Add extension realloc helper (#2821)

Author: CriesofCarrots

token/program-2022/src/extension/mod.rs

@@ -491,6 +491,24 @@ impl<'data, S: BaseState> StateWithExtensionsMut<'data, S> {
     fn get_first_extension_type(&self) -> Result<Option<ExtensionType>, ProgramError> {
         get_first_extension_type(self.tlv_data)
     }
+
+    /// Compares the length of an extension with the currently used TLV buffer to determine if
+    /// reallocation is needed. If so, returns Some(v), where v is the difference between current
+    /// space and needed.
+    #[allow(dead_code)]
+    pub(crate) fn realloc_needed(
+        &self,
+        new_extension: ExtensionType,
+    ) -> Result<Option<usize>, ProgramError> {
+        let current_extensions = self.get_extension_types()?;
+        let needed_tlv_len = ExtensionType::get_total_tlv_len(&current_extensions);
+        let new_needed_tlv_len = needed_tlv_len.saturating_add(new_extension.get_type_len());
+        if self.tlv_data.len() >= new_needed_tlv_len {
+            Ok(None)
+        } else {
+            Ok(Some(new_needed_tlv_len - self.tlv_data.len())) // arithmetic safe because of if clause
+        }
+    }
 }
 
 /// Different kinds of accounts. Note that `Mint`, `Account`, and `Multisig` types
@@ -1344,4 +1362,69 @@ mod test {
 
         assert_eq!(state.get_extension_types().unwrap(), vec![]);
     }
+
+    #[test]
+    fn test_realloc_needed() {
+        // buffer exact size of existing extension
+        let mint_size = ExtensionType::get_account_len::<Mint>(&[ExtensionType::TransferFeeConfig]);
+        let mut buffer = vec![0; mint_size];
+        let mut state = StateWithExtensionsMut::<Mint>::unpack_uninitialized(&mut buffer).unwrap();
+        state.base = TEST_MINT;
+        state.pack_base();
+        state.init_account_type().unwrap();
+        assert_eq!(
+            state
+                .realloc_needed(ExtensionType::TransferFeeConfig)
+                .unwrap(),
+            None
+        );
+        state.init_extension::<TransferFeeConfig>().unwrap();
+        assert_eq!(
+            state
+                .realloc_needed(ExtensionType::MintCloseAuthority)
+                .unwrap(),
+            Some(ExtensionType::MintCloseAuthority.get_type_len())
+        );
+
+        // buffer with multisig len
+        let mint_size = ExtensionType::get_account_len::<Mint>(&[ExtensionType::MintPaddingTest]);
+        let mut buffer = vec![0; mint_size];
+        let mut state = StateWithExtensionsMut::<Mint>::unpack_uninitialized(&mut buffer).unwrap();
+        state.base = TEST_MINT;
+        state.pack_base();
+        state.init_account_type().unwrap();
+        assert_eq!(
+            state
+                .realloc_needed(ExtensionType::MintPaddingTest)
+                .unwrap(),
+            None
+        );
+        state.init_extension::<MintPaddingTest>().unwrap();
+        assert_eq!(
+            state
+                .realloc_needed(ExtensionType::MintCloseAuthority)
+                .unwrap(),
+            Some(ExtensionType::MintCloseAuthority.get_type_len() - size_of::<ExtensionType>())
+        );
+
+        // huge buffer
+        let mut buffer = vec![0; u16::MAX.into()];
+        let mut state = StateWithExtensionsMut::<Mint>::unpack_uninitialized(&mut buffer).unwrap();
+        state.base = TEST_MINT;
+        state.pack_base();
+        state.init_account_type().unwrap();
+        assert_eq!(
+            state
+                .realloc_needed(ExtensionType::TransferFeeConfig)
+                .unwrap(),
+            None
+        );
+        state.init_extension::<TransferFeeConfig>().unwrap();
+        assert_eq!(
+            state
+                .realloc_needed(ExtensionType::MintCloseAuthority)
+                .unwrap(),
+            None
+        );
+    }
 }