Print warnings when fetching pending blocks (openethereum#8711)

* Lots of println to figure out what eth_getBlockByNumber does/should do

* Remove debugging

* Print warnings when fetching pending blocks

When calling `eth_getBlockByNumber` with `pending`, we now print a deprecation warning and:

* if a pending block is found, use it to respond
* if no pending block is found, respond as if if was a request for `Latest`

Addresses issue openethereum#8703 (not sure if it's enough to close it tbh)

ethcore/src/miner/miner.rs

@@ -290,10 +290,12 @@ impl Miner {
 	{
 		self.sealing.lock().queue
 			.peek_last_ref()
-			.and_then(|b| if b.block().header().number() > latest_block_number {
-				Some(f(b))
-			} else {
-				None
+			.and_then(|b| {
+				if b.block().header().number() > latest_block_number {
+					Some(f(b))
+				} else {
+					None
+				}
 			})
 	}
 

rpc/src/v1/impls/eth.rs

@@ -108,6 +108,7 @@ pub struct EthClient<C, SN: ?Sized, S: ?Sized, M, EM> where
 	eip86_transition: u64,
 }
 
+#[derive(Debug)]
 enum BlockNumberOrId {
 	Number(BlockNumber),
 	Id(BlockId),
@@ -184,29 +185,38 @@ impl<C, SN: ?Sized, S: ?Sized, M, EM, T: StateInfo + 'static> EthClient<C, SN, S
 		let (block, difficulty, extra, is_pending) = match id {
 			BlockNumberOrId::Number(BlockNumber::Pending) => {
 				let info = self.client.chain_info();
-				let pending_block = self.miner.pending_block(info.best_block_number);
-				let difficulty = {
-					let latest_difficulty = self.client.block_total_difficulty(BlockId::Latest).expect("blocks in chain have details; qed");
-					let pending_difficulty = self.miner.pending_block_header(info.best_block_number).map(|header| *header.difficulty());
+				match self.miner.pending_block(info.best_block_number) {
+					Some(pending_block) => {
+						warn!("`Pending` is deprecated and may be removed in future versions.");
 
-				 	if let Some(difficulty) = pending_difficulty {
-						difficulty + latest_difficulty
-					} else {
-						latest_difficulty
-					}
-				};
+						let difficulty = {
+							let latest_difficulty = self.client.block_total_difficulty(BlockId::Latest).expect("blocks in chain have details; qed");
+							let pending_difficulty = self.miner.pending_block_header(info.best_block_number).map(|header| *header.difficulty());
+
+							if let Some(difficulty) = pending_difficulty {
+								difficulty + latest_difficulty
+							} else {
+								latest_difficulty
+							}
+						};
 
-				let extra = pending_block.as_ref().map(|b| self.client.engine().extra_info(&b.header));
+						let extra = self.client.engine().extra_info(&pending_block.header);
 
-				(pending_block.map(|b| encoded::Block::new(b.rlp_bytes())), Some(difficulty), extra, true)
+						(Some(encoded::Block::new(pending_block.rlp_bytes())), Some(difficulty), Some(extra), true)
+					},
+					None => {
+						warn!("`Pending` is deprecated and may be removed in future versions. Falling back to `Latest`");
+						client_query(BlockId::Latest)
+					}
+				}
 			},
 
 			BlockNumberOrId::Number(num) => {
 				let id = match num {
 					BlockNumber::Latest => BlockId::Latest,
 					BlockNumber::Earliest => BlockId::Earliest,
 					BlockNumber::Num(n) => BlockId::Number(n),
-					BlockNumber::Pending => unreachable!(), // Already covered
+					BlockNumber::Pending => unreachable!() // Already covered
 				};
 
 				client_query(id)