light-client-verifier: reuse sign_bytes buffer

Rather than allocating a new vector each time canonical vote is
encoded, allocate a vector in NonAbsentCommitVotes once and then reuse
it each time sign_bytes are needed.  This saves on doing multiple
allocations and deallocations.

For overly verbose description of the length calculation:

* CanonicalVote.  Note that length prefix doesn’t count bytes used for
  encoding the length, e.g. if the CanonicalVote message is 127 bytes
  than the length prefix is one byte and thus sign_bytes is 128 bytes.
  The table assumes all fields are set.

      |                 | tag | value |
      |-----------------+-----+-------+
      | (length)        |     |   1–2 | 2 if (length) is > 127
      | type            |   1 |     1 | SignedMsgType is at most 32
      | height          |   1 |     8 | The field is sfixed64
      | round           |   1 |     8 | The field is sfixed64
      | block_id        |   1 | 73–77 | See CanonicalBlockId below
      | timestamp       |   1 |  7—13 | See Timestamp below
      | chain_id        |   1 |   1+N | Assuming N < 128
      |-----------------+-----+-------+
      | (total)         |   105–116+N | If N≤13 than (length) is
      |                 |             | 1 byte and this is 115+N

* CanonicalBlockId.

      |                 | tag | value |
      |-----------------+-----+-------+
      | (length)        |     |     1 |
      | hash            |   1 |    33 | Assuming 32-byte hash.
      | part_set_header |   1 | 37–41 | See CanonicalPartSetHeader
      |-----------------+-----+-------+
      | (total)         |       73–77 |

* CanonicalPartSetHeader.

      |                 | tag | value |
      |-----------------+-----+-------+
      | (length)        |     |     1 |
      | total           |   1 |   1–5 | Varint of u32; at most 5 bytes.
      | hash            |   1 |    33 | Assuming 32-byte hash.
      |-----------------+-----+-------+
      | (total)         |       37–41 |

* Timestamp.  In unlikely event vote was made exactly at a second,
  nanos is zero and thus the field is not present.  Assuming uniform
  probability of the nanos, in a bit over 73% of cases the filed will
  occupy it’s maximum size.

      |                 | tag | value |
      |-----------------+-----+-------+
      | (length)        |     |     1 |
      | seconds         |   1 |     5 | Assuming timestamps around now.
      | nanos           | 0–1 |   0–5 |
      |-----------------+-----+-------+
      | (total)         |     |  7–13 |

.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

@@ -264,6 +264,11 @@ 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 {
@@ -297,7 +302,12 @@ impl NonAbsentCommitVotes {
                 pair[0].validator_id(),
             ))
         } else {
-            Ok(Self { votes })
+            // As of protocol 0.38, 160 buffer is enough for chain identifier up
+            // to 44 bytes.
+            Ok(Self {
+                votes,
+                sign_bytes: Vec::with_capacity(160),
+            })
         }
     }
 
@@ -319,14 +329,18 @@ 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)
+                .unwrap();
+            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