solana: Add check for no registered transceivers

Add guard on transfer instructions to prevent a transfer from being
processed when no transceivers are registered. This is done by checking
the `next_transceiver_id` field in the Config. This field
auto-increments when a transceiver is added. If this value is 0, then
the program is in a state where no transceivers have been registered.

Author: johnsaigle

solana/programs/example-native-token-transfers/src/error.rs

@@ -37,4 +37,6 @@ pub enum NTTError {
     DisabledTransceiver,
     #[msg("InvalidDeployer")]
     InvalidDeployer,
+    #[msg("NoRegisteredTransceivers")]
+    NoRegisteredTransceivers,
 }

solana/programs/example-native-token-transfers/src/instructions/transfer.rs

@@ -120,6 +120,15 @@ pub fn transfer_burn(ctx: Context<TransferBurn>, args: TransferArgs) -> Result<(
     );
 
     let accs = ctx.accounts;
+
+    // TODO If functionality is added to disable transceivers, this check should be modified to
+    // ensure that, if all transceivers have been disabled, that an error is returned. Otherwise it
+    // may be possible to permanently lose funds.
+    // No transceivers have been registered yet
+    if accs.common.config.next_transceiver_id == 0 {
+        return Err(NTTError::NoRegisteredTransceivers.into());
+    }
+
     let TransferArgs {
         mut amount,
         recipient_chain,
@@ -213,6 +222,15 @@ pub fn transfer_lock(ctx: Context<TransferLock>, args: TransferArgs) -> Result<(
     );
 
     let accs = ctx.accounts;
+
+    // TODO If functionality is added to disable transceivers, this check should be modified to
+    // ensure that, if all transceivers have been disabled, that an error is returned. Otherwise it
+    // may be possible to permanently lose funds.
+    // No transceivers have been registered yet
+    if accs.common.config.next_transceiver_id == 0 {
+        return Err(NTTError::NoRegisteredTransceivers.into());
+    }
+
     let TransferArgs {
         mut amount,
         recipient_chain,