value-log 1.9.0

Value log implementation for key-value separated LSM storage
Documentation 
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// Copyright (c) 2024-present, fjall-rs
// This source code is licensed under both the Apache 2.0 and MIT License
// (found in the LICENSE-* files in the repository)

use super::meta::Metadata;
use crate::coding::{Decode, DecodeError, Encode, EncodeError};
use byteorder::{BigEndian, ReadBytesExt, WriteBytesExt};
use std::{
    fs::File,
    io::{BufReader, Read, Seek, Write},
    path::Path,
};

pub const TRAILER_MAGIC: &[u8] = &[b'V', b'L', b'O', b'G', b'T', b'R', b'L', 1];
pub const TRAILER_SIZE: usize = 256;

#[derive(Debug)]
#[allow(clippy::module_name_repetitions)]
pub struct SegmentFileTrailer {
    pub metadata: Metadata,
    pub metadata_ptr: u64,
}

impl SegmentFileTrailer {
    pub fn from_file<P: AsRef<Path>>(path: P) -> crate::Result<Self> {
        let file = File::open(path)?;
        let mut reader = BufReader::new(file);
        reader.seek(std::io::SeekFrom::End(-(TRAILER_SIZE as i64)))?;

        // Get metadata ptr
        let metadata_ptr = reader.read_u64::<BigEndian>()?;

        // IMPORTANT: Subtract sizeof(meta_ptr) ------v
        let remaining_padding = TRAILER_SIZE - std::mem::size_of::<u64>() - TRAILER_MAGIC.len();
        reader.seek_relative(remaining_padding as i64)?;

        // Check trailer magic
        let mut magic = [0u8; TRAILER_MAGIC.len()];
        reader.read_exact(&mut magic)?;

        if magic != TRAILER_MAGIC {
            return Err(crate::Error::Decode(DecodeError::InvalidHeader(
                "SegmentTrailer",
            )));
        }

        // Jump to metadata and parse
        reader.seek(std::io::SeekFrom::Start(metadata_ptr))?;
        let metadata = Metadata::decode_from(&mut reader)?;

        Ok(Self {
            metadata,
            metadata_ptr,
        })
    }
}

impl Encode for SegmentFileTrailer {
    fn encode_into<W: Write>(&self, writer: &mut W) -> Result<(), EncodeError> {
        let mut v = Vec::with_capacity(TRAILER_SIZE);

        v.write_u64::<BigEndian>(self.metadata_ptr)?;

        // Pad with remaining bytes
        v.resize(TRAILER_SIZE - TRAILER_MAGIC.len(), 0);

        v.write_all(TRAILER_MAGIC)?;

        assert_eq!(
            v.len(),
            TRAILER_SIZE,
            "segment file trailer has invalid size"
        );

        writer.write_all(&v)?;

        Ok(())
    }
}