Add Error message when sync is still in progress.

parity/cli/mod.rs

@@ -463,6 +463,10 @@ usage! {
 
 
 		["API and Console Options – HTTP JSON-RPC"]
+			FLAG flag_allow_missing_blocks: (bool) = false, or |c: &Config| c.rpc.as_ref()?.allow_missing_blocks.clone(),
+			"--allow-missing-blocks",
+			"RPC calls will return 'null' instead of an error if ancient block sync is still in progress and the block information requested could not be found",
+
 			FLAG flag_no_jsonrpc: (bool) = false, or |c: &Config| c.rpc.as_ref()?.disable.clone(),
 			"--no-jsonrpc",
 			"Disable the HTTP JSON-RPC API server.",
@@ -1230,6 +1234,7 @@ struct Rpc {
 	keep_alive: Option<bool>,
 	experimental_rpcs: Option<bool>,
 	poll_lifetime: Option<u32>,
+	allow_missing_blocks: Option<bool>,
 }
 
 #[derive(Default, Debug, PartialEq, Deserialize)]
@@ -1697,6 +1702,7 @@ mod tests {
 			arg_jsonrpc_threads: 4,
 			arg_jsonrpc_max_payload: None,
 			arg_poll_lifetime: 60u32,
+			flag_allow_missing_blocks: false,
 
 			// WS
 			flag_no_ws: false,
@@ -1973,6 +1979,7 @@ mod tests {
 				keep_alive: None,
 				experimental_rpcs: None,
 				poll_lifetime: None,
+				allow_missing_blocks: None
 			}),
 			ipc: Some(Ipc {
 				disable: None,

parity/cli/tests/config.full.toml

@@ -64,6 +64,7 @@ interface = "local"
 cors = ["null"]
 apis = ["web3", "eth", "net", "parity", "traces", "rpc", "secretstore"]
 hosts = ["none"]
+allow_missing_blocks = false
 
 [websockets]
 disable = false

parity/configuration.rs

@@ -370,6 +370,7 @@ impl Configuration {
 				miner_extras: self.miner_extras()?,
 				stratum: self.stratum_options()?,
 				update_policy: update_policy,
+				allow_missing_blocks: self.args.flag_allow_missing_blocks,
 				mode: mode,
 				tracing: tracing,
 				fat_db: fat_db,
@@ -1390,6 +1391,7 @@ mod tests {
 		let args = vec!["parity"];
 		let conf = parse(&args);
 		let mut expected = RunCmd {
+			allow_missing_blocks: false,
 			cache_config: Default::default(),
 			dirs: Default::default(),
 			spec: Default::default(),