blockstore: store first received coding shred index in ErasureMeta (s…

…olana-labs#961)

* blockstore: store first received coding shred index in ErasureMeta

* pr feedback: use u64 instead of usize

ledger/src/blockstore_meta.rs

@@ -125,9 +125,8 @@ pub struct ErasureMeta {
     set_index: u64,
     /// First coding index in the FEC set
     first_coding_index: u64,
-    /// Size of shards in this erasure set
-    #[serde(rename = "size")]
-    __unused_size: usize,
+    /// Index of the first received coding shred in the FEC set
+    first_received_coding_index: u64,
     /// Erasure configuration for this erasure set
     config: ErasureConfig,
 }
@@ -348,11 +347,12 @@ impl ErasureMeta {
                     num_coding: usize::from(shred.num_coding_shreds().ok()?),
                 };
                 let first_coding_index = u64::from(shred.first_coding_index()?);
+                let first_received_coding_index = u64::from(shred.index());
                 let erasure_meta = ErasureMeta {
                     set_index: u64::from(shred.fec_set_index()),
                     config,
                     first_coding_index,
-                    __unused_size: 0,
+                    first_received_coding_index,
                 };
                 Some(erasure_meta)
             }
@@ -365,7 +365,7 @@ impl ErasureMeta {
         let Some(mut other) = Self::from_coding_shred(shred) else {
             return false;
         };
-        other.__unused_size = self.__unused_size;
+        other.first_received_coding_index = self.first_received_coding_index;
         self == &other
     }
 
@@ -392,6 +392,11 @@ impl ErasureMeta {
         self.first_coding_index..self.first_coding_index + num_coding
     }
 
+    #[allow(dead_code)]
+    pub(crate) fn first_received_coding_shred_index(&self) -> Option<u32> {
+        u32::try_from(self.first_received_coding_index).ok()
+    }
+
     pub(crate) fn next_fec_set_index(&self) -> Option<u32> {
         let num_data = u64::try_from(self.config.num_data).ok()?;
         self.set_index
@@ -568,7 +573,7 @@ mod test {
             set_index,
             first_coding_index: set_index,
             config: erasure_config,
-            __unused_size: 0,
+            first_received_coding_index: 0,
         };
         let mut rng = thread_rng();
         let mut index = Index::new(0);
@@ -721,4 +726,54 @@ mod test {
 
         assert_eq!(actual, expected);
     }
+
+    #[test]
+    fn test_erasure_meta_transition() {
+        #[derive(Debug, Deserialize, PartialEq, Serialize)]
+        struct OldErasureMeta {
+            set_index: u64,
+            first_coding_index: u64,
+            #[serde(rename = "size")]
+            __unused_size: usize,
+            config: ErasureConfig,
+        }
+
+        let set_index = 64;
+        let erasure_config = ErasureConfig {
+            num_data: 8,
+            num_coding: 16,
+        };
+        let mut old_erasure_meta = OldErasureMeta {
+            set_index,
+            first_coding_index: set_index,
+            __unused_size: 0,
+            config: erasure_config,
+        };
+        let mut new_erasure_meta = ErasureMeta {
+            set_index,
+            first_coding_index: set_index,
+            first_received_coding_index: 0,
+            config: erasure_config,
+        };
+
+        assert_eq!(
+            bincode::serialized_size(&old_erasure_meta).unwrap(),
+            bincode::serialized_size(&new_erasure_meta).unwrap(),
+        );
+
+        assert_eq!(
+            bincode::deserialize::<ErasureMeta>(&bincode::serialize(&old_erasure_meta).unwrap())
+                .unwrap(),
+            new_erasure_meta
+        );
+
+        new_erasure_meta.first_received_coding_index = u64::from(u32::max_value());
+        old_erasure_meta.__unused_size = usize::try_from(u32::max_value()).unwrap();
+
+        assert_eq!(
+            bincode::deserialize::<OldErasureMeta>(&bincode::serialize(&new_erasure_meta).unwrap())
+                .unwrap(),
+            old_erasure_meta
+        );
+    }
 }