light-client-verifier: reuse sign_bytes buffer (#1413)

.changelog/unreleased/improvements/1413-reuse-sign-bytes.md

@@ -0,0 +1,3 @@
+- `[tendermint-light-client-verifier]` Reuse buffer used to store
+  sign_bytes to reduce number of allocations and deallocations.
+  ([\#1413](https://github.com/informalsystems/tendermint-rs/pull/1413))

light-client-verifier/src/operations/voting_power.rs

@@ -264,9 +264,26 @@ impl NonAbsentCommitVote {
 struct NonAbsentCommitVotes {
     /// Votes sorted by validator address.
     votes: Vec<NonAbsentCommitVote>,
+    /// Internal buffer for storing sign_bytes.
+    ///
+    /// The buffer is reused for each canonical vote so that we allocate it
+    /// once.
+    sign_bytes: Vec<u8>,
 }
 
 impl NonAbsentCommitVotes {
+    /// Initial capacity of the `sign_bytes` buffer.
+    ///
+    /// The buffer will be resized if it happens to be too small so this value
+    /// isn’t critical for correctness.  It’s a matter of performance to avoid
+    /// reallocations.
+    ///
+    /// Note: As of protocol 0.38, maximum length of the sign bytes is `115 + (N
+    /// > 13) + N` where `N` is the length of the chain id.  Chain id can be at
+    /// most 50 bytes (see [`tendermint::chain::id::MAX_LEN`]) thus the largest
+    /// buffer we’ll ever need is 166 bytes long.
+    const SIGN_BYTES_INITIAL_CAPACITY: usize = 166;
+
     pub fn new(signed_header: &SignedHeader) -> Result<Self, VerificationError> {
         let mut votes = signed_header
             .commit
@@ -297,7 +314,10 @@ impl NonAbsentCommitVotes {
                 pair[0].validator_id(),
             ))
         } else {
-            Ok(Self { votes })
+            Ok(Self {
+                votes,
+                sign_bytes: Vec::with_capacity(Self::SIGN_BYTES_INITIAL_CAPACITY),
+            })
         }
     }
 
@@ -319,14 +339,18 @@ impl NonAbsentCommitVotes {
         };
 
         if !vote.verified {
-            let sign_bytes = vote.signed_vote.sign_bytes();
+            self.sign_bytes.truncate(0);
+            vote.signed_vote
+                .sign_bytes_into(&mut self.sign_bytes)
+                .expect("buffer is resized if needed and encoding never fails");
+            let sign_bytes = self.sign_bytes.as_slice();
             validator
-                .verify_signature::<V>(&sign_bytes, vote.signed_vote.signature())
+                .verify_signature::<V>(sign_bytes, vote.signed_vote.signature())
                 .map_err(|_| {
                     VerificationError::invalid_signature(
                         vote.signed_vote.signature().as_bytes().to_vec(),
                         Box::new(validator.clone()),
-                        sign_bytes,
+                        sign_bytes.to_vec(),
                     )
                 })?;
         }

tendermint/src/vote.rs

@@ -329,6 +329,10 @@ impl SignedVote {
         Protobuf::<RawCanonicalVote>::encode_length_delimited_vec(self.vote.clone())
     }
 
+    pub fn sign_bytes_into(&self, buf: &mut impl BufMut) -> Result<(), ProtobufError> {
+        Protobuf::<RawCanonicalVote>::encode_length_delimited(self.vote.clone(), buf)
+    }
+
     /// Return the actual signature on the canonicalized vote.
     pub fn signature(&self) -> &Signature {
         &self.signature