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2014 Aug 20
6
Full MVCC in Brass
...On Unix platforms, Xapian already uses fcntl byte range locking to
lock byte 0 of "flintlock" to protect against concurrent writers.
Create a new "brasslock" that extends this to use byte R+1 to
represent a lock on revision R.
1) In the writer: Lock byte 0 like usual. Then let RSAFE be the
current database revision minus 1 (omitting the latest revision
ensures it's available to readers that start during this writer.) Use
F_GETLK to test for conflicting shared locks on revision range [0,
RSAFE]. If F_GETLK returns a conflicting lock, set RSAFE to one less
than the lower b...