131 lines
5.4 KiB
Plaintext
131 lines
5.4 KiB
Plaintext
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The 5 states of an historical rollback lock as implemented by the
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xLock, xUnlock, and xCheckReservedLock methods of the sqlite3_io_methods
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objec are:
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UNLOCKED
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SHARED
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RESERVED
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PENDING
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EXCLUSIVE
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The wal-index file has a similar locking hierarchy implemented using
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the xShmLock method of the sqlite3_vfs object, but with 7
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states. Each connection to a wal-index file must be in one of
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the following 7 states:
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UNLOCKED
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READ
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READ_FULL
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WRITE
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PENDING
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CHECKPOINT
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RECOVER
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These roughly correspond to the 5 states of a rollback lock except
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that SHARED is split out into 2 states: READ and READ_FULL and
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there is an extra RECOVER state used for wal-index reconstruction.
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The meanings of the various wal-index locking states is as follows:
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UNLOCKED - The wal-index is not in use.
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READ - Some prefix of the wal-index is being read. Additional
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wal-index information can be appended at any time. The
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newly appended content will be ignored by the holder of
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the READ lock.
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READ_FULL - The entire wal-index is being read. No new information
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can be added to the wal-index. The holder of a READ_FULL
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lock promises never to read pages from the database file
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that are available anywhere in the wal-index.
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WRITE - It is OK to append to the wal-index file and to adjust
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the header to indicate the new "last valid frame".
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PENDING - Waiting on all READ locks to clear so that a
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CHECKPOINT lock can be acquired.
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CHECKPOINT - It is OK to write any WAL data into the database file
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and zero the last valid frame field of the wal-index
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header. The wal-index file itself may not be changed
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other than to zero the last valid frame field in the
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header.
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RECOVER - Held during wal-index recovery. Used to prevent a
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race if multiple clients try to recover a wal-index at
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the same time.
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A particular lock manager implementation may coalesce one or more of
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the wal-index locking states, though with a reduction in concurrency.
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For example, an implemention might implement only exclusive locking,
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in which case all states would be equivalent to CHECKPOINT, meaning that
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only one reader or one writer or one checkpointer could be active at a
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time. Or, an implementation might combine READ and READ_FULL into
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a single state equivalent to READ, meaning that a writer could
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coexist with a reader, but no reader or writers could coexist with a
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checkpointer.
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The lock manager must obey the following rules:
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(1) A READ cannot coexist with CHECKPOINT.
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(2) A READ_FULL cannot coexist with WRITE.
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(3) None of WRITE, PENDING, CHECKPOINT, or RECOVER can coexist.
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The SQLite core will obey the next set of rules. These rules are
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assertions on the behavior of the SQLite core which might be verified
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during testing using an instrumented lock manager.
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(5) No part of the wal-index will be read without holding either some
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kind of SHM lock or an EXCLUSIVE lock on the original database.
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The original database is the file named in the 2nd parameter to
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the xShmOpen method.
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(6) A holder of a READ_FULL will never read any page of the database
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file that is contained anywhere in the wal-index.
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(7) No part of the wal-index other than the header will be written nor
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will the size of the wal-index grow without holding a WRITE or
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an EXCLUSIVE on the original database file.
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(8) The wal-index header will not be written without holding one of
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WRITE, CHECKPOINT, or RECOVER on the wal-index or an EXCLUSIVE on
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the original database files.
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(9) A CHECKPOINT or RECOVER must be held on the wal-index, or an
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EXCLUSIVE on the original database file, in order to reset the
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last valid frame counter in the header of the wal-index back to zero.
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(10) A WRITE can only increase the last valid frame pointer in the header.
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The SQLite core will only ever send requests for UNLOCK, READ, WRITE,
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CHECKPOINT, or RECOVER to the lock manager. The SQLite core will never
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request a READ_FULL or PENDING lock though the lock manager may deliver
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those locking states in response to READ and CHECKPOINT requests,
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respectively, if and only if the requested READ or CHECKPOINT cannot
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be delivered.
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The following are the allowed lock transitions:
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Original-State Request New-State
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-------------- ---------- ----------
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(11a) UNLOCK READ READ
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(11b) UNLOCK READ READ_FULL
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(11c) UNLOCK CHECKPOINT PENDING
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(11d) UNLOCK CHECKPOINT CHECKPOINT
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(11e) READ UNLOCK UNLOCK
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(11f) READ WRITE WRITE
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(11g) READ RECOVER RECOVER
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(11h) READ_FULL UNLOCK UNLOCK
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(11i) READ_FULL WRITE WRITE
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(11j) READ_FULL RECOVER RECOVER
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(11k) WRITE READ READ
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(11l) PENDING UNLOCK UNLOCK
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(11m) PENDING CHECKPOINT CHECKPOINT
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(11n) CHECKPOINT UNLOCK UNLOCK
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(11o) RECOVER READ READ
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These 15 transitions are all that needs to be supported. The lock
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manager implementation can assert that fact. The other 27 possible
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transitions among the 7 locking states will never occur.
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