fix(Transactions): Fix resource consumption on empty write transaction

[Zach-Johnson]

Problem

If a write transaction is opened without doing any updates, e.g.

txn := db.NewTransaction(true)
txn.Commit()

the read mark is never marked done, because txn.Discard has not been called and neither has the commit logic because of the early return in Commit():

if len(txn.pendingWrites) == 0 {
    return nil // Nothing to do.
}

This causes unbounded storage growth until the service is restarted. The watermark process in y/watermark.go never receives a mark for that transaction, and so never updates the DoneUntil. This value is used in levels.go for compaction to determine the discardTS value, and since it never updates, compaction can never occur.

Solution

The solution here is to call txn.Discard in the case when there are no writes on the transaction. This marks the read as done and allows the watermark process to properly update.

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[harshil-goel]

Great find!

[Zach-Johnson]

Great find!

Thanks @harshil-goel - is it possible to get a release cut for this sometime soon?

[harshil-goel]

If all the tests pass, it will be a part of the next release