Fix export when starting from block 0 (#406)

* More explicit variable names

* Fix export when from block is 0

Fix #402

substrate/polkadot/cli/src/lib.rs

@@ -376,18 +376,22 @@ fn export_blocks<E>(matches: &clap::ArgMatches, exit: E) -> error::Result<()>
 		let _ = exit.wait();
 		let _ = exit_send.send(());
 	});
-	info!("Exporting blocks");
-	let mut block: u32 = match matches.value_of("from") {
+	let mut from_block: u32 = match matches.value_of("from") {
 		Some(v) => v.parse().map_err(|_| "Invalid --from argument")?,
 		None => 1,
 	};
 
-	let last = match matches.value_of("to") {
+	if from_block < 1 {
+		from_block = 1;
+	}
+
+	let to_block = match matches.value_of("to") {
 		Some(v) => v.parse().map_err(|_| "Invalid --to argument")?,
 		None => client.info()?.chain.best_number as u32,
 	};
+	info!("Exporting blocks from #{} to #{}", from_block, to_block);
 
-	if last < block {
+	if to_block < from_block {
 		return Err("Invalid block range specified".into());
 	}
 
@@ -399,30 +403,30 @@ fn export_blocks<E>(matches: &clap::ArgMatches, exit: E) -> error::Result<()>
 	};
 
 	if !json {
-		file.write(&(last - block + 1).encode())?;
+		file.write(&(to_block - from_block + 1).encode())?;
 	}
 
 	loop {
 		if exit_recv.try_recv().is_ok() {
 			break;
 		}
-		match client.block(&BlockId::number(block as u64))? {
-			Some(block) => {
+		match client.block(&BlockId::number(from_block as u64))? {
+			Some(from_block) => {
 				if json {
-					serde_json::to_writer(&mut *file, &block).map_err(|e| format!("Eror writing JSON: {}", e))?;
+					serde_json::to_writer(&mut *file, &from_block).map_err(|e| format!("Eror writing JSON: {}", e))?;
 				} else {
-					file.write(&block.encode())?;
+					file.write(&from_block.encode())?;
 				}
 			},
 			None => break,
 		}
-		if block % 10000 == 0 {
-			info!("#{}", block);
+		if from_block % 10000 == 0 {
+			info!("#{}", from_block);
 		}
-		if block == last {
+		if from_block == to_block {
 			break;
 		}
-		block += 1;
+		from_block += 1;
 	}
 	Ok(())
 }