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SQLITE3_UNLOCK_NOTIFY(3)   Library Functions Manual   SQLITE3_UNLOCK_NOTIFY(3)

     sqlite3_unlock_notify - Unlock Notification

     sqlite3_unlock_notify(sqlite3 *pBlocked,
         void (*xNotify)(void **apArg, int nArg),
         void *pNotifyArg                            );

     When running in shared-cache mode, a database operation may fail with an
     SQLITE_LOCKED error if the required locks on the shared-cache or
     individual tables within the shared-cache cannot be obtained.  See SQLite
     Shared-Cache Mode for a description of shared-cache locking.  This API
     may be used to register a callback that SQLite will invoke when the
     connection currently holding the required lock relinquishes it.  This API
     is only available if the library was compiled with the
     SQLITE_ENABLE_UNLOCK_NOTIFY C-preprocessor symbol defined.

     Shared-cache locks are released when a database connection concludes its
     current transaction, either by committing it or rolling it back.

     When a connection (known as the blocked connection) fails to obtain a
     shared-cache lock and SQLITE_LOCKED is returned to the caller, the
     identity of the database connection (the blocking connection) that has
     locked the required resource is stored internally.  After an application
     receives an SQLITE_LOCKED error, it may call the sqlite3_unlock_notify()
     method with the blocked connection handle as the first argument to
     register for a callback that will be invoked when the blocking
     connections current transaction is concluded.  The callback is invoked
     from within the sqlite3_step or sqlite3_close call that concludes the
     blocking connections transaction.

     If sqlite3_unlock_notify() is called in a multi-threaded application,
     there is a chance that the blocking connection will have already
     concluded its transaction by the time sqlite3_unlock_notify() is invoked.
     If this happens, then the specified callback is invoked immediately, from
     within the call to sqlite3_unlock_notify().

     If the blocked connection is attempting to obtain a write-lock on a
     shared-cache table, and more than one other connection currently holds a
     read-lock on the same table, then SQLite arbitrarily selects one of the
     other connections to use as the blocking connection.

     There may be at most one unlock-notify callback registered by a blocked
     connection.  If sqlite3_unlock_notify() is called when the blocked
     connection already has a registered unlock-notify callback, then the new
     callback replaces the old.  If sqlite3_unlock_notify() is called with a
     NULL pointer as its second argument, then any existing unlock-notify
     callback is canceled.  The blocked connections unlock-notify callback may
     also be canceled by closing the blocked connection using sqlite3_close().

     The unlock-notify callback is not reentrant.  If an application invokes
     any sqlite3_xxx API functions from within an unlock-notify callback, a
     crash or deadlock may be the result.

     Unless deadlock is detected (see below), sqlite3_unlock_notify() always
     returns SQLITE_OK.

     Callback Invocation Details

     When an unlock-notify callback is registered, the application provides a
     single void* pointer that is passed to the callback when it is invoked.
     However, the signature of the callback function allows SQLite to pass it
     an array of void* context pointers.  The first argument passed to an
     unlock-notify callback is a pointer to an array of void* pointers, and
     the second is the number of entries in the array.

     When a blocking connections transaction is concluded, there may be more
     than one blocked connection that has registered for an unlock-notify
     callback.  If two or more such blocked connections have specified the
     same callback function, then instead of invoking the callback function
     multiple times, it is invoked once with the set of void* context pointers
     specified by the blocked connections bundled together into an array.
     This gives the application an opportunity to prioritize any actions
     related to the set of unblocked database connections.

     Deadlock Detection

     Assuming that after registering for an unlock-notify callback a database
     waits for the callback to be issued before taking any further action (a
     reasonable assumption), then using this API may cause the application to
     deadlock.  For example, if connection X is waiting for connection Y's
     transaction to be concluded, and similarly connection Y is waiting on
     connection X's transaction, then neither connection will proceed and the
     system may remain deadlocked indefinitely.

     To avoid this scenario, the sqlite3_unlock_notify() performs deadlock
     detection.  If a given call to sqlite3_unlock_notify() would put the
     system in a deadlocked state, then SQLITE_LOCKED is returned and no
     unlock-notify callback is registered.  The system is said to be in a
     deadlocked state if connection A has registered for an unlock-notify
     callback on the conclusion of connection B's transaction, and connection
     B has itself registered for an unlock-notify callback when connection A's
     transaction is concluded.  Indirect deadlock is also detected, so the
     system is also considered to be deadlocked if connection B has registered
     for an unlock-notify callback on the conclusion of connection C's
     transaction, where connection C is waiting on connection A.  Any number
     of levels of indirection are allowed.

     The "DROP TABLE" Exception

     When a call to sqlite3_step() returns SQLITE_LOCKED, it is almost always
     appropriate to call sqlite3_unlock_notify().  There is however, one
     exception.  When executing a "DROP TABLE" or "DROP INDEX" statement,
     SQLite checks if there are any currently executing SELECT statements that
     belong to the same connection.  If there are, SQLITE_LOCKED is returned.
     In this case there is no "blocking connection", so invoking
     sqlite3_unlock_notify() results in the unlock-notify callback being
     invoked immediately.  If the application then re-attempts the "DROP
     TABLE" or "DROP INDEX" query, an infinite loop might be the result.

     One way around this problem is to check the extended error code returned
     by an sqlite3_step() call.  If there is a blocking connection, then the
     extended error code is set to SQLITE_LOCKED_SHAREDCACHE.  Otherwise, in
     the special "DROP TABLE/INDEX" case, the extended error code is just

     sqlite3_close(3), sqlite3_step(3), SQLITE_OK(3)

NetBSD 8.0                      March 11, 2017                      NetBSD 8.0