Patch to support the Unlock Notify feature for shared-cache mode: blocking prepare and step

Originally reported on Google Code with ID 36

I've just begun to investigate using sqlite4java as a database backend for a web application.

I noticed in shared cache mode that I was getting a lot SQLITE_LOCKED errors.

I realized I needed sqlite3_unlock_notify() to implement blocking prepare() and step()
and that sqlite4java hadn't implemented it yet.

So here's a patch for it!

I implemented it based on the documentation/code presented at http://www.sqlite.org/unlock_notify.html.

Here's how to enable blocking prepare() and step() (I added Javadoc documention too):

SQLiteConnection db = new SQLiteConnection(new File("test.db"));

// must be in shared cache mode to use unlock notify blocking
db.openV2(SQLITE_OPEN_READWRITE | SQLITE_OPEN_CREATE | SQLITE_OPEN_SHAREDCACHE);

// call setBlocking(true) to turn on blocking of prepare() and step()
db.setBlocking(true);

// prepare() and step() will now attempt to block when locked in accordance with sqlite3_unlock_notify()
documentation

I have tested on my web application in a test environment and I no longer get the SQLITE_LOCKED
(or SQLITE_LOCKED_SHARECACHE) errors so it's looking good, but granted it hasn't been
battle-tested yet in any kind of high-traffic production environment.

It's my hope you can merge this into the mainline after reviewing it.

Reported by bkoehm on 2011-09-22 21:39:20

<hr> * Attachment: 20110922-unlockNotifyPatch.diff

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