Improve CheckMetadataHash: make it constant time and compile error on wrong env variable

Cargo.toml

@@ -683,6 +683,7 @@ color-print = { version = "0.3.4" }
 colored = { version = "2.0.4" }
 comfy-table = { version = "7.1.0", default-features = false }
 console = { version = "0.15.8" }
+const-hex = { version = "1.10.0", default-features = false }
 contracts-rococo-runtime = { path = "cumulus/parachains/runtimes/contracts/contracts-rococo" }
 coretime-rococo-emulated-chain = { path = "cumulus/parachains/integration-tests/emulated/chains/parachains/coretime/coretime-rococo" }
 coretime-rococo-runtime = { path = "cumulus/parachains/runtimes/coretime/coretime-rococo" }

prdoc/pr_6141.prdoc

@@ -0,0 +1,11 @@
+title: Improve `CheckMetadataHash` transaction extension weight and logic
+
+doc:
+  - audience: Runtime Dev
+    description: |
+      The compilation now panics if the optional compile-time environment variable `RUNTIME_METADATA_HASH` contains an invalid value.
+      The weight for the `CheckMetadataHash` transaction extension is more accurate as it is almost compile-time.
+
+crates:
+- name: frame-metadata-hash-extension
+  bump: minor

substrate/frame/metadata-hash-extension/Cargo.toml

@@ -17,6 +17,7 @@ frame-support = { workspace = true }
 frame-system = { workspace = true }
 log = { workspace = true }
 docify = { workspace = true }
+const-hex = { workspace = true }
 
 [dev-dependencies]
 substrate-wasm-builder = { features = ["metadata-hash"], workspace = true, default-features = true }
@@ -31,6 +32,7 @@ sp-tracing = { workspace = true, default-features = true }
 default = ["std"]
 std = [
 	"codec/std",
+	"const-hex/std",
 	"frame-support/std",
 	"frame-system/std",
 	"log/std",

substrate/frame/metadata-hash-extension/src/lib.rs

@@ -39,7 +39,7 @@ extern crate alloc;
 extern crate self as frame_metadata_hash_extension;
 
 use codec::{Decode, Encode};
-use frame_support::DebugNoBound;
+use frame_support::{pallet_prelude::Weight, DebugNoBound};
 use frame_system::Config;
 use scale_info::TypeInfo;
 use sp_runtime::{
@@ -68,12 +68,24 @@ enum MetadataHash {
 	Custom([u8; 32]),
 }
 
+const RUNTIME_METADATA: Option<[u8; 32]> = if let Some(hex) = option_env!("RUNTIME_METADATA_HASH") {
+	match const_hex::const_decode_to_array(hex.as_bytes()) {
+		Ok(hex) => Some(hex),
+		Err(_) => panic!(
+			"Invalid RUNTIME_METADATA_HASH environment variable: it must be a 32 \
+			bytes value in hexadecimal: e.g. 0x123ABCabd...123ABCabc. Upper case or lower case, \
+			0x prefix is optional."
+		),
+	}
+} else {
+	None
+};
+
 impl MetadataHash {
 	/// Returns the metadata hash.
 	fn hash(&self) -> Option<[u8; 32]> {
 		match self {
-			Self::FetchFromEnv =>
-				option_env!("RUNTIME_METADATA_HASH").map(array_bytes::hex2array_unchecked),
+			Self::FetchFromEnv => RUNTIME_METADATA,
 			Self::Custom(hash) => Some(*hash),
 		}
 	}
@@ -155,5 +167,11 @@ impl<T: Config + Send + Sync> TransactionExtension<T::RuntimeCall> for CheckMeta
 	type Val = ();
 	type Pre = ();
 
-	impl_tx_ext_default!(T::RuntimeCall; weight validate prepare);
+	fn weight(&self, _: &T::RuntimeCall) -> Weight {
+		// The weight is the weight of implicit, it consists of a few match operation, it is
+		// negligible.
+		Weight::zero()
+	}
+
+	impl_tx_ext_default!(T::RuntimeCall; validate prepare);
 }