vhdx/

reader.rs

use std::fs::File;
use std::io::{self, Read, Seek, SeekFrom};
use std::path::Path;

use crate::backing::Backing;
use crate::bat::Bat;
use crate::error::{Result, VhdxError};
use crate::header::{parse_active_header, REGION_TABLE1_OFFSET, REGION_TABLE2_OFFSET};
use crate::log::LogOverlay;
use crate::metadata::{parse_metadata, VhdxMetadata};
use crate::region::parse_region_table;
use crate::FILE_MAGIC;

/// Read-only VHDX container reader.
///
/// Implements `Read + Seek` over the virtual sector stream.
///
/// # Bounded memory
/// The reader holds the BAT (a small `Vec<u64>`), the parsed metadata, and a
/// log overlay (empty for a clean image, ~log-sized for a dirty one) — never the
/// whole container. Block bytes are fetched on demand from a positioned
/// [`Backing`], so peak RSS does not scale with image size. [`VhdxReader::open`]
/// takes the bounded [`Backing::File`] path; [`VhdxReader::from_bytes`] keeps the
/// legacy in-RAM path (backed by [`Backing::Mem`]).
#[derive(Debug)]
pub struct VhdxReader {
    backing: Backing,
    overlay: LogOverlay,
    bat: Bat,
    meta: VhdxMetadata,
    pos: u64,
    parent: Option<Box<VhdxReader>>,
}

/// Minimum container size: covers magic, both headers, and both region tables.
const MIN_CONTAINER_SIZE: u64 = 0x0025_0000;

impl VhdxReader {
    /// Open a VHDX container from a path with **bounded** memory.
    ///
    /// Reads only the small fixed structures (headers, region table, metadata,
    /// BAT, and — for a dirty image — the log region) at construction; the
    /// payload blocks are read on demand. A 2 TB image no longer means a 2 TB
    /// heap.
    pub fn open(path: &Path) -> Result<Self> {
        let file = File::open(path)?;
        Self::from_backing(Backing::File(file), None)
    }

    /// Construct a reader over an arbitrary positioned [`Backing`].
    ///
    /// The escape hatch behind [`open`](Self::open) (and the issen zip bridge):
    /// a [`Backing::Sub`] reads a STORED zip entry in place, a [`Backing::Mem`]
    /// reads inflated/owned bytes. Parsing and reads are identical across all
    /// three backings.
    pub fn from_backing(backing: Backing, parent: Option<Box<VhdxReader>>) -> Result<Self> {
        Self::parse_backing(backing, parent)
    }

    /// Open a VHDX container from owned bytes (legacy in-RAM path).
    ///
    /// The whole buffer is held in RAM (via [`Backing::Mem`]) and the dirty log,
    /// if any, is replayed in place before parsing — byte-identical to the
    /// historical behaviour. Prefer [`open`](Self::open) for files on disk.
    pub fn from_bytes(data: Vec<u8>) -> Result<Self> {
        Self::parse_bytes(data, None)
    }

    /// Open a differencing (child) disk with its parent chain (in-RAM path).
    ///
    /// Reads absent blocks from `parent` instead of returning zeros.
    pub fn from_bytes_with_parent(data: Vec<u8>, parent: VhdxReader) -> Result<Self> {
        Self::parse_bytes(data, Some(Box::new(parent)))
    }

    /// Open a differencing (child) disk over an arbitrary backing with its
    /// parent chain.
    pub fn from_backing_with_parent(backing: Backing, parent: VhdxReader) -> Result<Self> {
        Self::parse_backing(backing, Some(Box::new(parent)))
    }

    /// In-RAM parse: replay the log in place, then parse from the buffer and
    /// keep it as the [`Backing::Mem`].
    fn parse_bytes(mut data: Vec<u8>, parent: Option<Box<VhdxReader>>) -> Result<Self> {
        if data.len() < 8 || &data[0..8] != FILE_MAGIC {
            return Err(VhdxError::BadMagic);
        }
        if (data.len() as u64) < MIN_CONTAINER_SIZE {
            return Err(VhdxError::ContainerTooSmall(MIN_CONTAINER_SIZE));
        }
        crate::log::apply(&mut data)?;
        // The log is already folded into `data`, so the overlay is empty and
        // the backing serves the committed bytes directly.
        let backing = Backing::from_bytes(data);
        Self::assemble(backing, LogOverlay::default(), parent)
    }

    /// Bounded parse: read the small fixed structures via positioned reads, then
    /// build the log overlay (empty for a clean image).
    fn parse_backing(backing: Backing, parent: Option<Box<VhdxReader>>) -> Result<Self> {
        let mut magic = [0u8; 8];
        let n = backing.read_at(&mut magic, 0)?;
        if n < 8 || &magic != FILE_MAGIC {
            return Err(VhdxError::BadMagic);
        }
        if backing.len() < MIN_CONTAINER_SIZE {
            return Err(VhdxError::ContainerTooSmall(MIN_CONTAINER_SIZE));
        }
        let overlay = {
            // The header lives in the first 1 MB; read that slice to select the
            // active header and (if dirty) drive the log overlay.
            let head = backing.read_exact_at(0, MIN_CONTAINER_SIZE as usize)?;
            let header = parse_active_header(&head)?;
            crate::log::build_overlay(&backing, &header)?
        };
        Self::assemble(backing, overlay, parent)
    }

    /// Shared tail: parse region table → metadata → BAT from small positioned
    /// reads (the overlay patches any log-dirtied structure bytes), validate,
    /// and assemble the reader.
    fn assemble(
        backing: Backing,
        overlay: LogOverlay,
        parent: Option<Box<VhdxReader>>,
    ) -> Result<Self> {
        // Region tables sit at fixed offsets within the first 1 MB; read that
        // prefix once (small, bounded) and parse the region table from it,
        // applying the log overlay so a log-patched region table is honoured.
        let mut head = backing.read_exact_at(0, MIN_CONTAINER_SIZE as usize)?;
        overlay.patch(&mut head, 0);

        let container_len = backing.len();
        let regions = parse_region_table(&head, REGION_TABLE1_OFFSET as usize, container_len)
            .or_else(|_| parse_region_table(&head, REGION_TABLE2_OFFSET as usize, container_len))?;

        // Metadata + BAT can live past the first 1 MB; read each region by its
        // own offset/length and patch with the overlay.
        let meta_region = read_region(
            &backing,
            &overlay,
            regions.metadata.file_offset,
            regions.metadata.length,
        )?;
        let meta = parse_metadata(&meta_region, 0, regions.metadata.length)?;
        meta.validate()?;
        if meta.has_parent && parent.is_none() {
            return Err(VhdxError::DifferencingNotSupported);
        }

        let bat_region = read_region(
            &backing,
            &overlay,
            regions.bat.file_offset,
            regions.bat.length,
        )?;
        let bat = Bat::parse(&bat_region, 0, regions.bat.length, meta.clone())?;

        Ok(Self {
            backing,
            overlay,
            bat,
            meta,
            pos: 0,
            parent,
        })
    }

    pub fn virtual_disk_size(&self) -> u64 {
        self.meta.virtual_disk_size
    }

    pub fn logical_sector_size(&self) -> u32 {
        self.meta.logical_sector_size
    }
}

/// Read a region `[file_offset, file_offset+length)` from the backing into a
/// fresh buffer and apply the log overlay, so region/metadata/BAT parsing sees
/// the committed state — exactly as the in-place `apply` path would.
///
/// The returned buffer is offset-zero based, so callers parse with offset `0`.
fn read_region(
    backing: &Backing,
    overlay: &LogOverlay,
    file_offset: u64,
    length: u32,
) -> Result<Vec<u8>> {
    let end = file_offset.saturating_add(u64::from(length));
    if backing.len() < end {
        return Err(VhdxError::OffsetOutOfBounds);
    }
    let mut buf = backing.read_exact_at(file_offset, length as usize)?;
    if buf.len() < length as usize {
        return Err(VhdxError::OffsetOutOfBounds);
    }
    overlay.patch(&mut buf, file_offset);
    Ok(buf)
}

impl Read for VhdxReader {
    fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
        if self.pos >= self.meta.virtual_disk_size {
            return Ok(0);
        }
        let remaining = self.meta.virtual_disk_size - self.pos;
        let to_read = buf.len().min(remaining as usize);
        let block_size = u64::from(self.meta.block_size);
        let mut written = 0;

        while written < to_read {
            let virtual_byte = self.pos + written as u64;
            let block_end = ((virtual_byte / block_size) + 1) * block_size;
            let this_chunk = (to_read - written).min((block_end - virtual_byte) as usize);

            match self.bat.file_offset_for_byte(virtual_byte) {
                Ok(file_off) => {
                    let dst = &mut buf[written..written + this_chunk];
                    let n = self.backing.read_at(dst, file_off)?;
                    if n < this_chunk {
                        return Err(io::Error::new(
                            io::ErrorKind::UnexpectedEof,
                            "VHDX data truncated",
                        ));
                    }
                    // Fold any committed-log sectors over the on-disk block.
                    self.overlay.patch(dst, file_off);
                }
                Err(VhdxError::BlockNotPresent(_)) => {
                    if let Some(ref mut p) = self.parent {
                        p.seek(SeekFrom::Start(virtual_byte))
                            .map_err(io::Error::other)?;
                        p.read_exact(&mut buf[written..written + this_chunk])?;
                    } else {
                        buf[written..written + this_chunk].fill(0);
                    }
                }
                Err(e) => return Err(io::Error::other(e.to_string())),
            }
            written += this_chunk;
        }

        self.pos += written as u64;
        Ok(written)
    }
}

impl Seek for VhdxReader {
    fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
        let new_pos = match pos {
            SeekFrom::Start(n) => n as i64,
            SeekFrom::Current(n) => self.pos as i64 + n,
            SeekFrom::End(n) => self.meta.virtual_disk_size as i64 + n,
        };
        if new_pos < 0 {
            return Err(io::Error::new(
                io::ErrorKind::InvalidInput,
                "seek before start",
            ));
        }
        self.pos = new_pos as u64;
        Ok(self.pos)
    }
}