removes fallible ErasureMeta.fec_set_index type-casts

fec_set_index is u32 by the definition in the shred struct. Blockstore
however uses u64 for ErasureMeta.fec_set_index for backward
compatibility reasons which results in weird typing and fallible
conversions from u64 to u32. Any ErasureMeta with a fec_set_index which
does not fit in u32 is invalid and should be discarded early.

The commit changes ErasureMeta.fec_set_index type to u32 while using
serde attributes to maintain backward compatibility for
(de)serialization.

ledger/src/blockstore_meta.rs

@@ -122,7 +122,11 @@ pub struct ShredIndex {
 /// Erasure coding information
 pub struct ErasureMeta {
     /// Which erasure set in the slot this is
-    fec_set_index: u64,
+    #[serde(
+        serialize_with = "serde_compat_cast::serialize::<_, u64, _>",
+        deserialize_with = "serde_compat_cast::deserialize::<_, u64, _>"
+    )]
+    fec_set_index: u32,
     /// First coding index in the FEC set
     first_coding_index: u64,
     /// Index of the first received coding shred in the FEC set
@@ -131,6 +135,39 @@ pub struct ErasureMeta {
     config: ErasureConfig,
 }
 
+// Helper module to serde values by type-casting to an intermediate
+// type for backward compatibility.
+mod serde_compat_cast {
+    use super::*;
+
+    // Serializes a value of type T by first type-casting to type R.
+    pub(super) fn serialize<S: Serializer, R, T: Copy>(
+        &val: &T,
+        serializer: S,
+    ) -> Result<S::Ok, S::Error>
+    where
+        R: TryFrom<T> + Serialize,
+        <R as TryFrom<T>>::Error: std::fmt::Display,
+    {
+        R::try_from(val)
+            .map_err(serde::ser::Error::custom)?
+            .serialize(serializer)
+    }
+
+    // Deserializes a value of type R and type-casts it to type T.
+    pub(super) fn deserialize<'de, D, R, T>(deserializer: D) -> Result<T, D::Error>
+    where
+        D: Deserializer<'de>,
+        R: Deserialize<'de>,
+        T: TryFrom<R>,
+        <T as TryFrom<R>>::Error: std::fmt::Display,
+    {
+        R::deserialize(deserializer)
+            .map(T::try_from)?
+            .map_err(serde::de::Error::custom)
+    }
+}
+
 #[derive(Clone, Copy, Debug, Eq, PartialEq, Serialize, Deserialize)]
 pub(crate) struct ErasureConfig {
     num_data: usize,
@@ -349,7 +386,7 @@ impl ErasureMeta {
                 let first_coding_index = u64::from(shred.first_coding_index()?);
                 let first_received_coding_index = u64::from(shred.index());
                 let erasure_meta = ErasureMeta {
-                    fec_set_index: u64::from(shred.fec_set_index()),
+                    fec_set_index: shred.fec_set_index(),
                     config,
                     first_coding_index,
                     first_received_coding_index,
@@ -384,7 +421,8 @@ impl ErasureMeta {
 
     pub(crate) fn data_shreds_indices(&self) -> Range<u64> {
         let num_data = self.config.num_data as u64;
-        self.fec_set_index..self.fec_set_index + num_data
+        let fec_set_index = u64::from(self.fec_set_index);
+        fec_set_index..fec_set_index + num_data
     }
 
     pub(crate) fn coding_shreds_indices(&self) -> Range<u64> {
@@ -397,11 +435,8 @@ impl ErasureMeta {
     }
 
     pub(crate) fn next_fec_set_index(&self) -> Option<u32> {
-        let num_data = u64::try_from(self.config.num_data).ok()?;
-        self.fec_set_index
-            .checked_add(num_data)
-            .map(u32::try_from)?
-            .ok()
+        let num_data = u32::try_from(self.config.num_data).ok()?;
+        self.fec_set_index.checked_add(num_data)
     }
 
     pub(crate) fn status(&self, index: &Index) -> ErasureMetaStatus {
@@ -575,7 +610,7 @@ mod test {
         };
         let e_meta = ErasureMeta {
             fec_set_index,
-            first_coding_index: fec_set_index,
+            first_coding_index: u64::from(fec_set_index),
             config: erasure_config,
             first_received_coding_index: 0,
         };
@@ -754,7 +789,7 @@ mod test {
             config: erasure_config,
         };
         let mut new_erasure_meta = ErasureMeta {
-            fec_set_index: set_index,
+            fec_set_index: u32::try_from(set_index).unwrap(),
             first_coding_index: set_index,
             first_received_coding_index: 0,
             config: erasure_config,