instead, drawing on algorithms from the Linux liblockfile
> package.
> A native implementation of Lock is theoretically possible if the system
> supports true flock() semantics, including refcounting. However, certain
> systems (e.g. Solaris) fake up flock() support using fcntl() locks -- and
> fcntl() locks have terrible behavior under fork().
> Lucene today uses native locking by default. The implementation has been
> carefully worked out so that it works under fcntl semantics, and it is assumed
> that nobody's going to fork() a Lucene process:
> [https://issues.apache.org/jira/browse/LUCENE-1877?focusedCommentId=12749363&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#action_12749363]
> Lucy in contrast, absolutely must support fork(), so it cannot default to
> using native locks when fcntl semantics are in effect.
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