Speed up big chainspec json(~1.5 GB) load (paritytech#10137)

* Speed up chainspec json load

* Update client/chain-spec/src/chain_spec.rs

* Update client/chain-spec/src/chain_spec.rs

* Update client/chain-spec/src/chain_spec.rs

* Load the chainspec through `mmap`

Co-authored-by: icodezjb <icodezjb@users.noreply.github.com>
Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Jan Bujak <jan@parity.io>

client/chain-spec/Cargo.toml

@@ -22,3 +22,4 @@ serde_json = "1.0.68"
 sp-runtime = { version = "4.0.0-dev", path = "../../primitives/runtime" }
 sc-telemetry = { version = "4.0.0-dev", path = "../telemetry" }
 codec = { package = "parity-scale-codec", version = "2.0.0" }
+memmap2 = "0.5.0"

client/chain-spec/src/chain_spec.rs

@@ -285,10 +285,20 @@ impl<G, E: serde::de::DeserializeOwned> ChainSpec<G, E> {
 
 	/// Parse json file into a `ChainSpec`
 	pub fn from_json_file(path: PathBuf) -> Result<Self, String> {
+		// We mmap the file into memory first, as this is *a lot* faster than using
+		// `serde_json::from_reader`. See https://github.com/serde-rs/json/issues/160
 		let file = File::open(&path)
 			.map_err(|e| format!("Error opening spec file `{}`: {}", path.display(), e))?;
+
+		// SAFETY: `mmap` is fundamentally unsafe since technically the file can change
+		//         underneath us while it is mapped; in practice it's unlikely to be a problem
+		let bytes = unsafe {
+			memmap2::Mmap::map(&file)
+				.map_err(|e| format!("Error mmaping spec file `{}`: {}", path.display(), e))?
+		};
+
 		let client_spec =
-			json::from_reader(file).map_err(|e| format!("Error parsing spec file: {}", e))?;
+			json::from_slice(&bytes).map_err(|e| format!("Error parsing spec file: {}", e))?;
 		Ok(ChainSpec { client_spec, genesis: GenesisSource::File(path) })
 	}
 }