cu29_unifiedlog/

memmap.rs

//! This is the memory map file implementation for the unified logger for Copper.
//! It is std only.

use crate::{
    AllocatedSection, MAIN_MAGIC, MainHeader, SECTION_MAGIC, SectionHandle, SectionHeader,
    SectionStorage, UNIFIED_LOG_FORMAT_VERSION, UnifiedLogRead, UnifiedLogStatus, UnifiedLogWrite,
};

use crate::SECTION_HEADER_COMPACT_SIZE;

use AllocatedSection::Section;
use bincode::config::standard;
use bincode::enc::EncoderImpl;
use bincode::enc::write::SliceWriter;
use bincode::error::EncodeError;
use bincode::{Encode, decode_from_slice, encode_into_slice};
use core::slice::from_raw_parts_mut;
use cu29_traits::{
    CuError, CuResult, ObservedWriter, UnifiedLogType, abort_observed_encode,
    begin_observed_encode, finish_observed_encode,
};
use memmap2::{Mmap, MmapMut};
use std::fs::{File, OpenOptions};
use std::io::Read;
use std::mem::ManuallyDrop;
use std::path::{Path, PathBuf};
use std::{io, mem};

pub struct MmapSectionStorage {
    buffer: &'static mut [u8],
    offset: usize,
    block_size: usize,
}

impl MmapSectionStorage {
    pub fn new(buffer: &'static mut [u8], block_size: usize) -> Self {
        Self {
            buffer,
            offset: 0,
            block_size,
        }
    }

    pub fn buffer_ptr(&self) -> *const u8 {
        &self.buffer[0] as *const u8
    }
}

impl SectionStorage for MmapSectionStorage {
    fn initialize<E: Encode>(&mut self, header: &E) -> Result<usize, EncodeError> {
        self.post_update_header(header)?;
        self.offset = self.block_size;
        Ok(self.offset)
    }

    fn post_update_header<E: Encode>(&mut self, header: &E) -> Result<usize, EncodeError> {
        encode_into_slice(header, &mut self.buffer[0..], standard())
    }

    fn append<E: Encode>(&mut self, entry: &E) -> Result<usize, EncodeError> {
        begin_observed_encode();
        let result = (|| {
            let mut encoder = EncoderImpl::new(
                ObservedWriter::new(SliceWriter::new(&mut self.buffer[self.offset..])),
                standard(),
            );
            entry.encode(&mut encoder)?;
            Ok(encoder.into_writer().into_inner().bytes_written())
        })();
        let size = match result {
            Ok(size) => {
                debug_assert_eq!(size, finish_observed_encode());
                size
            }
            Err(err) => {
                abort_observed_encode();
                return Err(err);
            }
        };
        self.offset += size;
        Ok(size)
    }

    fn flush(&mut self) -> CuResult<usize> {
        // Flushing is handled at the slab level for mmap-backed storage.
        Ok(self.offset)
    }
}

///
/// Holds the read or write side of the datalogger.
pub enum MmapUnifiedLogger {
    Read(MmapUnifiedLoggerRead),
    Write(MmapUnifiedLoggerWrite),
}

/// Use this builder to create a new DataLogger.
pub struct MmapUnifiedLoggerBuilder {
    file_base_name: Option<PathBuf>,
    preallocated_size: Option<usize>,
    write: bool,
    create: bool,
    append: bool,
}

impl Default for MmapUnifiedLoggerBuilder {
    fn default() -> Self {
        Self::new()
    }
}

impl MmapUnifiedLoggerBuilder {
    pub fn new() -> Self {
        Self {
            file_base_name: None,
            preallocated_size: None,
            write: false,
            create: false, // This is the safest default
            append: false,
        }
    }

    /// If "something/toto.copper" is given, it will find or create "something/toto_0.copper",  "something/toto_1.copper" etc.
    pub fn file_base_name(mut self, file_path: &Path) -> Self {
        self.file_base_name = Some(file_path.to_path_buf());
        self
    }

    pub fn preallocated_size(mut self, preallocated_size: usize) -> Self {
        self.preallocated_size = Some(preallocated_size);
        self
    }

    pub fn write(mut self, write: bool) -> Self {
        self.write = write;
        self
    }

    pub fn create(mut self, create: bool) -> Self {
        self.create = create;
        self
    }

    /// When `write` and `create` are both set, resume writing at the end of an
    /// existing log instead of truncating it.
    ///
    /// Requires a cleanly closed log (one ending with a permanent end-of-log
    /// marker). Returns an error if the log does not exist or is incomplete.
    pub fn append(mut self, append: bool) -> Self {
        self.append = append;
        self
    }

    pub fn build(self) -> io::Result<MmapUnifiedLogger> {
        let page_size = page_size::get();

        if self.write && self.create {
            let file_path = self.file_base_name.ok_or_else(|| {
                io::Error::new(
                    io::ErrorKind::InvalidInput,
                    "File path is required for write mode",
                )
            })?;
            let preallocated_size = self.preallocated_size.ok_or_else(|| {
                io::Error::new(
                    io::ErrorKind::InvalidInput,
                    "Preallocated size is required for write mode",
                )
            })?;
            let ulw = if self.append {
                MmapUnifiedLoggerWrite::append(&file_path, preallocated_size)?
            } else {
                MmapUnifiedLoggerWrite::new(&file_path, preallocated_size, page_size)?
            };
            Ok(MmapUnifiedLogger::Write(ulw))
        } else {
            let file_path = self.file_base_name.ok_or_else(|| {
                io::Error::new(io::ErrorKind::InvalidInput, "File path is required")
            })?;
            let ulr = MmapUnifiedLoggerRead::new(&file_path)?;
            Ok(MmapUnifiedLogger::Read(ulr))
        }
    }
}

struct SlabEntry {
    file: File,
    mmap_buffer: ManuallyDrop<MmapMut>,
    current_global_position: usize,
    sections_offsets_in_flight: Vec<usize>,
    flushed_until_offset: usize,
    page_size: usize,
    temporary_end_marker: Option<usize>,
    #[cfg(test)]
    closed_sections: Vec<(usize, usize)>,
    #[cfg(test)]
    flushed_ranges: Vec<(usize, usize)>,
    #[cfg(all(test, feature = "mmap-fsync"))]
    sync_call_count: usize,
}

impl Drop for SlabEntry {
    fn drop(&mut self) {
        self.flush_until(self.current_global_position);
        // SAFETY: We own the mapping and must drop it before trimming the file.
        unsafe { ManuallyDrop::drop(&mut self.mmap_buffer) };
        if let Err(error) = self.file.set_len(self.current_global_position as u64) {
            eprintln!("Failed to trim datalogger file: {}", error);
        }
        self.sync_file();

        if !self.sections_offsets_in_flight.is_empty() {
            eprintln!("Error: Slab not full flushed.");
        }
    }
}

impl SlabEntry {
    fn new(file: File, page_size: usize) -> io::Result<Self> {
        let mmap_buffer = ManuallyDrop::new(
            // SAFETY: The file descriptor is valid and mapping is confined to this struct.
            unsafe { MmapMut::map_mut(&file) }
                .map_err(|e| io::Error::new(e.kind(), format!("Failed to map file: {e}")))?,
        );
        Ok(Self {
            file,
            mmap_buffer,
            current_global_position: 0,
            sections_offsets_in_flight: Vec::with_capacity(16),
            flushed_until_offset: 0,
            page_size,
            temporary_end_marker: None,
            #[cfg(test)]
            closed_sections: Vec::new(),
            #[cfg(test)]
            flushed_ranges: Vec::new(),
            #[cfg(all(test, feature = "mmap-fsync"))]
            sync_call_count: 0,
        })
    }

    fn flush_range(&mut self, start: usize, len: usize) {
        if len == 0 {
            return;
        }
        self.mmap_buffer
            .flush_async_range(start, len)
            .expect("Failed to flush memory map");
        self.sync_file();
        #[cfg(test)]
        self.record_flushed_range(start, len);
    }

    fn sync_file(&mut self) {
        #[cfg(feature = "mmap-fsync")]
        {
            self.file.sync_all().expect("Failed to fsync log file");
            #[cfg(test)]
            {
                self.sync_call_count += 1;
            }
        }
    }
    /// Unsure the underlying mmap is flush to disk until the given position.
    fn flush_until(&mut self, until_position: usize) {
        // This is tolerated under linux, but crashes on macos
        if (self.flushed_until_offset == until_position) || (until_position == 0) {
            return;
        }
        self.flush_range(
            self.flushed_until_offset,
            until_position - self.flushed_until_offset,
        );
        self.flushed_until_offset = until_position;
    }

    fn clear_temporary_end_marker(&mut self) {
        if let Some(marker_start) = self.temporary_end_marker.take() {
            self.current_global_position = marker_start;
            if self.flushed_until_offset > marker_start {
                self.flushed_until_offset = marker_start;
            }
        }
    }

    fn write_end_marker(&mut self, temporary: bool) -> CuResult<()> {
        let block_size = SECTION_HEADER_COMPACT_SIZE as usize;
        let marker_start = self.align_to_next_page(self.current_global_position);
        let total_marker_size = block_size; // header only
        let marker_end = marker_start + total_marker_size;
        if marker_end > self.mmap_buffer.len() {
            return Err("Not enough space to write end-of-log marker".into());
        }

        let header = SectionHeader {
            magic: SECTION_MAGIC,
            block_size: SECTION_HEADER_COMPACT_SIZE,
            entry_type: UnifiedLogType::LastEntry,
            offset_to_next_section: total_marker_size as u32,
            used: 0,
            is_open: temporary,
        };

        encode_into_slice(
            &header,
            &mut self.mmap_buffer
                [marker_start..marker_start + SECTION_HEADER_COMPACT_SIZE as usize],
            standard(),
        )
        .map_err(|e| CuError::new_with_cause("Failed to encode end-of-log header", e))?;

        self.temporary_end_marker = Some(marker_start);
        self.current_global_position = marker_end;
        Ok(())
    }

    fn is_it_my_section(&self, section: &SectionHandle<MmapSectionStorage>) -> bool {
        let storage = section.get_storage();
        let ptr = storage.buffer_ptr();
        (ptr >= self.mmap_buffer.as_ptr())
            && (ptr as usize)
                < (self.mmap_buffer.as_ref().as_ptr() as usize + self.mmap_buffer.as_ref().len())
    }

    /// Flush the section to disk.
    /// the flushing is permanent and the section is considered closed.
    fn flush_section(&mut self, section: &mut SectionHandle<MmapSectionStorage>) {
        section
            .post_update_header()
            .expect("Failed to update section header");

        let storage = section.get_storage();
        let ptr = storage.buffer_ptr();

        if ptr < self.mmap_buffer.as_ptr()
            || ptr as usize > self.mmap_buffer.as_ptr() as usize + self.mmap_buffer.len()
        {
            panic!("Invalid section buffer, not in the slab");
        }

        let base = self.mmap_buffer.as_ptr() as usize;
        let section_start = ptr as usize - base;
        let section_len = section.header.offset_to_next_section as usize;
        #[cfg(test)]
        self.record_closed_section(section_start, section_len);
        self.sections_offsets_in_flight
            .retain(|&x| x != section_start);

        if self.sections_offsets_in_flight.is_empty() {
            self.flush_until(self.current_global_position);
            return;
        }
        let next_open_offset = self.sections_offsets_in_flight[0];
        if self.flushed_until_offset < next_open_offset {
            self.flush_until(next_open_offset);
        }
        if section_start + section_len > self.flushed_until_offset {
            // A long-lived early section can otherwise pin later closed sections
            // behind the prefix cursor until shutdown.
            self.flush_range(section_start, section_len);
        }
    }

    #[cfg(test)]
    fn record_closed_section(&mut self, start: usize, len: usize) {
        self.closed_sections.push((start, len));
    }

    #[cfg(test)]
    fn record_flushed_range(&mut self, start: usize, len: usize) {
        let mut merged_start = start;
        let mut merged_end = start + len;
        let mut merged_ranges = Vec::with_capacity(self.flushed_ranges.len() + 1);
        let mut inserted = false;

        for (range_start, range_len) in self.flushed_ranges.drain(..) {
            let range_end = range_start + range_len;
            if range_end < merged_start {
                merged_ranges.push((range_start, range_len));
                continue;
            }
            if merged_end < range_start {
                if !inserted {
                    merged_ranges.push((merged_start, merged_end - merged_start));
                    inserted = true;
                }
                merged_ranges.push((range_start, range_len));
                continue;
            }

            merged_start = merged_start.min(range_start);
            merged_end = merged_end.max(range_end);
        }

        if !inserted {
            merged_ranges.push((merged_start, merged_end - merged_start));
        }

        self.flushed_ranges = merged_ranges;
    }

    #[cfg(test)]
    fn pending_closed_bytes(&self) -> usize {
        let mut pending = 0;

        for (section_start, section_len) in &self.closed_sections {
            let section_end = section_start + section_len;
            let mut cursor = *section_start;

            for (range_start, range_len) in &self.flushed_ranges {
                let range_end = range_start + range_len;
                if range_end <= cursor {
                    continue;
                }
                if *range_start >= section_end {
                    break;
                }
                if *range_start > cursor {
                    pending += *range_start - cursor;
                }
                cursor = cursor.max(range_end);
                if cursor >= section_end {
                    break;
                }
            }

            if cursor < section_end {
                pending += section_end - cursor;
            }
        }

        pending
    }

    #[inline]
    fn align_to_next_page(&self, ptr: usize) -> usize {
        (ptr + self.page_size - 1) & !(self.page_size - 1)
    }

    /// The returned slice is section_size or greater.
    fn add_section(
        &mut self,
        entry_type: UnifiedLogType,
        requested_section_size: usize,
    ) -> AllocatedSection<MmapSectionStorage> {
        // align current_position to the next page
        self.current_global_position = self.align_to_next_page(self.current_global_position);
        let section_size = self.align_to_next_page(requested_section_size) as u32;

        // We need to have enough space to store the section in that slab
        if self.current_global_position + section_size as usize > self.mmap_buffer.len() {
            return AllocatedSection::NoMoreSpace;
        }

        #[cfg(feature = "compact")]
        let block_size = SECTION_HEADER_COMPACT_SIZE;

        #[cfg(not(feature = "compact"))]
        let block_size = self.page_size as u16;

        let section_header = SectionHeader {
            magic: SECTION_MAGIC,
            block_size,
            entry_type,
            offset_to_next_section: section_size,
            used: 0u32,
            is_open: true,
        };

        // save the position to keep track for in flight sections
        self.sections_offsets_in_flight
            .push(self.current_global_position);
        let end_of_section = self.current_global_position + requested_section_size;
        let user_buffer = &mut self.mmap_buffer[self.current_global_position..end_of_section];

        // SAFETY: We have exclusive access to user_buffer for the handle's lifetime.
        let handle_buffer =
            unsafe { from_raw_parts_mut(user_buffer.as_mut_ptr(), user_buffer.len()) };
        let storage = MmapSectionStorage::new(handle_buffer, block_size as usize);

        self.current_global_position = end_of_section;

        Section(SectionHandle::create(section_header, storage).expect("Failed to create section"))
    }

    #[cfg(test)]
    fn used(&self) -> usize {
        self.current_global_position
    }
}

/// A write side of the datalogger.
pub struct MmapUnifiedLoggerWrite {
    /// the front slab is the current active slab for any new section.
    front_slab: SlabEntry,
    /// the back slab is the previous slab that is being flushed.
    back_slabs: Vec<SlabEntry>,
    /// base file path to create the backing files from.
    base_file_path: PathBuf,
    /// allocation size for the backing files.
    slab_size: usize,
    /// current suffix for the backing files.
    front_slab_suffix: usize,
}

fn build_slab_path(base_file_path: &Path, slab_index: usize) -> io::Result<PathBuf> {
    let mut file_path = base_file_path.to_path_buf();
    let stem = file_path.file_stem().ok_or_else(|| {
        io::Error::new(
            io::ErrorKind::InvalidInput,
            "Base file path has no file name",
        )
    })?;
    let stem = stem.to_str().ok_or_else(|| {
        io::Error::new(
            io::ErrorKind::InvalidInput,
            "Base file name is not valid UTF-8",
        )
    })?;
    let extension = file_path.extension().ok_or_else(|| {
        io::Error::new(
            io::ErrorKind::InvalidInput,
            "Base file path has no extension",
        )
    })?;
    let extension = extension.to_str().ok_or_else(|| {
        io::Error::new(
            io::ErrorKind::InvalidInput,
            "Base file extension is not valid UTF-8",
        )
    })?;
    if stem.is_empty() {
        return Err(io::Error::new(
            io::ErrorKind::InvalidInput,
            "Base file name is empty",
        ));
    }
    let file_name = format!("{stem}_{slab_index}.{extension}");
    file_path.set_file_name(file_name);
    Ok(file_path)
}

fn make_slab_file(base_file_path: &Path, slab_size: usize, slab_suffix: usize) -> io::Result<File> {
    let file_path = build_slab_path(base_file_path, slab_suffix)?;
    let file = OpenOptions::new()
        .read(true)
        .write(true)
        .create(true)
        .truncate(true)
        .open(&file_path)
        .map_err(|e| {
            io::Error::new(
                e.kind(),
                format!("Failed to open file {}: {e}", file_path.display()),
            )
        })?;
    file.set_len(slab_size as u64).map_err(|e| {
        io::Error::new(
            e.kind(),
            format!("Failed to set file length for {}: {e}", file_path.display()),
        )
    })?;
    Ok(file)
}

fn remove_existing_alias(base_file_path: &Path) -> io::Result<()> {
    match std::fs::symlink_metadata(base_file_path) {
        Ok(meta) => {
            if meta.is_dir() {
                return Err(io::Error::new(
                    io::ErrorKind::AlreadyExists,
                    format!(
                        "Cannot create base log alias at {} because a directory already exists there",
                        base_file_path.display()
                    ),
                ));
            }
            std::fs::remove_file(base_file_path).map_err(|e| {
                io::Error::new(
                    e.kind(),
                    format!(
                        "Failed to remove existing base log alias {}: {e}",
                        base_file_path.display()
                    ),
                )
            })
        }
        Err(e) if e.kind() == io::ErrorKind::NotFound => Ok(()),
        Err(e) => Err(io::Error::new(
            e.kind(),
            format!(
                "Failed to inspect existing base log alias {}: {e}",
                base_file_path.display()
            ),
        )),
    }
}

fn create_base_alias_link(base_file_path: &Path) -> io::Result<()> {
    let first_slab_path = build_slab_path(base_file_path, 0)?;
    remove_existing_alias(base_file_path)?;

    #[cfg(unix)]
    {
        use std::os::unix::fs::symlink;
        let relative_target = Path::new(first_slab_path.file_name().ok_or_else(|| {
            io::Error::new(
                io::ErrorKind::InvalidInput,
                "First slab file has no name component",
            )
        })?);
        symlink(relative_target, base_file_path).map_err(|e| {
            io::Error::new(
                e.kind(),
                format!(
                    "Failed to create base log alias {} -> {}: {e}",
                    base_file_path.display(),
                    first_slab_path.display()
                ),
            )
        })
    }

    #[cfg(windows)]
    {
        use std::os::windows::fs::symlink_file;
        let relative_target = Path::new(first_slab_path.file_name().ok_or_else(|| {
            io::Error::new(
                io::ErrorKind::InvalidInput,
                "First slab file has no name component",
            )
        })?);
        match symlink_file(relative_target, base_file_path) {
            Ok(()) => Ok(()),
            Err(symlink_err) => std::fs::hard_link(&first_slab_path, base_file_path).map_err(
                |hard_link_err| {
                    io::Error::other(format!(
                        "Failed to create base log alias {}. Symlink error: {symlink_err}. Hard-link fallback error: {hard_link_err}",
                        base_file_path.display()
                    ))
                },
            ),
        }?;
        Ok(())
    }

    #[cfg(not(any(unix, windows)))]
    {
        std::fs::hard_link(&first_slab_path, base_file_path).map_err(|e| {
            io::Error::new(
                e.kind(),
                format!(
                    "Failed to create base log alias {} -> {}: {e}",
                    base_file_path.display(),
                    first_slab_path.display()
                ),
            )
        })
    }
}

impl UnifiedLogWrite<MmapSectionStorage> for MmapUnifiedLoggerWrite {
    /// The returned slice is section_size or greater.
    fn add_section(
        &mut self,
        entry_type: UnifiedLogType,
        requested_section_size: usize,
    ) -> CuResult<SectionHandle<MmapSectionStorage>> {
        self.garbage_collect_backslabs(); // Take the opportunity to keep up and close stale back slabs.
        self.front_slab.clear_temporary_end_marker();
        let maybe_section = self
            .front_slab
            .add_section(entry_type, requested_section_size);

        match maybe_section {
            AllocatedSection::NoMoreSpace => {
                // move the front slab to the back slab.
                let new_slab = self.create_slab()?;
                // keep the slab until all its sections has been flushed.
                self.back_slabs
                    .push(mem::replace(&mut self.front_slab, new_slab));
                match self
                    .front_slab
                    .add_section(entry_type, requested_section_size)
                {
                    AllocatedSection::NoMoreSpace => Err(CuError::from("out of space")),
                    Section(section) => {
                        self.place_end_marker(true)?;
                        Ok(section)
                    }
                }
            }
            Section(section) => {
                self.place_end_marker(true)?;
                Ok(section)
            }
        }
    }

    fn flush_section(&mut self, section: &mut SectionHandle<MmapSectionStorage>) {
        section.mark_closed();
        for slab in self.back_slabs.iter_mut() {
            if slab.is_it_my_section(section) {
                slab.flush_section(section);
                return;
            }
        }
        self.front_slab.flush_section(section);
    }

    fn status(&self) -> UnifiedLogStatus {
        UnifiedLogStatus {
            total_used_space: self.front_slab.current_global_position,
            total_allocated_space: self.slab_size * self.front_slab_suffix,
        }
    }
}

impl MmapUnifiedLoggerWrite {
    fn next_slab(&mut self) -> io::Result<File> {
        let next_suffix = self.front_slab_suffix + 1;
        let file = make_slab_file(&self.base_file_path, self.slab_size, next_suffix)?;
        self.front_slab_suffix = next_suffix;
        Ok(file)
    }

    fn new(base_file_path: &Path, slab_size: usize, page_size: usize) -> io::Result<Self> {
        let file = make_slab_file(base_file_path, slab_size, 0)?;
        create_base_alias_link(base_file_path)?;
        let mut front_slab = SlabEntry::new(file, page_size)?;

        // This is the first slab so add the main header.
        let main_header = MainHeader {
            magic: MAIN_MAGIC,
            format_version: UNIFIED_LOG_FORMAT_VERSION,
            first_section_offset: page_size as u16,
            page_size: page_size as u16,
        };
        let nb_bytes = encode_into_slice(&main_header, &mut front_slab.mmap_buffer[..], standard())
            .map_err(|e| io::Error::other(format!("Failed to encode main header: {e}")))?;
        assert!(nb_bytes < page_size);
        front_slab.current_global_position = page_size; // align to the next page

        Ok(Self {
            front_slab,
            back_slabs: Vec::new(),
            base_file_path: base_file_path.to_path_buf(),
            slab_size,
            front_slab_suffix: 0,
        })
    }

    /// Resume writing at the end of a cleanly closed log instead of zapping it.
    ///
    /// Scans the existing log for the permanent end-of-log marker, reopens the
    /// last slab for writing (re-extending it to `slab_size`, since it is trimmed
    /// on clean shutdown), and positions the cursor right after that marker so new
    /// sections overwrite it. The main header and earlier slabs are left untouched.
    fn append(base_file_path: &Path, slab_size: usize) -> io::Result<Self> {
        // Locate the end-of-log marker using the read side.
        let mut reader = MmapUnifiedLoggerRead::new(base_file_path)?;
        let end = reader.end_of_log().map_err(io::Error::other)?;
        let last_slab_index = end.slab_index;
        let resume_offset = end.offset;
        // Preserve the original page alignment recorded in the main header.
        let original_page_size = reader.raw_main_header().page_size as usize;
        drop(reader);

        let slab_path = build_slab_path(base_file_path, last_slab_index)?;
        let file = OpenOptions::new()
            .read(true)
            .write(true)
            .open(&slab_path)
            .map_err(|e| {
                io::Error::new(
                    e.kind(),
                    format!(
                        "Failed to open slab {} for append: {e}",
                        slab_path.display()
                    ),
                )
            })?;

        // The last slab is trimmed to its used size on clean shutdown; give it
        // back room for new sections.
        let current_len = file
            .metadata()
            .map_err(|e| {
                io::Error::new(
                    e.kind(),
                    format!("Failed to read metadata for {}", slab_path.display()),
                )
            })?
            .len();
        if (current_len as usize) < slab_size {
            file.set_len(slab_size as u64).map_err(|e| {
                io::Error::new(
                    e.kind(),
                    format!(
                        "Failed to extend slab {} for append: {e}",
                        slab_path.display()
                    ),
                )
            })?;
        }

        let mut front_slab = SlabEntry::new(file, original_page_size)?;
        front_slab.current_global_position = resume_offset;
        front_slab.flushed_until_offset = resume_offset;

        Ok(Self {
            front_slab,
            back_slabs: Vec::new(),
            base_file_path: base_file_path.to_path_buf(),
            slab_size,
            front_slab_suffix: last_slab_index,
        })
    }

    fn garbage_collect_backslabs(&mut self) {
        self.back_slabs
            .retain_mut(|slab| !slab.sections_offsets_in_flight.is_empty());
    }

    fn place_end_marker(&mut self, temporary: bool) -> CuResult<()> {
        match self.front_slab.write_end_marker(temporary) {
            Ok(_) => Ok(()),
            Err(_) => {
                // Not enough space in the current slab, roll to a new one.
                let new_slab = self.create_slab()?;
                self.back_slabs
                    .push(mem::replace(&mut self.front_slab, new_slab));
                self.front_slab.write_end_marker(temporary)
            }
        }
    }

    pub fn stats(&self) -> (usize, Vec<usize>, usize) {
        (
            self.front_slab.current_global_position,
            self.front_slab.sections_offsets_in_flight.clone(),
            self.back_slabs.len(),
        )
    }

    fn create_slab(&mut self) -> CuResult<SlabEntry> {
        let file = self
            .next_slab()
            .map_err(|e| CuError::new_with_cause("Failed to create slab file", e))?;
        SlabEntry::new(file, self.front_slab.page_size)
            .map_err(|e| CuError::new_with_cause("Failed to create slab memory map", e))
    }
}

impl Drop for MmapUnifiedLoggerWrite {
    fn drop(&mut self) {
        #[cfg(debug_assertions)]
        eprintln!("Flushing the unified Logger ... "); // Note this cannot be a structured log writing in this log.

        self.front_slab.clear_temporary_end_marker();
        if let Err(e) = self.place_end_marker(false) {
            panic!("Failed to flush the unified logger: {}", e);
        }
        self.front_slab
            .flush_until(self.front_slab.current_global_position);
        self.garbage_collect_backslabs();
        #[cfg(debug_assertions)]
        eprintln!("Unified Logger flushed."); // Note this cannot be a structured log writing in this log.
    }
}

fn open_slab_index(
    base_file_path: &Path,
    slab_index: usize,
) -> io::Result<(File, Mmap, u16, Option<MainHeader>)> {
    let mut options = OpenOptions::new();
    let options = options.read(true);

    let file_path = build_slab_path(base_file_path, slab_index)?;
    let file = options.open(&file_path).map_err(|e| {
        io::Error::new(
            e.kind(),
            format!("Failed to open slab file {}: {e}", file_path.display()),
        )
    })?;
    // SAFETY: The file is kept open for the lifetime of the mapping.
    let mmap = unsafe { Mmap::map(&file) }
        .map_err(|e| io::Error::new(e.kind(), format!("Failed to map slab file: {e}")))?;
    let mut prolog = 0u16;
    let mut maybe_main_header: Option<MainHeader> = None;
    if slab_index == 0 {
        let main_header: MainHeader;
        let _read: usize;
        (main_header, _read) = decode_from_slice(&mmap[..], standard()).map_err(|e| {
            io::Error::new(
                io::ErrorKind::InvalidData,
                format!("Failed to decode main header: {e}"),
            )
        })?;
        if main_header.magic != MAIN_MAGIC {
            return Err(io::Error::new(
                io::ErrorKind::InvalidData,
                "Invalid magic number in main header",
            ));
        }
        if main_header.format_version != UNIFIED_LOG_FORMAT_VERSION {
            return Err(io::Error::new(
                io::ErrorKind::InvalidData,
                format!(
                    "Unsupported unified log format version {} in main header; this reader supports version {}",
                    main_header.format_version, UNIFIED_LOG_FORMAT_VERSION
                ),
            ));
        }
        prolog = main_header.first_section_offset;
        maybe_main_header = Some(main_header);
    }
    Ok((file, mmap, prolog, maybe_main_header))
}

/// A read side of the memory map based unified logger.
pub struct MmapUnifiedLoggerRead {
    base_file_path: PathBuf,
    main_header: MainHeader,
    current_mmap_buffer: Mmap,
    current_file: File,
    current_slab_index: usize,
    current_reading_position: usize,
}

/// Absolute position inside a unified log (slab index + byte offset).
#[derive(Clone, Copy, Debug, PartialEq, Eq)]
pub struct LogPosition {
    pub slab_index: usize,
    pub offset: usize,
}

impl UnifiedLogRead for MmapUnifiedLoggerRead {
    fn read_next_section_type(&mut self, datalogtype: UnifiedLogType) -> CuResult<Option<Vec<u8>>> {
        // TODO: eventually implement a 0 copy of this too.
        loop {
            if self.current_reading_position >= self.current_mmap_buffer.len() {
                self.next_slab().map_err(|e| {
                    CuError::new_with_cause("Failed to read next slab, is the log complete?", e)
                })?;
            }

            let header_result = self.read_section_header();
            let header = header_result.map_err(|error| {
                CuError::new_with_cause(
                    &format!(
                        "Could not read a sections header: {}/{}:{}",
                        self.base_file_path.as_os_str().to_string_lossy(),
                        self.current_slab_index,
                        self.current_reading_position,
                    ),
                    error,
                )
            })?;

            // Reached the end of file
            if header.entry_type == UnifiedLogType::LastEntry {
                return Ok(None);
            }

            // Found a section of the requested type
            if header.entry_type == datalogtype {
                let result = Some(self.read_section_content(&header)?);
                self.current_reading_position += header.offset_to_next_section as usize;
                return Ok(result);
            }

            // Keep reading until we find the requested type
            self.current_reading_position += header.offset_to_next_section as usize;
        }
    }

    /// Reads the section from the section header pos.
    fn raw_read_section(&mut self) -> CuResult<(SectionHeader, Vec<u8>)> {
        if self.current_reading_position >= self.current_mmap_buffer.len() {
            self.next_slab().map_err(|e| {
                CuError::new_with_cause("Failed to read next slab, is the log complete?", e)
            })?;
        }

        let read_result = self.read_section_header();

        match read_result {
            Err(error) => Err(CuError::new_with_cause(
                &format!(
                    "Could not read a sections header: {}/{}:{}",
                    self.base_file_path.as_os_str().to_string_lossy(),
                    self.current_slab_index,
                    self.current_reading_position,
                ),
                error,
            )),
            Ok(header) => {
                let data = self.read_section_content(&header)?;
                self.current_reading_position += header.offset_to_next_section as usize;
                Ok((header, data))
            }
        }
    }
}

impl MmapUnifiedLoggerRead {
    /// Advance past the next section without copying its payload.
    pub fn raw_skip_section(&mut self) -> CuResult<SectionHeader> {
        if self.current_reading_position >= self.current_mmap_buffer.len() {
            self.next_slab().map_err(|e| {
                CuError::new_with_cause("Failed to read next slab, is the log complete?", e)
            })?;
        }

        let header = self.read_section_header().map_err(|error| {
            CuError::new_with_cause(
                &format!(
                    "Could not read a sections header: {}/{}:{}",
                    self.base_file_path.as_os_str().to_string_lossy(),
                    self.current_slab_index,
                    self.current_reading_position,
                ),
                error,
            )
        })?;
        self.current_reading_position += header.offset_to_next_section as usize;
        Ok(header)
    }

    pub fn new(base_file_path: &Path) -> io::Result<Self> {
        let (file, mmap, prolog, header) = open_slab_index(base_file_path, 0)?;
        let main_header = header.ok_or_else(|| {
            io::Error::new(io::ErrorKind::InvalidData, "Missing main header in slab 0")
        })?;

        Ok(Self {
            base_file_path: base_file_path.to_path_buf(),
            main_header,
            current_file: file,
            current_mmap_buffer: mmap,
            current_slab_index: 0,
            current_reading_position: prolog as usize,
        })
    }

    /// Current cursor position (start of next section header).
    pub fn position(&self) -> LogPosition {
        LogPosition {
            slab_index: self.current_slab_index,
            offset: self.current_reading_position,
        }
    }

    /// Seek to an absolute position (start of a section header).
    pub fn seek(&mut self, pos: LogPosition) -> CuResult<()> {
        if pos.slab_index != self.current_slab_index {
            let (file, mmap, _prolog, _header) =
                open_slab_index(&self.base_file_path, pos.slab_index).map_err(|e| {
                    CuError::new_with_cause(
                        &format!("Failed to open slab {} for seek", pos.slab_index),
                        e,
                    )
                })?;
            self.current_file = file;
            self.current_mmap_buffer = mmap;
            self.current_slab_index = pos.slab_index;
        }
        self.current_reading_position = pos.offset;
        Ok(())
    }

    fn next_slab(&mut self) -> io::Result<()> {
        self.current_slab_index += 1;
        let (file, mmap, prolog, _) =
            open_slab_index(&self.base_file_path, self.current_slab_index)?;
        self.current_file = file;
        self.current_mmap_buffer = mmap;
        self.current_reading_position = prolog as usize;
        Ok(())
    }

    pub fn raw_main_header(&self) -> &MainHeader {
        &self.main_header
    }

    /// Scan forward to the permanent end-of-log marker and return its position
    /// (the offset at which a writer may resume appending).
    pub fn end_of_log(&mut self) -> CuResult<LogPosition> {
        loop {
            if self.current_reading_position >= self.current_mmap_buffer.len() {
                self.next_slab().map_err(|e| {
                    CuError::new_with_cause(
                        "Failed to advance to the next slab while locating the end of the log",
                        e,
                    )
                })?;
            }

            let header = self.read_section_header()?;
            if header.entry_type == UnifiedLogType::LastEntry {
                return Ok(self.position());
            }
            self.current_reading_position += header.offset_to_next_section as usize;
        }
    }

    pub fn scan_section_bytes(&mut self, datalogtype: UnifiedLogType) -> CuResult<u64> {
        let mut total = 0u64;

        loop {
            if self.current_reading_position >= self.current_mmap_buffer.len() {
                self.next_slab().map_err(|e| {
                    CuError::new_with_cause("Failed to read next slab, is the log complete?", e)
                })?;
            }

            let header = self.read_section_header()?;

            if header.entry_type == UnifiedLogType::LastEntry {
                return Ok(total);
            }

            if header.entry_type == datalogtype {
                total = total.saturating_add(header.used as u64);
            }

            self.current_reading_position += header.offset_to_next_section as usize;
        }
    }

    /// Reads the section content from the section header pos.
    fn read_section_content(&mut self, header: &SectionHeader) -> CuResult<Vec<u8>> {
        // TODO: we could optimize by asking the buffer to fill
        let mut section_data = vec![0; header.used as usize];
        let start_of_data = self.current_reading_position + header.block_size as usize;
        section_data.copy_from_slice(
            &self.current_mmap_buffer[start_of_data..start_of_data + header.used as usize],
        );

        Ok(section_data)
    }

    fn read_section_header(&mut self) -> CuResult<SectionHeader> {
        let section_header: SectionHeader;
        (section_header, _) = decode_from_slice(
            &self.current_mmap_buffer[self.current_reading_position..],
            standard(),
        )
        .map_err(|e| {
            CuError::new_with_cause(
                &format!(
                    "Could not read a sections header: {}/{}:{}",
                    self.base_file_path.as_os_str().to_string_lossy(),
                    self.current_slab_index,
                    self.current_reading_position,
                ),
                e,
            )
        })?;
        if section_header.magic != SECTION_MAGIC {
            return Err("Invalid magic number in section header".into());
        }

        Ok(section_header)
    }
}

/// This a convenience wrapper around the UnifiedLoggerRead to implement the Read trait.
pub struct UnifiedLoggerIOReader {
    logger: MmapUnifiedLoggerRead,
    log_type: UnifiedLogType,
    buffer: Vec<u8>,
    buffer_pos: usize,
}

impl UnifiedLoggerIOReader {
    pub fn new(logger: MmapUnifiedLoggerRead, log_type: UnifiedLogType) -> Self {
        Self {
            logger,
            log_type,
            buffer: Vec::new(),
            buffer_pos: 0,
        }
    }

    /// returns true if there is more data to read.
    fn fill_buffer(&mut self) -> io::Result<bool> {
        match self.logger.read_next_section_type(self.log_type) {
            Ok(Some(section)) => {
                self.buffer = section;
                self.buffer_pos = 0;
                Ok(true)
            }
            Ok(None) => Ok(false), // No more sections of this type
            Err(e) => Err(io::Error::other(e.to_string())),
        }
    }
}

impl Read for UnifiedLoggerIOReader {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        if self.buffer_pos >= self.buffer.len() && !self.fill_buffer()? {
            // This means we hit the last section.
            return Ok(0);
        }

        // If we still have no data after trying to fill the buffer, we're at EOF
        if self.buffer_pos >= self.buffer.len() {
            return Ok(0);
        }

        // Copy as much as we can from the buffer to `buf`
        let len = std::cmp::min(buf.len(), self.buffer.len() - self.buffer_pos);
        buf[..len].copy_from_slice(&self.buffer[self.buffer_pos..self.buffer_pos + len]);
        self.buffer_pos += len;
        Ok(len)
    }
}

#[cfg(feature = "std")]
#[cfg(test)]
mod tests {
    use super::*;
    use crate::stream_write;
    use bincode::de::read::SliceReader;
    use bincode::{Decode, Encode, decode_from_reader, decode_from_slice};
    use cu29_traits::WriteStream;
    use std::io::{Seek, SeekFrom, Write};
    use std::path::PathBuf;
    use std::sync::{Arc, Mutex};
    use tempfile::TempDir;

    const LARGE_SLAB: usize = 100 * 1024; // 100KB
    const SMALL_SLAB: usize = 16 * 2 * 1024; // 16KB is the page size on MacOS for example

    fn make_a_logger(
        tmp_dir: &TempDir,
        slab_size: usize,
    ) -> (Arc<Mutex<MmapUnifiedLoggerWrite>>, PathBuf) {
        let file_path = tmp_dir.path().join("test.bin");
        let MmapUnifiedLogger::Write(data_logger) = MmapUnifiedLoggerBuilder::new()
            .write(true)
            .create(true)
            .file_base_name(&file_path)
            .preallocated_size(slab_size)
            .build()
            .expect("Failed to create logger")
        else {
            panic!("Failed to create logger")
        };

        (Arc::new(Mutex::new(data_logger)), file_path)
    }

    #[test]
    fn test_truncation_and_sections_creations() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let file_path = tmp_dir.path().join("test.bin");
        let _used = {
            let MmapUnifiedLogger::Write(mut logger) = MmapUnifiedLoggerBuilder::new()
                .write(true)
                .create(true)
                .file_base_name(&file_path)
                .preallocated_size(100000)
                .build()
                .expect("Failed to create logger")
            else {
                panic!("Failed to create logger")
            };
            logger
                .add_section(UnifiedLogType::StructuredLogLine, 1024)
                .unwrap();
            logger
                .add_section(UnifiedLogType::CopperList, 2048)
                .unwrap();
            let used = logger.front_slab.used();
            assert!(used < 4 * page_size::get()); // ie. 3 headers, 1 page max per
            // logger drops

            used
        };

        let _file = OpenOptions::new()
            .read(true)
            .open(tmp_dir.path().join("test_0.bin"))
            .expect("Could not reopen the file");
        // Check if we have correctly truncated the file
        // TODO: recompute this math
        //assert_eq!(
        //    file.metadata().unwrap().len(),
        //    (used + size_of::<SectionHeader>()) as u64
        //);
    }

    #[test]
    fn test_unsupported_main_header_format_version_is_rejected() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let file_path = tmp_dir.path().join("test.bin");
        {
            let MmapUnifiedLogger::Write(_logger) = MmapUnifiedLoggerBuilder::new()
                .write(true)
                .create(true)
                .file_base_name(&file_path)
                .preallocated_size(100000)
                .build()
                .expect("Failed to create logger")
            else {
                panic!("Failed to create logger")
            };
        }

        let mut file = OpenOptions::new()
            .read(true)
            .write(true)
            .open(tmp_dir.path().join("test_0.bin"))
            .expect("Could not reopen the slab");
        let unsupported_version = UNIFIED_LOG_FORMAT_VERSION + 1;
        file.seek(SeekFrom::Start(MAIN_MAGIC.len() as u64))
            .expect("Could not seek to format version");
        file.write_all(&[unsupported_version])
            .expect("Could not write unsupported format version");
        drop(file);

        let err = match MmapUnifiedLoggerBuilder::new()
            .file_base_name(&file_path)
            .build()
        {
            Ok(_) => panic!("Reader accepted unsupported unified log format version"),
            Err(err) => err,
        };

        assert_eq!(err.kind(), io::ErrorKind::InvalidData);
        assert_eq!(
            err.to_string(),
            format!(
                "Unsupported unified log format version {unsupported_version} in main header; this reader supports version {UNIFIED_LOG_FORMAT_VERSION}"
            )
        );
    }

    #[test]
    fn test_base_alias_exists_and_matches_first_slab() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let file_path = tmp_dir.path().join("test.bin");
        let _logger = MmapUnifiedLoggerBuilder::new()
            .write(true)
            .create(true)
            .file_base_name(&file_path)
            .preallocated_size(LARGE_SLAB)
            .build()
            .expect("Failed to create logger");

        let first_slab = build_slab_path(&file_path, 0).expect("Failed to build first slab path");
        assert!(file_path.exists(), "base alias does not exist");
        assert!(first_slab.exists(), "first slab does not exist");

        let alias_bytes = std::fs::read(&file_path).expect("Failed to read base alias");
        let slab_bytes = std::fs::read(&first_slab).expect("Failed to read first slab");
        assert_eq!(alias_bytes, slab_bytes);
    }

    #[test]
    fn test_one_section_self_cleaning() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let (logger, _) = make_a_logger(&tmp_dir, LARGE_SLAB);
        {
            let _stream = stream_write::<(), MmapSectionStorage>(
                logger.clone(),
                UnifiedLogType::StructuredLogLine,
                1024,
            );
            assert_eq!(
                logger
                    .lock()
                    .unwrap()
                    .front_slab
                    .sections_offsets_in_flight
                    .len(),
                1
            );
        }
        assert_eq!(
            logger
                .lock()
                .unwrap()
                .front_slab
                .sections_offsets_in_flight
                .len(),
            0
        );
        let logger = logger.lock().unwrap();
        assert_eq!(
            logger.front_slab.flushed_until_offset,
            logger.front_slab.current_global_position
        );
    }

    #[test]
    fn test_temporary_end_marker_is_created() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let (logger, _) = make_a_logger(&tmp_dir, LARGE_SLAB);
        {
            let mut stream = stream_write::<u32, MmapSectionStorage>(
                logger.clone(),
                UnifiedLogType::StructuredLogLine,
                1024,
            )
            .unwrap();
            stream.log(&42u32).unwrap();
        }

        let logger_guard = logger.lock().unwrap();
        let slab = &logger_guard.front_slab;
        let marker_start = slab
            .temporary_end_marker
            .expect("temporary end-of-log marker missing");
        let (eof_header, _) =
            decode_from_slice::<SectionHeader, _>(&slab.mmap_buffer[marker_start..], standard())
                .expect("Could not decode end-of-log marker header");
        assert_eq!(eof_header.entry_type, UnifiedLogType::LastEntry);
        assert!(eof_header.is_open);
        assert_eq!(eof_header.used, 0);
    }

    #[test]
    fn test_final_end_marker_is_not_temporary() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let (logger, f) = make_a_logger(&tmp_dir, LARGE_SLAB);
        {
            let mut stream = stream_write::<u32, MmapSectionStorage>(
                logger.clone(),
                UnifiedLogType::CopperList,
                1024,
            )
            .unwrap();
            stream.log(&1u32).unwrap();
        }
        drop(logger);

        let MmapUnifiedLogger::Read(mut reader) = MmapUnifiedLoggerBuilder::new()
            .file_base_name(&f)
            .build()
            .expect("Failed to build reader")
        else {
            panic!("Failed to create reader");
        };

        loop {
            let (header, _data) = reader
                .raw_read_section()
                .expect("Failed to read section while searching for EOF");
            if header.entry_type == UnifiedLogType::LastEntry {
                assert!(!header.is_open);
                break;
            }
        }
    }

    #[test]
    fn test_two_sections_self_cleaning_in_order() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let (logger, _) = make_a_logger(&tmp_dir, LARGE_SLAB);
        let s1 = stream_write::<(), MmapSectionStorage>(
            logger.clone(),
            UnifiedLogType::StructuredLogLine,
            1024,
        );
        assert_eq!(
            logger
                .lock()
                .unwrap()
                .front_slab
                .sections_offsets_in_flight
                .len(),
            1
        );
        let s2 = stream_write::<(), MmapSectionStorage>(
            logger.clone(),
            UnifiedLogType::StructuredLogLine,
            1024,
        );
        assert_eq!(
            logger
                .lock()
                .unwrap()
                .front_slab
                .sections_offsets_in_flight
                .len(),
            2
        );
        drop(s2);
        assert_eq!(
            logger
                .lock()
                .unwrap()
                .front_slab
                .sections_offsets_in_flight
                .len(),
            1
        );
        drop(s1);
        let lg = logger.lock().unwrap();
        assert_eq!(lg.front_slab.sections_offsets_in_flight.len(), 0);
        assert_eq!(
            lg.front_slab.flushed_until_offset,
            lg.front_slab.current_global_position
        );
    }

    #[test]
    fn test_two_sections_self_cleaning_out_of_order() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let (logger, _) = make_a_logger(&tmp_dir, LARGE_SLAB);
        let s1 = stream_write::<(), MmapSectionStorage>(
            logger.clone(),
            UnifiedLogType::StructuredLogLine,
            1024,
        );
        assert_eq!(
            logger
                .lock()
                .unwrap()
                .front_slab
                .sections_offsets_in_flight
                .len(),
            1
        );
        let s2 = stream_write::<(), MmapSectionStorage>(
            logger.clone(),
            UnifiedLogType::StructuredLogLine,
            1024,
        );
        assert_eq!(
            logger
                .lock()
                .unwrap()
                .front_slab
                .sections_offsets_in_flight
                .len(),
            2
        );
        drop(s1);
        assert_eq!(
            logger
                .lock()
                .unwrap()
                .front_slab
                .sections_offsets_in_flight
                .len(),
            1
        );
        drop(s2);
        let lg = logger.lock().unwrap();
        assert_eq!(lg.front_slab.sections_offsets_in_flight.len(), 0);
        assert_eq!(
            lg.front_slab.flushed_until_offset,
            lg.front_slab.current_global_position
        );
    }

    #[test]
    fn test_closed_section_flushes_behind_open_earlier_section() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let (logger, _) = make_a_logger(&tmp_dir, LARGE_SLAB);
        let s1 = stream_write::<(), MmapSectionStorage>(
            logger.clone(),
            UnifiedLogType::StructuredLogLine,
            1024,
        )
        .unwrap();
        {
            let mut s2 = stream_write::<u32, MmapSectionStorage>(
                logger.clone(),
                UnifiedLogType::CopperList,
                1024,
            )
            .unwrap();
            s2.log(&42u32).unwrap();
        }

        let logger_guard = logger.lock().unwrap();
        assert_eq!(logger_guard.front_slab.sections_offsets_in_flight.len(), 1);
        assert!(
            logger_guard.front_slab.flushed_until_offset
                < logger_guard.front_slab.current_global_position
        );
        assert_eq!(logger_guard.front_slab.pending_closed_bytes(), 0);
        drop(logger_guard);
        drop(s1);
    }

    #[test]
    fn test_append_preserves_existing_and_adds_new_sections() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let file_path = tmp_dir.path().join("test.bin");

        // First run: one section with three entries, then a clean close.
        {
            let MmapUnifiedLogger::Write(logger) = MmapUnifiedLoggerBuilder::new()
                .write(true)
                .create(true)
                .file_base_name(&file_path)
                .preallocated_size(LARGE_SLAB)
                .build()
                .expect("Failed to create logger")
            else {
                panic!("Failed to create logger")
            };
            let logger = Arc::new(Mutex::new(logger));
            {
                let mut stream = stream_write::<u32, MmapSectionStorage>(
                    logger.clone(),
                    UnifiedLogType::StructuredLogLine,
                    1024,
                )
                .unwrap();
                stream.log(&1u32).unwrap();
                stream.log(&2u32).unwrap();
                stream.log(&3u32).unwrap();
            }
        } // logger drops -> clean close

        // Second run: append one more section with two entries.
        {
            let MmapUnifiedLogger::Write(logger) = MmapUnifiedLoggerBuilder::new()
                .write(true)
                .create(true)
                .append(true)
                .file_base_name(&file_path)
                .preallocated_size(LARGE_SLAB)
                .build()
                .expect("Failed to append to logger")
            else {
                panic!("Failed to append to logger")
            };
            let logger = Arc::new(Mutex::new(logger));
            {
                let mut stream = stream_write::<u32, MmapSectionStorage>(
                    logger.clone(),
                    UnifiedLogType::StructuredLogLine,
                    1024,
                )
                .unwrap();
                stream.log(&4u32).unwrap();
                stream.log(&5u32).unwrap();
            }
        }

        // Read back both sections: the original three entries then the two appended.
        let MmapUnifiedLogger::Read(mut dl) = MmapUnifiedLoggerBuilder::new()
            .file_base_name(&file_path)
            .build()
            .expect("Failed to build logger")
        else {
            panic!("Failed to build logger")
        };

        let first = dl
            .read_next_section_type(UnifiedLogType::StructuredLogLine)
            .expect("Failed to read first section")
            .expect("Missing first section");
        let mut reader = SliceReader::new(&first[..]);
        assert_eq!(
            decode_from_reader::<u32, _, _>(&mut reader, standard()).unwrap(),
            1
        );
        assert_eq!(
            decode_from_reader::<u32, _, _>(&mut reader, standard()).unwrap(),
            2
        );
        assert_eq!(
            decode_from_reader::<u32, _, _>(&mut reader, standard()).unwrap(),
            3
        );

        let second = dl
            .read_next_section_type(UnifiedLogType::StructuredLogLine)
            .expect("Failed to read second section")
            .expect("Missing second section");
        let mut reader = SliceReader::new(&second[..]);
        assert_eq!(
            decode_from_reader::<u32, _, _>(&mut reader, standard()).unwrap(),
            4
        );
        assert_eq!(
            decode_from_reader::<u32, _, _>(&mut reader, standard()).unwrap(),
            5
        );

        assert!(
            dl.read_next_section_type(UnifiedLogType::StructuredLogLine)
                .expect("Failed to read past sections")
                .is_none()
        );
    }

    #[test]
    fn test_write_then_read_one_section() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let (logger, f) = make_a_logger(&tmp_dir, LARGE_SLAB);
        {
            let mut stream =
                stream_write(logger.clone(), UnifiedLogType::StructuredLogLine, 1024).unwrap();
            stream.log(&1u32).unwrap();
            stream.log(&2u32).unwrap();
            stream.log(&3u32).unwrap();
        }
        drop(logger);
        let MmapUnifiedLogger::Read(mut dl) = MmapUnifiedLoggerBuilder::new()
            .file_base_name(&f)
            .build()
            .expect("Failed to build logger")
        else {
            panic!("Failed to build logger");
        };
        let section = dl
            .read_next_section_type(UnifiedLogType::StructuredLogLine)
            .expect("Failed to read section");
        assert!(section.is_some());
        let section = section.unwrap();
        let mut reader = SliceReader::new(&section[..]);
        let v1: u32 = decode_from_reader(&mut reader, standard()).unwrap();
        let v2: u32 = decode_from_reader(&mut reader, standard()).unwrap();
        let v3: u32 = decode_from_reader(&mut reader, standard()).unwrap();
        assert_eq!(v1, 1);
        assert_eq!(v2, 2);
        assert_eq!(v3, 3);
    }

    #[cfg(feature = "mmap-fsync")]
    #[test]
    fn test_fsync_feature_syncs_on_section_flush() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let (logger, _) = make_a_logger(&tmp_dir, LARGE_SLAB);
        {
            let mut stream =
                stream_write(logger.clone(), UnifiedLogType::StructuredLogLine, 1024).unwrap();
            stream.log(&1u32).unwrap();
        }

        let logger = logger.lock().unwrap();
        assert!(
            logger.front_slab.sync_call_count > 0,
            "expected mmap-fsync to issue at least one sync_all call"
        );
    }

    /// Mimic a basic CopperList implementation.

    #[derive(Debug, Encode, Decode)]
    enum CopperListStateMock {
        Free,
        ProcessingTasks,
        BeingSerialized,
    }

    #[derive(Encode, Decode)]
    struct CopperList<P: bincode::enc::Encode> {
        state: CopperListStateMock,
        payload: P, // This is generated from the runtime.
    }

    #[test]
    fn test_copperlist_list_like_logging() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let (logger, f) = make_a_logger(&tmp_dir, LARGE_SLAB);
        {
            let mut stream =
                stream_write(logger.clone(), UnifiedLogType::CopperList, 1024).unwrap();
            let cl0 = CopperList {
                state: CopperListStateMock::Free,
                payload: (1u32, 2u32, 3u32),
            };
            let cl1 = CopperList {
                state: CopperListStateMock::ProcessingTasks,
                payload: (4u32, 5u32, 6u32),
            };
            stream.log(&cl0).unwrap();
            stream.log(&cl1).unwrap();
        }
        drop(logger);

        let MmapUnifiedLogger::Read(mut dl) = MmapUnifiedLoggerBuilder::new()
            .file_base_name(&f)
            .build()
            .expect("Failed to build logger")
        else {
            panic!("Failed to build logger");
        };
        let section = dl
            .read_next_section_type(UnifiedLogType::CopperList)
            .expect("Failed to read section");
        assert!(section.is_some());
        let section = section.unwrap();

        let mut reader = SliceReader::new(&section[..]);
        let cl0: CopperList<(u32, u32, u32)> = decode_from_reader(&mut reader, standard()).unwrap();
        let cl1: CopperList<(u32, u32, u32)> = decode_from_reader(&mut reader, standard()).unwrap();
        assert_eq!(cl0.payload.1, 2);
        assert_eq!(cl1.payload.2, 6);
    }

    #[test]
    fn test_multi_slab_end2end() {
        let tmp_dir = TempDir::new().expect("could not create a tmp dir");
        let (logger, f) = make_a_logger(&tmp_dir, SMALL_SLAB);
        {
            let mut stream =
                stream_write(logger.clone(), UnifiedLogType::CopperList, 1024).unwrap();
            let cl0 = CopperList {
                state: CopperListStateMock::Free,
                payload: (1u32, 2u32, 3u32),
            };
            // large enough so we are sure to create a few slabs
            for _ in 0..10000 {
                stream.log(&cl0).unwrap();
            }
        }
        drop(logger);

        let MmapUnifiedLogger::Read(mut dl) = MmapUnifiedLoggerBuilder::new()
            .file_base_name(&f)
            .build()
            .expect("Failed to build logger")
        else {
            panic!("Failed to build logger");
        };
        let mut total_readback = 0;
        loop {
            let section = dl.read_next_section_type(UnifiedLogType::CopperList);
            if section.is_err() {
                break;
            }
            let section = section.unwrap();
            if section.is_none() {
                break;
            }
            let section = section.unwrap();

            let mut reader = SliceReader::new(&section[..]);
            loop {
                let maybe_cl: Result<CopperList<(u32, u32, u32)>, _> =
                    decode_from_reader(&mut reader, standard());
                if maybe_cl.is_ok() {
                    total_readback += 1;
                } else {
                    break;
                }
            }
        }
        assert_eq!(total_readback, 10000);
    }
}