Update the block RPCs to return null when missing, not an error

Promotes clarity.

substrate/node/src/rpc/blockchain.rs

@@ -37,10 +37,12 @@ pub(crate) fn module<
   module.register_method(
     "blockchain/is_finalized",
     |params, client, _ext| -> Result<_, Error> {
-      let block_hash = block_hash(&**client, &params)?;
+      let Some(block_hash) = block_hash(&**client, &params)? else {
+        return Ok(false);
+      };
       let finalized = client.info().finalized_number;
       let Ok(Some(number)) = client.number(block_hash) else {
-        Err(Error::Missing("failed to fetch block's number"))?
+        return Ok(false);
       };
       let Ok(status) = client.block_status(block_hash) else {
         Err(Error::Internal("failed to fetch block's status"))?
@@ -53,18 +55,22 @@ pub(crate) fn module<
   )?;
 
   module.register_method("blockchain/block", |params, client, _ext| -> Result<_, Error> {
-    let block_hash = block_hash(&**client, &params)?;
+    let Some(block_hash) = block_hash(&**client, &params)? else {
+      return Ok(None);
+    };
     let Ok(Some(block)) = client.block(block_hash) else {
-      Err(Error::Missing("couldn't find requested block"))?
+      return Ok(None);
     };
 
-    Ok(hex::encode(borsh::to_vec(&serai_abi::Block::from(block.block)).unwrap()))
+    Ok(Some(hex::encode(borsh::to_vec(&serai_abi::Block::from(block.block)).unwrap())))
   })?;
 
   module.register_method("blockchain/events", |params, client, _ext| -> Result<_, Error> {
-    let block_hash = block_hash(&**client, &params)?;
+    let Some(block_hash) = block_hash(&**client, &params)? else {
+      Err(Error::InvalidStateReference)?
+    };
     let Ok(events) = client.runtime_api().events(block_hash) else {
-      Err(Error::Missing("couldn't fetch the events for the requested block"))?
+      Err(Error::InvalidStateReference)?
     };
     Ok(
       events

substrate/node/src/rpc/utils.rs

@@ -6,7 +6,7 @@ use serai_abi::{primitives::prelude::*, SubstrateBlock as Block};
 pub(super) enum Error {
   Internal(&'static str),
   InvalidRequest(&'static str),
-  Missing(&'static str),
+  InvalidStateReference,
 }
 
 impl From<Error> for jsonrpsee::types::error::ErrorObjectOwned {
@@ -18,9 +18,11 @@ impl From<Error> for jsonrpsee::types::error::ErrorObjectOwned {
       Error::InvalidRequest(str) => {
         jsonrpsee::types::error::ErrorObjectOwned::owned(-2, str, Option::<()>::None)
       }
-      Error::Missing(str) => {
-        jsonrpsee::types::error::ErrorObjectOwned::owned(-3, str, Option::<()>::None)
-      }
+      Error::InvalidStateReference => jsonrpsee::types::error::ErrorObjectOwned::owned(
+        -4,
+        "the block used as the reference was not locally held",
+        Option::<()>::None,
+      ),
     }
   }
 }
@@ -30,7 +32,7 @@ pub(super) fn block_hash<
 >(
   client: &C,
   params: &jsonrpsee::types::params::Params,
-) -> Result<<Block as sp_runtime::traits::Block>::Hash, Error> {
+) -> Result<Option<<Block as sp_runtime::traits::Block>::Hash>, Error> {
   #[derive(sp_core::serde::Deserialize)]
   #[serde(crate = "sp_core::serde")]
   struct BlockByHash {
@@ -50,13 +52,13 @@ pub(super) fn block_hash<
     }) else {
       return Err(Error::InvalidRequest("requested block hash wasn't a valid hash"));
     };
-    block_hash
+    Some(block_hash)
   } else {
     let Ok(block_number) = params.parse::<BlockByNumber>() else {
       return Err(Error::InvalidRequest("requested block wasn't a valid hash nor number"));
     };
-    let Ok(Some(block_hash)) = client.block_hash(block_number.block) else {
-      return Err(Error::Missing("no block hash for that block number"));
+    let Ok(block_hash) = client.block_hash(block_number.block) else {
+      return Err(Error::Internal("couldn't fetch block hash for block number"));
     };
     block_hash
   })

substrate/node/src/rpc/validator_sets.rs

@@ -72,7 +72,9 @@ pub(crate) fn module<
   module.register_method(
     "validator-sets/current_session",
     |params, client, _ext| -> Result<_, Error> {
-      let block_hash = block_hash(&**client, &params)?;
+      let Some(block_hash) = block_hash(&**client, &params)? else {
+        Err(Error::InvalidStateReference)?
+      };
       let network = network(&params)?;
       let Ok(session) = client.runtime_api().current_session(block_hash, network) else {
         Err(Error::Internal("couldn't fetch the session for the requested network"))?
@@ -84,7 +86,9 @@ pub(crate) fn module<
   module.register_method(
     "validator-sets/current_stake",
     |params, client, _ext| -> Result<_, Error> {
-      let block_hash = block_hash(&**client, &params)?;
+      let Some(block_hash) = block_hash(&**client, &params)? else {
+        Err(Error::InvalidStateReference)?
+      };
       let network = network(&params)?;
       let Ok(stake) = client.runtime_api().current_stake(block_hash, network) else {
         Err(Error::Internal("couldn't fetch the total allocated stake for the requested network"))?
@@ -94,7 +98,9 @@ pub(crate) fn module<
   );
 
   module.register_method("validator-sets/keys", |params, client, _ext| -> Result<_, Error> {
-    let block_hash = block_hash(&**client, &params)?;
+    let Some(block_hash) = block_hash(&**client, &params)? else {
+      Err(Error::InvalidStateReference)?
+    };
     let set = set(&params)?;
     let Ok(set) = ExternalValidatorSet::try_from(set) else {
       Err(Error::InvalidRequest("requested keys for a non-external validator set"))?