Update SlotHashesSysvar API for more flexibility

sdk/program/Cargo.toml

@@ -81,6 +81,7 @@ itertools = { workspace = true }
 serde_json = { workspace = true }
 serial_test = { workspace = true }
 static_assertions = { workspace = true }
+test-case = { workspace = true }
 
 [build-dependencies]
 rustc_version = { workspace = true }

sdk/program/src/sysvar/slot_hashes.rs

@@ -52,7 +52,6 @@ use {
         clock::Slot,
         hash::Hash,
         program_error::ProgramError,
-        slot_hashes::MAX_ENTRIES,
         sysvar::{get_sysvar, Sysvar, SysvarId},
     },
     bytemuck_derive::{Pod, Zeroable},
@@ -72,21 +71,62 @@ impl Sysvar for SlotHashes {
     }
 }
 
+/// A bytemuck-compatible representation of a `SlotHash`.
 #[derive(Copy, Clone, Default, Pod, Zeroable)]
 #[repr(C)]
-struct PodSlotHash {
-    slot: Slot,
-    hash: Hash,
+pub struct PodSlotHash {
+    pub slot: Slot,
+    pub hash: Hash,
 }
 
+const U64_SIZE: usize = std::mem::size_of::<u64>();
+
 /// API for querying the `SlotHashes` sysvar.
-pub struct SlotHashesSysvar;
+pub struct SlotHashesSysvar {
+    data: Vec<u8>,
+    slot_hashes_start: usize,
+    slot_hashes_end: usize,
+}
 
 impl SlotHashesSysvar {
+    /// Fetch the slot hashes sysvar data and use it to return an instance of
+    /// `SlotHashesSysvar`.
+    pub fn fetch() -> Result<Self, ProgramError> {
+        // First fetch all the sysvar data.
+        let sysvar_len = SlotHashes::size_of();
+        let mut data = vec![0; sysvar_len];
+        get_sysvar(
+            &mut data,
+            &SlotHashes::id(),
+            /* offset */ 0,
+            /* length */ sysvar_len as u64,
+        )?;
+
+        // Read the sysvar's vector length (u64).
+        let slot_hash_count = data
+            .get(..U64_SIZE)
+            .and_then(|bytes| bytes.try_into().ok())
+            .map(u64::from_le_bytes)
+            .ok_or(ProgramError::InvalidAccountData)?;
+
+        // From the vector length, determine the expected length of the data.
+        let length = (slot_hash_count as usize)
+            .checked_mul(std::mem::size_of::<PodSlotHash>())
+            .ok_or(ProgramError::ArithmeticOverflow)?;
+        let slot_hashes_start = U64_SIZE;
+        let slot_hashes_end = slot_hashes_start.saturating_add(length);
+
+        Ok(Self {
+            data,
+            slot_hashes_start,
+            slot_hashes_end,
+        })
+    }
+
     /// Get a value from the sysvar entries by its key.
     /// Returns `None` if the key is not found.
-    pub fn get(slot: &Slot) -> Result<Option<Hash>, ProgramError> {
-        get_pod_slot_hashes().map(|pod_hashes| {
+    pub fn get(&self, slot: &Slot) -> Result<Option<Hash>, ProgramError> {
+        self.as_slice().map(|pod_hashes| {
             pod_hashes
                 .binary_search_by(|PodSlotHash { slot: this, .. }| slot.cmp(this))
                 .map(|idx| pod_hashes[idx].hash)
@@ -96,31 +136,21 @@ impl SlotHashesSysvar {
 
     /// Get the position of an entry in the sysvar by its key.
     /// Returns `None` if the key is not found.
-    pub fn position(slot: &Slot) -> Result<Option<usize>, ProgramError> {
-        get_pod_slot_hashes().map(|pod_hashes| {
+    pub fn position(&self, slot: &Slot) -> Result<Option<usize>, ProgramError> {
+        self.as_slice().map(|pod_hashes| {
             pod_hashes
                 .binary_search_by(|PodSlotHash { slot: this, .. }| slot.cmp(this))
                 .ok()
         })
     }
-}
-
-fn get_pod_slot_hashes() -> Result<Vec<PodSlotHash>, ProgramError> {
-    let mut pod_hashes = vec![PodSlotHash::default(); MAX_ENTRIES];
-    {
-        let data = bytemuck::try_cast_slice_mut::<PodSlotHash, u8>(&mut pod_hashes)
-            .map_err(|_| ProgramError::InvalidAccountData)?;
-
-        // Ensure the created buffer is aligned to 8.
-        if data.as_ptr().align_offset(8) != 0 {
-            return Err(ProgramError::InvalidAccountData);
-        }
 
-        let offset = 8; // Vector length as `u64`.
-        let length = (SlotHashes::size_of() as u64).saturating_sub(offset);
-        get_sysvar(data, &SlotHashes::id(), offset, length)?;
+    /// Return the slot hashes sysvar as a vector of `PodSlotHash`.
+    pub fn as_slice(&self) -> Result<&[PodSlotHash], ProgramError> {
+        self.data
+            .get(self.slot_hashes_start..self.slot_hashes_end)
+            .and_then(|data| bytemuck::try_cast_slice(data).ok())
+            .ok_or(ProgramError::InvalidAccountData)
     }
-    Ok(pod_hashes)
 }
 
 #[cfg(test)]
@@ -134,6 +164,7 @@ mod tests {
             sysvar::tests::mock_get_sysvar_syscall,
         },
         serial_test::serial,
+        test_case::test_case,
     };
 
     #[test]
@@ -149,56 +180,76 @@ mod tests {
         );
     }
 
+    fn mock_slot_hashes(slot_hashes: &SlotHashes) {
+        // The data is always `SlotHashes::size_of()`.
+        let mut data = vec![0; SlotHashes::size_of()];
+        bincode::serialize_into(&mut data[..], slot_hashes).unwrap();
+        mock_get_sysvar_syscall(&data);
+    }
+
+    #[allow(clippy::arithmetic_side_effects)]
+    #[test_case(0)]
+    #[test_case(1)]
+    #[test_case(2)]
+    #[test_case(5)]
+    #[test_case(10)]
+    #[test_case(64)]
+    #[test_case(128)]
+    #[test_case(192)]
+    #[test_case(256)]
+    #[test_case(384)]
+    #[test_case(MAX_ENTRIES)]
     #[serial]
-    #[test]
-    fn test_slot_hashes_sysvar() {
+    fn test_slot_hashes_sysvar(num_entries: usize) {
         let mut slot_hashes = vec![];
-        for i in 0..MAX_ENTRIES {
+        for i in 0..num_entries {
             slot_hashes.push((
                 i as u64,
                 hash(&[(i >> 24) as u8, (i >> 16) as u8, (i >> 8) as u8, i as u8]),
             ));
         }
 
         let check_slot_hashes = SlotHashes::new(&slot_hashes);
-        mock_get_sysvar_syscall(&bincode::serialize(&check_slot_hashes).unwrap());
-
-        // `get`:
-        assert_eq!(
-            SlotHashesSysvar::get(&0).unwrap().as_ref(),
-            check_slot_hashes.get(&0),
-        );
-        assert_eq!(
-            SlotHashesSysvar::get(&256).unwrap().as_ref(),
-            check_slot_hashes.get(&256),
-        );
-        assert_eq!(
-            SlotHashesSysvar::get(&511).unwrap().as_ref(),
-            check_slot_hashes.get(&511),
-        );
-        // `None`.
-        assert_eq!(
-            SlotHashesSysvar::get(&600).unwrap().as_ref(),
-            check_slot_hashes.get(&600),
-        );
+        mock_slot_hashes(&check_slot_hashes);
+
+        // Fetch the slot hashes sysvar.
+        let slot_hashes_sysvar = SlotHashesSysvar::fetch().unwrap();
+        let pod_slot_hashes = slot_hashes_sysvar.as_slice().unwrap();
+
+        // `pod_slot_hashes` should match the slot hashes.
+        // Note slot hashes are stored largest slot to smallest.
+        for (i, pod_slot_hash) in pod_slot_hashes.iter().enumerate() {
+            let check = slot_hashes[num_entries - 1 - i];
+            assert_eq!(pod_slot_hash.slot, check.0);
+            assert_eq!(pod_slot_hash.hash, check.1);
+        }
 
-        // `position`:
-        assert_eq!(
-            SlotHashesSysvar::position(&0).unwrap(),
-            check_slot_hashes.position(&0),
-        );
-        assert_eq!(
-            SlotHashesSysvar::position(&256).unwrap(),
-            check_slot_hashes.position(&256),
-        );
-        assert_eq!(
-            SlotHashesSysvar::position(&511).unwrap(),
-            check_slot_hashes.position(&511),
-        );
-        // `None`.
-        assert_eq!(
-            SlotHashesSysvar::position(&600).unwrap(),
-            check_slot_hashes.position(&600),
-        );
+        // Check some arbitrary slots in the created slot hashes.
+        let num_entries = num_entries as Slot;
+        let check_slots = if num_entries == 0 {
+            vec![num_entries, num_entries + 100]
+        } else {
+            vec![
+                0,
+                num_entries / 4,
+                num_entries / 2,
+                num_entries - 1,
+                num_entries,
+                num_entries + 100,
+            ]
+        };
+
+        for slot in check_slots.iter() {
+            // `get`:
+            assert_eq!(
+                slot_hashes_sysvar.get(slot).unwrap().as_ref(),
+                check_slot_hashes.get(slot),
+            );
+            // `position`:
+            assert_eq!(
+                slot_hashes_sysvar.position(slot).unwrap(),
+                check_slot_hashes.position(slot),
+            );
+        }
     }
 }