SDK: Add sysvar to expose recent block hashes to programs (#6663)

* SDK: Add sysvar to expose recent block hashes to programs

* Blockhashes is one word

* Missed one

* Avoid allocs on update

* unwrap_or_else

* Use iterators

* Add microbench

* Revert "unwrap_or_else"

This reverts commit a8f8c3bfbe.

* Revert "Avoid allocs on update"

This reverts commit 486f01790c.

sdk/src/sysvar/mod.rs

@@ -5,6 +5,7 @@ use crate::pubkey::Pubkey;
 pub mod clock;
 pub mod epoch_schedule;
 pub mod fees;
+pub mod recent_blockhashes;
 pub mod rent;
 pub mod rewards;
 pub mod slot_hashes;
@@ -14,6 +15,7 @@ pub fn is_sysvar_id(id: &Pubkey) -> bool {
     clock::check_id(id)
         || epoch_schedule::check_id(id)
         || fees::check_id(id)
+        || recent_blockhashes::check_id(id)
         || rent::check_id(id)
         || rewards::check_id(id)
         || slot_hashes::check_id(id)

sdk/src/sysvar/recent_blockhashes.rs

@@ -0,0 +1,116 @@
+use crate::{account::Account, account_info::AccountInfo, hash::Hash, sysvar};
+use bincode::serialized_size;
+use std::collections::BinaryHeap;
+use std::iter::FromIterator;
+use std::ops::Deref;
+
+const MAX_ENTRIES: usize = 32;
+const ID: [u8; 32] = [
+    0x06, 0xa7, 0xd5, 0x17, 0x19, 0x2c, 0x56, 0x8e, 0xe0, 0x8a, 0x84, 0x5f, 0x73, 0xd2, 0x97, 0x88,
+    0xcf, 0x03, 0x5c, 0x31, 0x45, 0xb2, 0x1a, 0xb3, 0x44, 0xd8, 0x06, 0x2e, 0xa9, 0x40, 0x00, 0x00,
+];
+
+crate::solana_sysvar_id!(ID, "SysvarRecentB1ockHashes11111111111111111111");
+
+#[repr(C)]
+#[derive(Serialize, Deserialize, Debug, PartialEq)]
+pub struct RecentBlockhashes(Vec<Hash>);
+
+impl Default for RecentBlockhashes {
+    fn default() -> Self {
+        Self(Vec::with_capacity(MAX_ENTRIES))
+    }
+}
+
+impl<'a> FromIterator<&'a Hash> for RecentBlockhashes {
+    fn from_iter<I>(iter: I) -> Self
+    where
+        I: IntoIterator<Item = &'a Hash>,
+    {
+        let mut new = Self::default();
+        for i in iter {
+            new.0.push(*i)
+        }
+        new
+    }
+}
+
+impl RecentBlockhashes {
+    pub fn from_account(account: &Account) -> Option<Self> {
+        account.deserialize_data().ok()
+    }
+    pub fn to_account(&self, account: &mut Account) -> Option<()> {
+        account.serialize_data(self).unwrap();
+        Some(())
+    }
+    pub fn from_account_info(account: &AccountInfo) -> Option<Self> {
+        account.deserialize_data().ok()
+    }
+    pub fn to_account_info(&self, account: &mut AccountInfo) -> Option<()> {
+        account.serialize_data(self).ok()
+    }
+    pub fn size_of() -> usize {
+        serialized_size(&RecentBlockhashes(vec![Hash::default(); MAX_ENTRIES])).unwrap() as usize
+    }
+}
+
+impl Deref for RecentBlockhashes {
+    type Target = Vec<Hash>;
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+pub fn create_account(lamports: u64) -> Account {
+    Account::new(lamports, RecentBlockhashes::size_of(), &sysvar::id())
+}
+
+pub fn update_account<'a, I>(account: &mut Account, recent_blockhash_iter: I) -> Option<()>
+where
+    I: IntoIterator<Item = (u64, &'a Hash)>,
+{
+    let sorted = BinaryHeap::from_iter(recent_blockhash_iter);
+    let recent_blockhash_iter = sorted.into_iter().take(MAX_ENTRIES).map(|(_, hash)| hash);
+    let recent_blockhashes = RecentBlockhashes::from_iter(recent_blockhash_iter);
+    recent_blockhashes.to_account(account)
+}
+
+pub fn create_account_with_data<'a, I>(lamports: u64, recent_blockhash_iter: I) -> Account
+where
+    I: IntoIterator<Item = (u64, &'a Hash)>,
+{
+    let mut account = create_account(lamports);
+    update_account(&mut account, recent_blockhash_iter).unwrap();
+    account
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::hash::Hash;
+
+    #[test]
+    fn test_create_account_empty() {
+        let account = create_account_with_data(42, vec![].into_iter());
+        let recent_blockhashes = RecentBlockhashes::from_account(&account).unwrap();
+        assert_eq!(recent_blockhashes, RecentBlockhashes::default());
+    }
+
+    #[test]
+    fn test_create_account_full() {
+        let def_hash = Hash::default();
+        let account =
+            create_account_with_data(42, vec![(0u64, &def_hash); MAX_ENTRIES].into_iter());
+        let recent_blockhashes = RecentBlockhashes::from_account(&account).unwrap();
+        assert_eq!(recent_blockhashes.len(), MAX_ENTRIES);
+    }
+
+    #[test]
+    fn test_create_account_truncate() {
+        let def_hash = Hash::default();
+        let account =
+            create_account_with_data(42, vec![(0u64, &def_hash); MAX_ENTRIES + 1].into_iter());
+        let recent_blockhashes = RecentBlockhashes::from_account(&account).unwrap();
+        assert_eq!(recent_blockhashes.len(), MAX_ENTRIES);
+    }
+}