Add backend method first_block_hash (polkadot-evm#1561)

* add backend method first_block_hash

* fix comment

---------

Co-authored-by: Éloïs <c@elo.tf>

Author: TarekkMA

client/api/src/backend.rs

@@ -53,6 +53,9 @@ pub trait Backend<Block: BlockT>: Send + Sync {
 		self.log_indexer().is_indexed()
 	}
 
+	/// Get the hash of the oldest substrate block fully indexed by the backend.
+	async fn first_block_hash(&self) -> Result<Block::Hash, String>;
+
 	/// Get the hash of the latest substrate block fully indexed by the backend.
 	async fn latest_block_hash(&self) -> Result<Block::Hash, String>;
 }

client/db/src/kv/mod.rs

@@ -90,6 +90,10 @@ impl<Block: BlockT, C: HeaderBackend<Block>> fc_api::Backend<Block> for Backend<
 		&self.log_indexer
 	}
 
+	async fn first_block_hash(&self) -> Result<Block::Hash, String> {
+		Ok(self.client.info().genesis_hash)
+	}
+
 	async fn latest_block_hash(&self) -> Result<Block::Hash, String> {
 		Ok(self.client.info().best_hash)
 	}

client/db/src/sql/mod.rs

@@ -820,6 +820,15 @@ impl<Block: BlockT<Hash = H256>> fc_api::Backend<Block> for Backend<Block> {
 		self
 	}
 
+	async fn first_block_hash(&self) -> Result<Block::Hash, String> {
+		// Retrieves the block hash for the earliest indexed block, maybe it's not canon.
+		sqlx::query("SELECT substrate_block_hash FROM blocks ORDER BY block_number ASC LIMIT 1")
+			.fetch_one(self.pool())
+			.await
+			.map(|row| H256::from_slice(&row.get::<Vec<u8>, _>(0)[..]))
+			.map_err(|e| format!("Failed to fetch oldest block hash: {}", e))
+	}
+
 	async fn latest_block_hash(&self) -> Result<Block::Hash, String> {
 		// Retrieves the block hash for the latest indexed block, maybe it's not canon.
 		sqlx::query("SELECT substrate_block_hash FROM blocks ORDER BY block_number DESC LIMIT 1")