Fix SszBlock bug relating to excess ssz

If you gave SszBlock too many bytes it would hash them too.

lighthouse/state/block/block.rs

@@ -1,15 +1,12 @@
 use super::utils::types::Hash256;
-use super::attestation_record::{
-    AttestationRecord,
-    MIN_SSZ_ATTESTION_RECORD_LENGTH,
-};
+use super::attestation_record::AttestationRecord;
 use super::ssz::{ Encodable, SszStream };
 
 pub const MIN_SSZ_BLOCK_LENGTH: usize = {
     4 + 32 +    // parent_hash
     8 +         // slot_number
     4 + 32 +    // randao_reveal
-    4 + MIN_SSZ_ATTESTION_RECORD_LENGTH +  // attestations (minimum one)
+    4 +         // attestations (assuming zero)
     4 + 32 +    // pow_chain_ref
     4 + 32 +    // active_state_root
     4 + 32      // crystallized_state_root
@@ -71,8 +68,7 @@ mod tests {
 
     #[test]
     pub fn test_block_min_ssz_length() {
-        let mut b = Block::zero();
-        b.attestations = vec![AttestationRecord::zero()];
+        let b = Block::zero();
 
         let mut ssz_stream = SszStream::new();
         ssz_stream.append(&b);

lighthouse/state/block/ssz_block.rs

@@ -7,6 +7,7 @@ use super::block::{
     MIN_SSZ_BLOCK_LENGTH,
     MAX_SSZ_BLOCK_LENGTH,
 };
+use super::attestation_record::MIN_SSZ_ATTESTION_RECORD_LENGTH;
 
 #[derive(Debug, PartialEq)]
 pub enum BlockValidatorError {
@@ -32,34 +33,41 @@ impl<'a> SszBlock<'a> {
     pub fn from_slice(vec: &'a [u8])
         -> Result<Self, BlockValidatorError>
     {
-        let ssz = &vec[..];
-        let len = vec.len();
+        let untrimmed_ssz = &vec[..];
         /*
-         * Ensure the SSZ is long enough to be a block.
+         * Ensure the SSZ is long enough to be a block with
+         * one attestation record (not necessarily a valid
+         * attestation record).
          */
-        if len < MIN_SSZ_BLOCK_LENGTH {
+        if vec.len() < MIN_SSZ_BLOCK_LENGTH + MIN_SSZ_ATTESTION_RECORD_LENGTH {
             return Err(BlockValidatorError::TooShort);
         }
         /*
          * Ensure the SSZ slice isn't longer than is possible for a block.
          */
-        if len > MAX_SSZ_BLOCK_LENGTH {
+        if vec.len() > MAX_SSZ_BLOCK_LENGTH {
             return Err(BlockValidatorError::TooLong);
         }
         /*
-         * Determine how many bytes are used to store attestation records
-         * and ensure that length is enough to store at least one attestation
-         * record.
+         * Determine how many bytes are used to store attestation records.
          */
-        let attestation_len = decode_length(ssz, 80, LENGTH_BYTES)
+        let attestation_len = decode_length(untrimmed_ssz, 80, LENGTH_BYTES)
             .map_err(|_| BlockValidatorError::TooShort)?;
-        if len < (76 + attestation_len + 96) {
+        /*
+         * The block only has one variable field, `attestations`, therefore
+         * the size of the block must be the minimum size, plus the length
+         * of the attestations.
+         */
+        let block_ssz_len = {
+            MIN_SSZ_BLOCK_LENGTH + attestation_len
+        };
+        if vec.len() < block_ssz_len {
             return Err(BlockValidatorError::TooShort);
         }
         Ok(Self{
-            ssz,
+            ssz: &untrimmed_ssz[0..block_ssz_len],
             attestation_len,
-            len,
+            len: block_ssz_len,
         })
     }
 
@@ -156,6 +164,19 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_ssz_block_single_attestation_record_one_byte_long() {
+        let mut b = Block::zero();
+        b.attestations = vec![AttestationRecord::zero()];
+        let mut ssz = get_block_ssz(&b);
+        let original_len = ssz.len();
+        ssz.push(42);
+
+        let ssz_block = SszBlock::from_slice(&ssz[..]).unwrap();
+
+        assert_eq!(ssz_block.len, original_len);
+    }
+
     #[test]
     fn test_ssz_block_single_attestation_record() {
         let mut b = Block::zero();