More than 2 lock modes

[pwnall]

A recurring theme I've seen in database papers is extending the lock manager with new modes to get more parallelism. The most popular need is intent locks. At least BerkeleyDB [1] and MSSQL [2] implement more than S and X.

Fortunately, all extended lock schemes can be specified using a conflict matrix, as shown in [1]. I think it'd be more intuitive to use an inverted representation compared to [1]. Specifically, matrix[i][j] is 0 if there's a conflict when the lock has an owner in mode i and a new owner requests mode j.

We could offer IS/IX/S/X by default, and let apps register custom matrices. Each acquire would have an optional matrix name parameter, and it'd get bounced if the lock has exists and was created with a different matrix.

Registration seems a bit bulky, but each app (or, realistically, db library) would only make a couple of registration calls during initialization. Registration should be idempotent (registering the same matrix with an existing name would be OK, registering a different matrix with the same name would fail) so apps that (unknowingly) use multiple instances of the same DB library can work without problems.

[1] https://docs.oracle.com/cd/E17276_01/html/programmer_reference/lock_stdmode.html
[2] https://technet.microsoft.com/en-us/library/ms175519(v=sql.105).aspx#Intent Locks

[inexorabletash]

Thanks! This is really cool. Looks like something we could layer on easily.