[stable]: backport #10691 and #10683 (#11143)

* Fix compiler warning (that will become an error) (#10683)

* Remove annoying compiler warnings

* Fix compiler warning (that will become an error)

Fixes #10648

I'm not sure this fix is Good™ but rustc seems happy enough.

There's a deeper issue which may or may not be related to this: the Engine is not shutdown properly and the `StepService` thread keeps running indefinitely after Ctrl-C (so `update_sealing()` is called repeatedly for 300sec). I don't think this is related to Clique as I've seen this happen on mainnet as well, but I wonder if the effects of it are worse for a PoA network where the node can create new blocks all on its own?

* Use saturating_sub

* WIP

* Fix warning, second attempt

The idea here is to avoid using `Arc::get_mut()` (which does not work: fails every time here) and instead trust `drop()` to do the right thing.

This is a conservative change. I think this can be reformed further, e.g. by `impl Drop for StepService` and halt the thread there, or even skip `join()`ing the thread entirely and trust the `AtomicBool` to signal shutdown. I also have doubts abut the `Option<StepService>`: seems a bit much to have an `Option` there and it makes things cumbersome.

* Refactor Clique stepping (#10691)

* Use Drop to shutdown stepper thread
Make period == 0 an error and remove the Option from step_service

* Remove StepService

Remove StepService and spawn the stepping thread in `Clique::new()`. Don't store the thread handle and instead trust the `AtomicBool` to signal shutdown time.
Don't check for `period > 0`: we assume a valid chainspec file.

* Don't shutdown the stepper thread at all, just let it run until exit
Also: fix a few warnings and tests

* Put kvdb_memorydb back

* Warn&exit when engine is dropped
Don't sleep too long!

* Don't delay stepping thread

* Better formatting

ethcore/src/client/client.rs

@@ -2558,16 +2558,6 @@ impl ProvingBlockChainClient for Client {
 
 impl SnapshotClient for Client {}
 
-impl Drop for Client {
-	fn drop(&mut self) {
-		if let Some(c) = Arc::get_mut(&mut self.engine) {
-			c.stop()
-		} else {
-			warn!(target: "shutdown", "unable to get mut ref for engine for shutdown.");
-		}
-	}
-}
-
 /// Returns `LocalizedReceipt` given `LocalizedTransaction`
 /// and a vector of receipts from given block up to transaction index.
 fn transaction_receipt(

ethcore/src/engines/clique/mod.rs

@@ -64,7 +64,7 @@ use std::collections::VecDeque;
 use std::sync::{Arc, Weak};
 use std::thread;
 use std::time;
-use std::time::{Duration, SystemTime, UNIX_EPOCH};
+use std::time::{Instant, Duration, SystemTime, UNIX_EPOCH};
 
 use block::ExecutedBlock;
 use bytes::Bytes;
@@ -88,11 +88,9 @@ use types::header::{ExtendedHeader, Header};
 
 use self::block_state::CliqueBlockState;
 use self::params::CliqueParams;
-use self::step_service::StepService;
 
 mod params;
 mod block_state;
-mod step_service;
 mod util;
 
 // TODO(niklasad1): extract tester types into a separate mod to be shared in the code base
@@ -167,7 +165,6 @@ pub struct Clique {
 	block_state_by_hash: RwLock<LruCache<H256, CliqueBlockState>>,
 	proposals: RwLock<HashMap<Address, VoteType>>,
 	signer: RwLock<Option<Box<EngineSigner>>>,
-	step_service: Option<Arc<StepService>>,
 }
 
 #[cfg(test)]
@@ -180,30 +177,45 @@ pub struct Clique {
 	pub block_state_by_hash: RwLock<LruCache<H256, CliqueBlockState>>,
 	pub proposals: RwLock<HashMap<Address, VoteType>>,
 	pub signer: RwLock<Option<Box<EngineSigner>>>,
-	pub step_service: Option<Arc<StepService>>,
 }
 
 impl Clique {
 	/// Initialize Clique engine from empty state.
-	pub fn new(our_params: CliqueParams, machine: EthereumMachine) -> Result<Arc<Self>, Error> {
-		let mut engine = Clique {
-			epoch_length: our_params.epoch,
-			period: our_params.period,
+	pub fn new(params: CliqueParams, machine: EthereumMachine) -> Result<Arc<Self>, Error> {
+		/// Step Clique at most every 2 seconds
+		const SEALING_FREQ: Duration = Duration::from_secs(2);
+
+		let engine = Clique {
+			epoch_length: params.epoch,
+			period: params.period,
 			client: Default::default(),
 			block_state_by_hash: RwLock::new(LruCache::new(STATE_CACHE_NUM)),
 			proposals: Default::default(),
 			signer: Default::default(),
 			machine,
-			step_service: None,
 		};
+		let engine = Arc::new(engine);
+		let weak_eng = Arc::downgrade(&engine);
 
-		let res = Arc::new(engine);
-
-		if our_params.period > 0 {
-			engine.step_service = Some(StepService::start(Arc::downgrade(&res) as Weak<Engine<_>>));
-		}
+		thread::Builder::new().name("StepService".into())
+			.spawn(move || {
+				loop {
+ 					let next_step_at = Instant::now() + SEALING_FREQ;
+					trace!(target: "miner", "StepService: triggering sealing");
+					if let Some(eng) = weak_eng.upgrade() {
+						eng.step()
+					} else {
+						warn!(target: "shutdown", "StepService: engine is dropped; exiting.");
+						break;
+					}
 
-		Ok(res)
+					let now = Instant::now();
+					if now < next_step_at {
+						thread::sleep(next_step_at - now);
+					}
+				}
+			})?;
+		Ok(engine)
 	}
 
 	#[cfg(test)]
@@ -221,7 +233,6 @@ impl Clique {
 			proposals: Default::default(),
 			signer: Default::default(),
 			machine: Spec::new_test_machine(),
-			step_service: None,
 		}
 	}
 
@@ -695,7 +706,7 @@ impl Engine<EthereumMachine> for Clique {
 			trace!(target: "engine", "populate_from_parent in sealing");
 
 			// It's unclear how to prevent creating new blocks unless we are authorized, the best way (and geth does this too)
-			// it's just to ignore setting an correct difficulty here, we will check authorization in next step in generate_seal anyway.
+			// it's just to ignore setting a correct difficulty here, we will check authorization in next step in generate_seal anyway.
 			if let Some(signer) = self.signer.read().as_ref() {
 				let state = match self.state(&parent) {
 					Err(e) =>  {
@@ -744,14 +755,6 @@ impl Engine<EthereumMachine> for Clique {
 		}
 	}
 
-	fn stop(&mut self) {
-		if let Some(mut s) = self.step_service.as_mut() {
-			Arc::get_mut(&mut s).map(|x| x.stop());
-		} else {
-			warn!(target: "engine", "Stopping `CliqueStepService` failed requires mutable access");
-		}
-	}
-
 	/// Clique timestamp is set to parent + period , or current time which ever is higher.
 	fn open_block_header_timestamp(&self, parent_timestamp: u64) -> u64 {
 		let now = time::SystemTime::now().duration_since(time::UNIX_EPOCH).unwrap_or_default();

ethcore/src/engines/mod.rs

@@ -425,9 +425,6 @@ pub trait Engine<M: Machine>: Sync + Send {
 	/// Trigger next step of the consensus engine.
 	fn step(&self) {}
 
-	/// Stops any services that the may hold the Engine and makes it safe to drop.
-	fn stop(&mut self) {}
-
 	/// Create a factory for building snapshot chunks and restoring from them.
 	/// Returning `None` indicates that this engine doesn't support snapshot creation.
 	fn snapshot_components(&self) -> Option<Box<SnapshotComponents>> {