consensus, eth: don't CPU mine by default during remote mining

consensus/ethash/ethash.go

@@ -555,11 +555,18 @@ func (ethash *Ethash) Threads() int {
 
 // SetThreads updates the number of mining threads currently enabled. Calling
 // this method does not start mining, only sets the thread count. If zero is
-// specified, the miner will use all cores of the machine.
+// specified, the miner will use all cores of the machine. Setting a thread
+// count below zero is allowed and will cause the miner to idle, without any
+// work being done.
 func (ethash *Ethash) SetThreads(threads int) {
 	ethash.lock.Lock()
 	defer ethash.lock.Unlock()
 
+	// If we're running a shared PoW, set the thread count on that instead
+	if ethash.shared != nil {
+		ethash.shared.SetThreads(threads)
+		return
+	}
 	// Update the threads and ping any running seal to pull in any changes
 	ethash.threads = threads
 	select {

consensus/ethash/sealer.go

@@ -61,6 +61,9 @@ func (ethash *Ethash) Seal(chain consensus.ChainReader, block *types.Block, stop
 	if threads == 0 {
 		threads = runtime.NumCPU()
 	}
+	if threads < 0 {
+		threads = 0 // Allows disabling local mining without extra logic around local/remote
+	}
 	var pend sync.WaitGroup
 	for i := 0; i < threads; i++ {
 		pend.Add(1)