refactor: make segment trailer serializing less error prone

src/segment/file_offsets.rs

@@ -5,7 +5,7 @@ use crate::{
 use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
 use std::io::{Read, Write};
 
-#[derive(Debug)]
+#[derive(Debug, Default)]
 pub struct FileOffsets {
     pub index_block_ptr: u64,
     pub tli_ptr: u64,
@@ -14,6 +14,14 @@ pub struct FileOffsets {
     pub metadata_ptr: u64,
 }
 
+impl FileOffsets {
+    /// Returns the on-disk size
+    #[must_use]
+    pub const fn serialized_len() -> usize {
+        5 * std::mem::size_of::<u64>()
+    }
+}
+
 impl Serializable for FileOffsets {
     fn serialize<W: Write>(&self, writer: &mut W) -> Result<(), SerializeError> {
         writer.write_u64::<BigEndian>(self.index_block_ptr)?;
@@ -42,3 +50,19 @@ impl Deserializable for FileOffsets {
         })
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use test_log::test;
+
+    #[test]
+    fn file_offsets_serialized_len() -> crate::Result<()> {
+        let mut buf = vec![];
+        FileOffsets::default().serialize(&mut buf)?;
+
+        assert_eq!(FileOffsets::serialized_len(), buf.len());
+
+        Ok(())
+    }
+}

src/segment/trailer.rs

@@ -31,8 +31,7 @@ impl SegmentFileTrailer {
         // Parse pointers
         let offsets = FileOffsets::deserialize(&mut reader)?;
 
-        // IMPORTANT: sizeof(offsets) ---------v
-        let remaining_padding = TRAILER_SIZE - 5 * std::mem::size_of::<u64>() - TRAILER_MAGIC.len();
+        let remaining_padding = TRAILER_SIZE - FileOffsets::serialized_len() - TRAILER_MAGIC.len();
         reader.seek_relative(remaining_padding as i64)?;
 
         // Check trailer magic