Fix review comments

Author: febo

interface/src/instruction.rs

@@ -491,6 +491,9 @@ pub enum TokenInstruction {
     ///
     /// Both the number of accounts and instruction data length are used to identify
     /// the slice of accounts and instruction data for each instruction.
+    ///
+    /// Note that it is not sound to have a `batch` instruction that contains other
+    /// `batch` instruction; an error will be raised when this is detected.
     Batch = 255,
     // Any new variants also need to be added to program-2022 `TokenInstruction`, so that the
     // latter remains a superset of this instruction set. New variants also need to be added to

p-token/src/processor/batch.rs

@@ -23,7 +23,7 @@ pub fn process_batch(mut accounts: &[AccountInfo], mut instruction_data: &[u8])
         let expected_accounts = unsafe { *instruction_data.get_unchecked(0) as usize };
         let data_offset = IX_HEADER_SIZE + unsafe { *instruction_data.get_unchecked(1) as usize };
 
-        if instruction_data.len() < data_offset || data_offset == 0 {
+        if instruction_data.len() < data_offset || data_offset == IX_HEADER_SIZE {
             return Err(ProgramError::InvalidInstructionData);
         }
 
@@ -34,13 +34,15 @@ pub fn process_batch(mut accounts: &[AccountInfo], mut instruction_data: &[u8])
         // Process the instruction.
 
         // SAFETY: The instruction data and accounts lengths are already validated so all
-        // the slices are guaranteed to be valid.
-        unsafe {
-            inner_process_instruction(
+        // slices are guaranteed to be valid.
+        let (ix_accounts, ix_data) = unsafe {
+            (
                 accounts.get_unchecked(..expected_accounts),
                 instruction_data.get_unchecked(IX_HEADER_SIZE..data_offset),
-            )?;
-        }
+            )
+        };
+
+        inner_process_instruction(ix_accounts, ix_data)?;
 
         if data_offset == instruction_data.len() {
             // The batch is complete.