Add Incremental Snapshot support to RPC (#19559)

#### Problem

There's no way to get incremental snapshot information from RPC.

#### Summary of Changes

- Add new RPC method, `getHighestSnapshotSlot` that returns a `SnapshotSlotInfo`, which contains both the highest full snapshot slot, and the highest incremental snapshot slot _based on_ the full snapshot.
- Deprecate old RPC method, `getSnapshotSlot`
- Update API docs

Fixes #19579

client/src/mock_sender.rs

@@ -8,8 +8,9 @@ use {
         rpc_response::{
             Response, RpcAccountBalance, RpcBlockProduction, RpcBlockProductionRange, RpcBlockhash,
             RpcConfirmedTransactionStatusWithSignature, RpcContactInfo, RpcFees, RpcPerfSample,
-            RpcResponseContext, RpcSimulateTransactionResult, RpcStakeActivation, RpcSupply,
-            RpcVersionInfo, RpcVoteAccountInfo, RpcVoteAccountStatus, StakeActivationState,
+            RpcResponseContext, RpcSimulateTransactionResult, RpcSnapshotSlotInfo,
+            RpcStakeActivation, RpcSupply, RpcVersionInfo, RpcVoteAccountInfo,
+            RpcVoteAccountStatus, StakeActivationState,
         },
         rpc_sender::RpcSender,
     },
@@ -222,6 +223,10 @@ impl RpcSender for MockSender {
             "getMaxShredInsertSlot" => json![0],
             "requestAirdrop" => Value::String(Signature::new(&[8; 64]).to_string()),
             "getSnapshotSlot" => Value::Number(Number::from(0)),
+            "getHighestSnapshotSlot" => json!(RpcSnapshotSlotInfo {
+                full: 100,
+                incremental: Some(110),
+            }),
             "getBlockHeight" => Value::Number(Number::from(1234)),
             "getSlotLeaders" => json!([PUBKEY]),
             "getBlockProduction" => {

client/src/rpc_client.rs

@@ -1018,13 +1018,16 @@ impl RpcClient {
         )
     }
 
-    /// Returns the highest slot that the node has a snapshot for.
+    /// Returns the highest slot information that the node has snapshots for.
+    ///
+    /// This will find the highest full snapshot slot, and the highest incremental snapshot slot
+    /// _based on_ the full snapshot slot, if there is one.
     ///
     /// # RPC Reference
     ///
-    /// This method corresponds directly to the [`getSnapshotSlot`] RPC method.
+    /// This method corresponds directly to the [`getHighestSnapshotSlot`] RPC method.
     ///
-    /// [`getSnapshotSlot`]: https://docs.solana.com/developing/clients/jsonrpc-api#getsnapshotslot
+    /// [`getHighestSnapshotSlot`]: https://docs.solana.com/developing/clients/jsonrpc-api#gethighestsnapshotslot
     ///
     /// # Examples
     ///
@@ -1034,9 +1037,18 @@ impl RpcClient {
     /// #     client_error::ClientError,
     /// # };
     /// # let rpc_client = RpcClient::new_mock("succeeds".to_string());
-    /// let slot = rpc_client.get_snapshot_slot()?;
+    /// let snapshot_slot_info = rpc_client.get_highest_snapshot_slot()?;
     /// # Ok::<(), ClientError>(())
     /// ```
+    pub fn get_highest_snapshot_slot(&self) -> ClientResult<RpcSnapshotSlotInfo> {
+        self.send(RpcRequest::GetHighestSnapshotSlot, Value::Null)
+    }
+
+    #[deprecated(
+        since = "1.8.0",
+        note = "Please use RpcClient::get_highest_snapshot_slot() instead"
+    )]
+    #[allow(deprecated)]
     pub fn get_snapshot_slot(&self) -> ClientResult<Slot> {
         self.send(RpcRequest::GetSnapshotSlot, Value::Null)
     }

client/src/rpc_request.rs

@@ -79,6 +79,11 @@ pub enum RpcRequest {
     )]
     GetRecentBlockhash,
     GetRecentPerformanceSamples,
+    GetHighestSnapshotSlot,
+    #[deprecated(
+        since = "1.8.0",
+        note = "Please use RpcRequest::GetHighestSnapshotSlot instead"
+    )]
     GetSnapshotSlot,
     GetSignaturesForAddress,
     GetSignatureStatuses,
@@ -151,6 +156,7 @@ impl fmt::Display for RpcRequest {
             RpcRequest::GetProgramAccounts => "getProgramAccounts",
             RpcRequest::GetRecentBlockhash => "getRecentBlockhash",
             RpcRequest::GetRecentPerformanceSamples => "getRecentPerformanceSamples",
+            RpcRequest::GetHighestSnapshotSlot => "getHighestSnapshotSlot",
             RpcRequest::GetSnapshotSlot => "getSnapshotSlot",
             RpcRequest::GetSignaturesForAddress => "getSignaturesForAddress",
             RpcRequest::GetSignatureStatuses => "getSignatureStatuses",

client/src/rpc_response.rs

@@ -435,3 +435,9 @@ impl From<ConfirmedTransactionStatusWithSignature> for RpcConfirmedTransactionSt
         }
     }
 }
+
+#[derive(Serialize, Deserialize, Clone, Copy, Debug, PartialEq)]
+pub struct RpcSnapshotSlotInfo {
+    pub full: Slot,
+    pub incremental: Option<Slot>,
+}