miner: avoid unnecessary work after payload resolution

[jrhea]

Issue

In buildPayload(), the background goroutine uses a select to wait on the recommit timer, the stop channel, and the end timer. When both timer.C and payload.stop are ready simultaneously, Go's select picks a case non-deterministically. This means the loop can enter the timer.C case and perform an unnecessary generateWork call even after the payload has been resolved.

Fix

Add a non-blocking check of payload.stop at the top of the timer.C case to exit immediately when the payload has already been delivered.

[rjl493456442]

When both timer.C and payload.stop are ready simultaneously

It's a very rare situation which is not expected to happen frequently.

Besides, there is no severe consequence of the work generation, the work will be silently ignored if the payload.stop has been closed right?

I feel it's the unnecessary complexity.

[jrhea]

When both timer.C and payload.stop are ready simultaneously

It's a very rare situation which is not expected to happen frequently.

Besides, there is no severe consequence of the work generation, the work will be silently ignored if the payload.stop has been closed right?

I feel it's the unnecessary complexity.

It happened very frequently in the traces I was monitoring; otherwise I wouldn't have bothered. I kept seeing traces where the block was delivered on iteration N and the trace would continue for iteration N+1. If you think about it, it makes sense that it happens often. Let's say the miner is working on a block and spends the entire 2 seconds trying to stuff as many transactions as it can into the block. meanwhile the stop signal arrives somewhere in those 2 seconds....then it is a coin flip whether or not the miner goes into that extra iteration after the block has been delivered.

[jrhea]

Besides, there is no severe consequence of the work generation, the work will be silently ignored if the payload.stop has been closed right?

agreed that it will be silently ignored. this isn't a hill I am going to die on though