Watching a statement while it runs - #22

rowsRead is how far the statement running now has got, and progress is a timer around it. A statement that takes a minute is one somebody is sitting in front of, and this client had nothing to tell them while it ran.

using watch = conn.progress((rows) => process.stdout.write(`\r${rows} rows read`));
const answer = await conn.query(statement);

The count is read the way open is, as an atomic beside the connection's lock rather than a question through it, so it answers on the loop's thread while the statement holds the lock on a threadpool thread. It counts rows read out of storage rather than rows answered, because the statement somebody is waiting on is exactly the one that reads a hundred million rows to answer one.

The watch itself is JavaScript, in zudb.cjs beside stream. A timer is all it takes, since the loop is free while the statement runs, and nothing calls into JavaScript from the thread doing the scanning: a thread safe function there would be a scan that stops to talk to the loop. The callback runs only when the count has moved, so a watch on an idle connection says nothing and one nobody stopped is not a callback ten times a second forever, and the timer does not hold the process open. It is stopped by stop() or by leaving the scope of the using, and the interval is { everyMs } with a tenth of a second by default.

One thing turned up on the way. The counter was never put back down between statements here, so it had been counting a connection's whole life rather than one statement, which is neither what Python does nor what the shell does. It is cleared now where the connection becomes a statement's, which is the one moment that can be done without racing the getter, and the three places that happens are the query, the stream and the profile. So conn.rowsRead after a statement is what that statement cost.

Ten tests, including that the number moves during the statement rather than after it, that a watch on an idle connection stays quiet, that a stopped one stops, that a using stops it at the end of the block, and that a program with a watch nobody stopped still exits. The whole suite is green locally, with the types, the API report and the formatting.