miner: avoid unnecessary work after payload resolution (#33943)

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.

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.

miner/payload_building.go

@@ -260,6 +260,17 @@ func (miner *Miner) buildPayload(args *BuildPayloadArgs, witness bool) (*Payload
 		for {
 			select {
 			case <-timer.C:
+				// When block building takes close to the full recommit interval,
+				// the timer fires near-instantly on the next iteration. If the
+				// payload was resolved during that build, both timer.C and
+				// payload.stop are ready and Go's select picks one at random.
+				// Check payload.stop first to avoid an unnecessary generateWork.
+				select {
+				case <-payload.stop:
+					log.Info("Stopping work on payload", "id", payload.id, "reason", "delivery")
+					return
+				default:
+				}
 				start := time.Now()
 				r := miner.generateWork(fullParams, witness)
 				if r.err == nil {