fix(lang): Store CPI return data in Vec

CHANGELOG.md

@@ -22,6 +22,7 @@ The minor version will be incremented upon a breaking change and the patch versi
 
 ### Fixes
 
+- lang: Avoid an unnecessary stack allocation when snapshotting CPI return data ([#4931](https://github.com/otter-sec/anchor/pull/4931)).
 - spl: Fix `anchor-spl` failing to build with only the `metadata` feature ([#4742](https://github.com/solana-foundation/anchor/pull/4742)).
 - client: Fix ignored commitment level ([#4666](https://github.com/solana-foundation/anchor/pull/4666)).
 - lang: Remove cloning `AccountInfo` to read lamports in `init_if_needed` codegen ([#4675](https://github.com/solana-foundation/anchor/pull/4675)).

lang/attribute/program/src/declare_program/mods/cpi.rs

@@ -110,16 +110,16 @@ fn gen_cpi_instructions(idl: &Idl) -> proc_macro2::TokenStream {
 
 fn gen_cpi_return_type() -> proc_macro2::TokenStream {
     quote! {
-        #[derive(Debug, Clone, Copy)]
+        #[derive(Debug, Clone)]
         pub struct Return<T> {
             phantom: ::std::marker::PhantomData<T>,
             program_id: anchor_lang::solana_program::pubkey::Pubkey,
-            return_data: anchor_lang::__private::CpiReturnData,
+            return_data: Option<anchor_lang::__private::CpiReturnData>,
         }
 
         impl<T: AnchorDeserialize> Return<T> {
             pub fn get(&self) -> T {
-                self.return_data.get(self.program_id)
+                self.return_data.as_ref().unwrap().get(self.program_id)
             }
 
             /// Read return data without validating the program_id.

lang/src/lib.rs

@@ -655,52 +655,37 @@ pub mod __private {
     impl<T> IsSameType<T> for T {}
 
     #[doc(hidden)]
-    #[derive(Debug, Clone, Copy)]
+    #[derive(Debug, Clone)]
     pub struct CpiReturnData {
-        program_id: Option<Pubkey>,
-        data_len: usize,
-        data: [u8; crate::solana_program::program::MAX_RETURN_DATA],
+        program_id: Pubkey,
+        data: Vec<u8>,
     }
 
     impl CpiReturnData {
         #[doc(hidden)]
-        pub fn new(return_data: Option<(Pubkey, Vec<u8>)>) -> Self {
-            let mut snapshot = Self {
-                program_id: None,
-                data_len: 0,
-                data: [0u8; crate::solana_program::program::MAX_RETURN_DATA],
-            };
-
-            if let Some((program_id, data)) = return_data {
-                let data_len = data.len();
-                snapshot.data[..data_len].copy_from_slice(&data);
-                snapshot.program_id = Some(program_id);
-                snapshot.data_len = data_len;
-            }
-
-            snapshot
+        pub fn new(program_id: Pubkey, data: Vec<u8>) -> Self {
+            CpiReturnData { program_id, data }
         }
 
         #[doc(hidden)]
-        pub fn snapshot() -> Self {
-            Self::new(crate::solana_program::program::get_return_data())
+        pub fn snapshot() -> Option<Self> {
+            let (program_id, data) = crate::solana_program::program::get_return_data()?;
+            Some(Self::new(program_id, data))
         }
 
         #[doc(hidden)]
         pub fn get<T: crate::AnchorDeserialize>(&self, expected_program_id: Pubkey) -> T {
-            let program_id = self.program_id.unwrap();
-            if program_id != expected_program_id {
+            if self.program_id != expected_program_id {
                 crate::solana_program::log::sol_log("CPI return data program_id mismatch");
                 panic!();
             }
 
-            T::try_from_slice(&self.data[..self.data_len]).unwrap()
+            T::try_from_slice(&self.data).unwrap()
         }
 
         #[doc(hidden)]
-        pub fn return_data(&self) -> Option<(Pubkey, &[u8])> {
-            self.program_id
-                .map(|program_id| (program_id, &self.data[..self.data_len]))
+        pub fn return_data(&self) -> (Pubkey, &[u8]) {
+            (self.program_id, &self.data)
         }
     }
 }

lang/syn/src/codegen/program/cpi.rs

@@ -96,16 +96,16 @@ pub fn generate(program: &Program) -> proc_macro2::TokenStream {
             use ::std::marker::PhantomData;
 
 
-            #[derive(Debug, Clone, Copy)]
+            #[derive(Debug, Clone)]
             pub struct Return<T> {
                 phantom: ::std::marker::PhantomData<T>,
                 program_id: anchor_lang::solana_program::pubkey::Pubkey,
-                return_data: anchor_lang::__private::CpiReturnData,
+                return_data: Option<anchor_lang::__private::CpiReturnData>,
             }
 
             impl<T: AnchorDeserialize> Return<T> {
                 pub fn get(&self) -> T {
-                    self.return_data.get(self.program_id)
+                    self.return_data.as_ref().unwrap().get(self.program_id)
                 }
 
                 /// Read return data without validating the program_id.

lang/tests/cpi_return_data.rs

@@ -5,17 +5,17 @@ fn test_cpi_return_data_snapshot_keeps_original_bytes() {
     let program_id = Pubkey::new_unique();
     let original = 10u64;
     let original_bytes = borsh::to_vec(&original).unwrap();
-    let snapshot = CpiReturnData::new(Some((program_id, original_bytes.clone())));
+    let snapshot = CpiReturnData::new(program_id, original_bytes.clone());
 
     let spoofed = 999u64;
     let spoofed_bytes = borsh::to_vec(&spoofed).unwrap();
-    let later = CpiReturnData::new(Some((program_id, spoofed_bytes.clone())));
+    let later = CpiReturnData::new(program_id, spoofed_bytes.clone());
 
-    let (snapshot_program_id, snapshot_bytes) = snapshot.return_data().unwrap();
+    let (snapshot_program_id, snapshot_bytes) = snapshot.return_data();
     assert_eq!(snapshot_program_id, program_id);
     assert_eq!(snapshot_bytes, original_bytes.as_slice());
 
-    let (later_program_id, later_bytes) = later.return_data().unwrap();
+    let (later_program_id, later_bytes) = later.return_data();
     assert_eq!(later_program_id, program_id);
     assert_eq!(later_bytes, spoofed_bytes.as_slice());
 
@@ -29,7 +29,7 @@ fn test_cpi_return_data_snapshot_rejects_program_id_mismatch() {
     let program_id = Pubkey::new_unique();
     let other_program_id = Pubkey::new_unique();
     let value = 10u64;
-    let snapshot = CpiReturnData::new(Some((other_program_id, borsh::to_vec(&value).unwrap())));
+    let snapshot = CpiReturnData::new(other_program_id, borsh::to_vec(&value).unwrap());
 
     let _ = snapshot.get::<u64>(program_id);
 }