update host fn to panic if header is not available and update misleading comment and some minor nits

Author: vedhavyas

Cargo.lock

@@ -19,7 +19,6 @@ targets = ["x86_64-unknown-linux-gnu"]
 codec = { package = "parity-scale-codec", version = "3.6.5", default-features = false, features = ["derive"] }
 frame-support = { version = "4.0.0-dev", default-features = false, git = "https://github.com/subspace/polkadot-sdk", rev = "d6b500960579d73c43fc4ef550b703acfa61c4c8" }
 frame-system = { version = "4.0.0-dev", default-features = false, git = "https://github.com/subspace/polkadot-sdk", rev = "d6b500960579d73c43fc4ef550b703acfa61c4c8" }
-log = { version = "0.4.20", default-features = false }
 scale-info = { version = "2.7.0", default-features = false, features = ["derive"] }
 sp-core = { version = "21.0.0", default-features = false, git = "https://github.com/subspace/polkadot-sdk", rev = "d6b500960579d73c43fc4ef550b703acfa61c4c8" }
 sp-mmr-primitives = { version = "4.0.0-dev", default-features = false, git = "https://github.com/subspace/polkadot-sdk", rev = "d6b500960579d73c43fc4ef550b703acfa61c4c8" }
@@ -33,7 +32,6 @@ std = [
     "codec/std",
     "frame-support/std",
     "frame-system/std",
-    "log/std",
     "scale-info/std",
     "sp-core/std",
     "sp-mmr-primitives/std",

crates/pallet-subspace-mmr/Cargo.toml

@@ -19,11 +19,10 @@
 #![feature(associated_type_bounds)]
 
 use frame_system::pallet_prelude::BlockNumberFor;
-use log::error;
 pub use pallet::*;
 use sp_mmr_primitives::{LeafDataProvider, OnNewRoot};
-use sp_runtime::traits::One;
-use sp_runtime::{DigestItem, Saturating};
+use sp_runtime::traits::{CheckedSub, One};
+use sp_runtime::DigestItem;
 use sp_subspace_mmr::subspace_mmr_runtime_interface::get_mmr_leaf_data;
 use sp_subspace_mmr::{LeafDataV0, MmrDigest, MmrLeaf};
 
@@ -52,16 +51,12 @@ impl<T: Config> LeafDataProvider for Pallet<T> {
     type LeafData = MmrLeaf<BlockNumberFor<T>, T::Hash>;
 
     fn leaf_data() -> Self::LeafData {
-        let block_number = frame_system::Pallet::<T>::block_number().saturating_sub(One::one());
+        let block_number = frame_system::Pallet::<T>::block_number()
+            .checked_sub(&One::one())
+            .expect("`block_number` will always be >= 1; qed");
         let block_hash = frame_system::Pallet::<T>::parent_hash();
-        // unfortunately, we Leaf data provider trait expects the impl to be infallible
-        // but our host function might fail for any reason but practically shouldn't since
-        // we are querying the immediate parent block
-        // We will just log an error in case of such ever happening
-        let leaf_data = get_mmr_leaf_data(block_hash.into()).unwrap_or_else(|| {
-            error!(target: "runtime::subspace_mmr", "Failed to fetch leaf data for Block hash{block_hash:?}");
-            Default::default()
-        });
+        let leaf_data = get_mmr_leaf_data(block_hash.into())
+            .expect("leaf data for parent hash must always be derived; qed");
         MmrLeaf::V0(LeafDataV0 {
             block_number,
             block_hash,

crates/pallet-subspace-mmr/src/lib.rs

@@ -47,8 +47,9 @@ where
         let header = self
             .consensus_client
             .header(consensus_block_hash.into())
-            .ok()
-            .flatten()?;
+            .expect(
+                "Database error is fatal in host function, there is no recovery from this; qed",
+            )?;
 
         Some(LeafData {
             state_root: H256::from_slice(header.state_root().as_ref()),

crates/sp-subspace-mmr/src/host_functions.rs

@@ -41,7 +41,7 @@ pub enum MmrLeaf<BlockNumber, Hash> {
 pub struct LeafDataV0<BlockNumber, Hash> {
     pub block_number: BlockNumber,
     pub block_hash: Hash,
-    /// Can be used to prove specific storage after block was imported
+    /// Can be used to prove specific storage after block was pruned
     pub state_root: Hash,
     /// Can be used to prove block body
     pub extrinsics_root: Hash,

crates/sp-subspace-mmr/src/lib.rs

@@ -11,7 +11,6 @@ use sp_runtime_interface::runtime_interface;
 #[runtime_interface]
 pub trait SubspaceMmrRuntimeInterface {
     /// Returns the MMR leaf.
-    #[allow(dead_code)]
     fn get_mmr_leaf_data(&mut self, consensus_block_hash: H256) -> Option<LeafData> {
         self.extension::<SubspaceMmrExtension>()
             .expect("No `SubspaceMmrExtension` associated for the current context!")

crates/sp-subspace-mmr/src/runtime_interface.rs

@@ -132,6 +132,7 @@ runtime-benchmarks = [
     "orml-vesting/runtime-benchmarks",
     "pallet-balances/runtime-benchmarks",
     "pallet-domains/runtime-benchmarks",
+    "pallet-mmr/runtime-benchmarks",
     "pallet-runtime-configs/runtime-benchmarks",
     "pallet-subspace/runtime-benchmarks",
     "pallet-timestamp/runtime-benchmarks",

crates/subspace-runtime/Cargo.toml

@@ -802,6 +802,7 @@ mod benches {
         [frame_system, SystemBench::<Runtime>]
         [pallet_balances, Balances]
         [pallet_domains, Domains]
+        [pallet_mmr, Mmr]
         [pallet_runtime_configs, RuntimeConfigs]
         [pallet_subspace, Subspace]
         [pallet_timestamp, Timestamp]

crates/subspace-runtime/src/lib.rs

@@ -183,8 +183,8 @@ impl From<SubstrateConfiguration> for Configuration {
                 }),
             telemetry_endpoints: configuration.telemetry_endpoints,
             default_heap_pages: None,
-            // Offchain worker is not used and indexing is used by MMR gadget to
-            // prune fork mmr leaves once a given block is finalized
+            // Offchain worker is not used
+            // indexing is used to store the mmr leaves from Runtime
             offchain_worker: OffchainWorkerConfig {
                 enabled: false,
                 indexing_enabled: true,