asdf_core/

reader.rs

//! Reading an ASDF file: its tree, and the data in its binary blocks.

use std::borrow::Cow;
use std::path::{Path, PathBuf};

use asdf_yaml::{Document, parse_document};

use crate::block::header::CHECKSUM_SIZE;
use crate::compression::Compression;
use crate::error::{Result, err};
use crate::layout::{BlockLocation, Layout, scan};

/// Where a reader's bytes come from.
enum Source {
    /// A memory-mapped file. Block data is read straight out of the mapping,
    /// so a large array costs no copy until it is decompressed or converted.
    ///
    /// Absent under Miri, which cannot execute `mmap`: `Reader::open` reads
    /// the file whole there instead, so nothing would construct this and
    /// `dead_code` would fire.
    #[cfg(not(miri))]
    Mapped(memmap2::Mmap),
    /// An in-memory buffer.
    Owned(Vec<u8>),
}

impl std::ops::Deref for Source {
    type Target = [u8];
    fn deref(&self) -> &[u8] {
        match self {
            #[cfg(not(miri))]
            Source::Mapped(m) => m,
            Source::Owned(v) => v,
        }
    }
}

impl std::fmt::Debug for Source {
    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
        let kind = match self {
            #[cfg(not(miri))]
            Source::Mapped(_) => "Mapped",
            Source::Owned(_) => "Owned",
        };
        write!(f, "{kind}({} bytes)", self.len())
    }
}

/// The outcome of verifying a block's checksum.
#[derive(Clone, Copy, PartialEq, Eq, Debug)]
pub enum ChecksumStatus {
    /// The header records no checksum; the all-zero value means "do not
    /// verify", so this is not a failure.
    Absent,
    /// The recorded digest matches.
    Valid,
    /// The recorded digest does not match.
    Invalid,
}

impl ChecksumStatus {
    /// Whether this status should be treated as a failure.
    ///
    /// An absent checksum is not one: the standard makes it optional.
    pub fn is_failure(self) -> bool {
        self == ChecksumStatus::Invalid
    }
}

/// The relative path an external `source` URI names, if it names a safe one.
///
/// Rejects anything that could escape the referring file's directory: an
/// absolute path, a `..` component, a Windows drive or UNC prefix, or a URI
/// with a scheme or authority. `file:` is not special-cased -- a relative
/// path is the only form the reference corpus uses and the only one worth
/// the risk.
fn external_relative_path(uri: &str) -> Result<PathBuf> {
    if uri.is_empty() {
        return Err(err!(InvalidArgument, "external source is an empty URI"));
    }
    if uri.contains("://") || uri.starts_with('/') || uri.starts_with('\\') {
        return Err(err!(
            InvalidArgument,
            "external source {uri:?} is not a relative path; only files beside the \
             referring one can be resolved"
        ));
    }

    // Percent-decoding is deliberately not done: a URI needing it is not one
    // the corpus produces, and decoding would reopen the escape it rejects.
    let path = Path::new(uri);
    for component in path.components() {
        use std::path::Component;
        match component {
            Component::Normal(_) | Component::CurDir => {}
            Component::ParentDir => {
                return Err(err!(
                    InvalidArgument,
                    "external source {uri:?} climbs out of the referring file's directory"
                ));
            }
            Component::RootDir | Component::Prefix(_) => {
                return Err(err!(InvalidArgument, "external source {uri:?} is not relative"));
            }
        }
    }
    Ok(path.to_path_buf())
}

/// An open ASDF file.
#[derive(Debug)]
pub struct Reader {
    source: Source,
    layout: Layout,
    /// Where the file came from, when it came from disk.
    ///
    /// Kept so that an array whose `source` names another file -- exploded
    /// form -- can be resolved relative to this one, as the standard says.
    path: Option<PathBuf>,
}

impl Reader {
    /// Open and scan a file from disk.
    ///
    /// The file is memory-mapped, so block data is not read until it is used.
    pub fn open(path: impl AsRef<Path>) -> Result<Self> {
        let path = path.as_ref();

        // Miri interprets rather than executes, so it has no `mmap`. Reading
        // the file whole is observably the same to every caller -- `Source`
        // hands out a `&[u8]` either way -- and it is what lets the FFI layer,
        // which is where the unsafe code actually lives, be checked at all.
        #[cfg(miri)]
        {
            let bytes = std::fs::read(path)?;
            let layout = scan(&bytes)?;
            return Ok(Self {
                source: Source::Owned(bytes),
                layout,
                path: Some(path.to_path_buf()),
            });
        }

        #[cfg(not(miri))]
        {
            let file = std::fs::File::open(path)?;
            Self::map(file, path)
        }
    }

    /// The memory-mapping half of [`Reader::open`], split out so the `miri`
    /// fallback above stays readable.
    #[cfg(not(miri))]
    fn map(file: std::fs::File, path: &Path) -> Result<Self> {
        // SAFETY: the only unsafe operation in the engine. Mapping is unsafe
        // because another process truncating the file can turn a later read
        // into SIGBUS. That hazard is inherent to memory-mapping and is the
        // same one libasdf accepts; ASDF files are written whole rather than
        // modified in place, so a concurrent truncation is not a case the
        // format contemplates. Mapping is what lets a multi-gigabyte array be
        // read without loading the file into memory, which is the point of
        // the format.
        #[allow(unsafe_code)]
        let mapped = unsafe { memmap2::Mmap::map(&file) }?;
        let layout = scan(&mapped)?;
        Ok(Self { source: Source::Mapped(mapped), layout, path: Some(path.to_path_buf()) })
    }

    /// Scan an in-memory file.
    pub fn from_bytes(bytes: Vec<u8>) -> Result<Self> {
        let layout = scan(&bytes)?;
        Ok(Self { source: Source::Owned(bytes), layout, path: None })
    }

    /// The path this file was opened from, if it came from disk.
    pub fn path(&self) -> Option<&Path> {
        self.path.as_deref()
    }

    /// The whole file's bytes.
    pub fn bytes(&self) -> &[u8] {
        &self.source
    }

    /// The scanned layout.
    pub fn layout(&self) -> &Layout {
        &self.layout
    }

    /// The YAML tree's text, if the file has a tree.
    pub fn tree_text(&self) -> Option<&str> {
        self.layout.tree_str(&self.source)
    }

    /// Parse the YAML tree.
    ///
    /// A file with no tree -- legitimate in exploded form -- yields `None`.
    pub fn tree(&self) -> Result<Option<Document>> {
        match self.tree_text() {
            None => Ok(None),
            Some(text) => Ok(Some(parse_document(text)?)),
        }
    }

    /// The number of binary blocks.
    pub fn block_count(&self) -> usize {
        self.layout.blocks.len()
    }

    /// A block's location and header.
    pub fn block(&self, index: usize) -> Result<&BlockLocation> {
        self.layout.blocks.get(index).ok_or_else(|| {
            err!(
                InvalidArgument,
                "block index {index} is out of range; the file has {} blocks",
                self.layout.blocks.len()
            )
        })
    }

    /// A block's bytes exactly as stored, without decompressing.
    ///
    /// For an uncompressed block this is the data itself; for a compressed
    /// one it is the compressed form.
    pub fn block_raw(&self, index: usize) -> Result<&[u8]> {
        let block = self.block(index)?;
        let start = usize::try_from(block.data_pos)
            .map_err(|_| err!(UnexpectedEof, "block {index} data offset overflows"))?;

        let len = if block.header.is_streamed() {
            // A streamed block runs to the end of the file; its size fields
            // are meaningless.
            self.source.len().saturating_sub(start)
        } else {
            usize::try_from(block.header.used_size)
                .map_err(|_| err!(UnexpectedEof, "block {index} used_size overflows"))?
        };

        // Checked, since both `start` and `len` come from the file and a
        // corrupt header can make the sum wrap.
        let end = start
            .checked_add(len)
            .ok_or_else(|| err!(UnexpectedEof, "block {index} size overflows"))?;
        self.source
            .get(start..end)
            .ok_or_else(|| err!(UnexpectedEof, "block {index} extends past the end of the file"))
    }

    /// The compression method a block uses.
    pub fn block_compression(&self, index: usize) -> Result<Compression> {
        Compression::from_name(self.block(index)?.header.compression_name())
    }

    /// A block's data, decompressed if necessary.
    ///
    /// An uncompressed block borrows straight from the file with no copy.
    pub fn block_data(&self, index: usize) -> Result<Cow<'_, [u8]>> {
        let raw = self.block_raw(index)?;
        let compression = self.block_compression(index)?;
        if compression == Compression::None {
            return Ok(Cow::Borrowed(raw));
        }
        let expected = usize::try_from(self.block(index)?.header.data_size)
            .map_err(|_| err!(UnexpectedEof, "block {index} data_size overflows"))?;
        Ok(Cow::Owned(compression.decompress(raw, expected)?))
    }

    /// Verify a block's MD5 checksum.
    ///
    /// The returned digest is what the data actually hashes to, which is
    /// useful for reporting a mismatch.
    ///
    /// # The Python asdf compatibility case
    ///
    /// For a *compressed* block, the specification means the checksum to
    /// cover the bytes as stored. Python asdf 5.x and earlier instead
    /// checksum the *uncompressed* data
    /// ([asdf#2015](https://github.com/asdf-format/asdf/issues/2015)).
    /// libasdf works around this by consulting the file's `asdf_library`
    /// metadata, and so do we: see [`Reader::has_python_checksum_bug`]. A
    /// compressed block whose stored bytes do not match is therefore retried
    /// against the decompressed bytes when the writer is known to be affected.
    pub fn verify_block_checksum(
        &self,
        index: usize,
    ) -> Result<(ChecksumStatus, [u8; CHECKSUM_SIZE])> {
        let header = &self.block(index)?.header;
        if !header.has_checksum() {
            return Ok((ChecksumStatus::Absent, [0; CHECKSUM_SIZE]));
        }
        let expected = header.checksum;

        let raw_digest = md5_of(self.block_raw(index)?);
        if raw_digest == expected {
            return Ok((ChecksumStatus::Valid, raw_digest));
        }

        // Only compressed blocks are affected, and only when the writer is
        // one of the versions known to be wrong.
        if self.block_compression(index)? != Compression::None && self.has_python_checksum_bug() {
            let decompressed = md5_of(&self.block_data(index)?);
            if decompressed == expected {
                return Ok((ChecksumStatus::Valid, decompressed));
            }
        }

        Ok((ChecksumStatus::Invalid, raw_digest))
    }

    /// Whether this file was written by a Python asdf version that
    /// checksums compressed blocks incorrectly.
    ///
    /// Matches libasdf's test: the `asdf_library` name is `asdf` and its
    /// major version is 5 or below.
    pub fn has_python_checksum_bug(&self) -> bool {
        const BUGGY_THROUGH_MAJOR: u32 = 5;

        let Ok(Some(doc)) = self.tree() else { return false };
        let Some(root) = doc.root() else { return false };
        let Some(library) = doc.mapping_get(root, "asdf_library") else {
            return false;
        };

        let name = doc
            .mapping_get(library, "name")
            .and_then(|id| doc.resolved(id).as_str().map(str::to_string));
        if name.as_deref() != Some("asdf") {
            return false;
        }

        doc.mapping_get(library, "version")
            .and_then(|id| doc.resolved(id).as_str().map(crate::Version::parse))
            .is_some_and(|v| v.major <= BUGGY_THROUGH_MAJOR)
    }
}

/// MD5 of a buffer.
fn md5_of(data: &[u8]) -> [u8; CHECKSUM_SIZE] {
    use md5::{Digest, Md5};
    let mut hasher = Md5::new();
    hasher.update(data);
    hasher.finalize().into()
}

/// Walking a tree to find every node carrying a given tag name.
///
/// The version suffix is ignored, so `core/ndarray-1.0.0` and
/// `core/ndarray-1.1.0` both match `core/ndarray`.
fn find_tagged(doc: &Document, name: &str) -> Vec<asdf_yaml::NodeId> {
    use asdf_yaml::NodeData;

    let mut out = Vec::new();
    let mut seen = std::collections::HashSet::new();
    let Some(root) = doc.root() else { return out };
    let mut stack = vec![root];

    while let Some(id) = stack.pop() {
        let resolved = doc.resolve(id);
        if !seen.insert(resolved) {
            continue;
        }
        if doc.tag_of(resolved).is_some_and(|t| t.split_version().0 == name) {
            out.push(resolved);
        }
        match &doc.node(resolved).data {
            NodeData::Sequence { items, .. } => stack.extend(items.iter().copied()),
            NodeData::Mapping { entries, .. } => {
                stack.extend(entries.iter().map(|e| e.value));
            }
            _ => {}
        }
    }
    out.sort();
    out
}

impl Reader {
    /// Resolve an ndarray's `source` to a block index in this file.
    fn block_index_for(&self, source: &crate::core::ndarray::Source) -> Option<usize> {
        match source {
            crate::core::ndarray::Source::Block(i) => Some(*i),
            crate::core::ndarray::Source::LastBlock => self.block_count().checked_sub(1),
            _ => None,
        }
    }

    /// Resolve an external array `source` and read the data it names.
    ///
    /// The standard makes `source` a URI relative to the file's own, and
    /// exploded form writes one array per file with the data in block 0.
    ///
    /// Resolution is deliberately narrow. The URI must be a relative path
    /// with no `..` component and no scheme, so a file can only reach others
    /// beneath its own directory: a tree is untrusted input, and following an
    /// arbitrary path out of it would let a crafted file name anything on the
    /// machine. A file read from memory has no directory to resolve against
    /// and so resolves nothing.
    pub fn external_block(&self, uri: &str) -> Result<Vec<u8>> {
        let Some(base) = self.path.as_deref().and_then(Path::parent) else {
            return Err(err!(
                InvalidArgument,
                "external source {uri:?} cannot be resolved: this file was not read from disk"
            ));
        };
        let relative = external_relative_path(uri)?;
        let target = base.join(relative);

        let referenced = Reader::open(&target).map_err(|e| {
            err!(InvalidArgument, "external source {uri:?} ({}): {e}", target.display())
        })?;
        if referenced.block_count() == 0 {
            return Err(err!(InvalidArgument, "external source {uri:?} has no blocks"));
        }
        Ok(referenced.block_data(0)?.into_owned())
    }

    /// Parse the tree with every block-backed `core/ndarray` replaced by its
    /// data inline.
    ///
    /// This is the transformation the ASDF Standard's reference corpus asks
    /// for before comparing a file against its expected YAML. An array whose
    /// data lives in another file -- exploded form's external `source` -- is
    /// resolved through [`Reader::external_block`], which needs this file to
    /// have been read from disk.
    ///
    /// Returns the transformed tree and the paths of any arrays that could
    /// not be inlined.
    pub fn tree_inlined(&self) -> Result<Option<(Document, Vec<String>)>> {
        use crate::core::elements::{decode_all, inline_ndarray};
        use crate::core::ndarray::Ndarray;

        let Some(mut doc) = self.tree()? else { return Ok(None) };
        let mut skipped = Vec::new();

        for id in find_tagged(&doc, "core/ndarray") {
            let nd = match Ndarray::parse(&doc, id) {
                Ok(nd) => nd,
                Err(e) => {
                    skipped.push(format!("{id:?}: {e}"));
                    continue;
                }
            };

            // Inline data is already where it needs to be.
            if matches!(nd.source, crate::core::ndarray::Source::Inline(_)) {
                continue;
            }

            // An external source names another file; its first block holds
            // the data, which is how exploded form is written.
            let data = if let crate::core::ndarray::Source::External(uri) = &nd.source {
                match self.external_block(uri) {
                    Ok(bytes) => Cow::Owned(bytes),
                    Err(e) => {
                        skipped.push(format!("{id:?}: {e}"));
                        continue;
                    }
                }
            } else {
                let Some(index) = self.block_index_for(&nd.source) else {
                    skipped.push(format!("{id:?}: data is outside this file ({:?})", nd.source));
                    continue;
                };
                match self.block_data(index) {
                    Ok(d) => d,
                    Err(e) => {
                        skipped.push(format!("{id:?}: block {index}: {e}"));
                        continue;
                    }
                }
            };

            let shape = match nd.resolved_shape(Some(data.len() as u64)) {
                Ok(s) => s,
                Err(e) => {
                    skipped.push(format!("{id:?}: {e}"));
                    continue;
                }
            };

            match decode_all(&nd, &shape, &data) {
                Ok(elements) => inline_ndarray(&mut doc, id, &elements, &shape)?,
                Err(e) => skipped.push(format!("{id:?}: {e}")),
            }
        }
        Ok(Some((doc, skipped)))
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::block::header::BlockHeader;
    use crate::layout::write_block_index;

    /// Build a file with one block, optionally compressed and checksummed.
    fn build(payload: &[u8], compression: Compression, checksum_over: Option<&[u8]>) -> Vec<u8> {
        let stored = compression.compress(payload).unwrap();
        let mut buf = Vec::new();
        buf.extend_from_slice(b"#ASDF 1.0.0\n#ASDF_STANDARD 1.6.0\n");
        buf.extend_from_slice(
            b"%YAML 1.1\n%TAG ! tag:stsci.edu:asdf/\n--- !core/asdf-1.1.0\nx: 1\n...\n",
        );

        let mut header = BlockHeader {
            allocated_size: stored.len() as u64,
            used_size: stored.len() as u64,
            data_size: payload.len() as u64,
            ..Default::default()
        };
        header.set_compression(compression.name()).unwrap();
        if let Some(over) = checksum_over {
            header.checksum = md5_of(over);
        }

        let offset = buf.len() as u64;
        header.write(&mut buf);
        buf.extend_from_slice(&stored);
        buf.extend_from_slice(&write_block_index(&[offset]));
        buf
    }

    #[test]
    fn external_source_uris_may_not_escape_the_directory() {
        // The only shape the standard's exploded form uses.
        assert!(external_relative_path("exploded0000.asdf").is_ok());
        assert!(external_relative_path("data/block0.asdf").is_ok());
        assert!(external_relative_path("./here.asdf").is_ok());

        // Everything that could reach outside the referring file's tree.
        for bad in [
            "",
            "/etc/passwd",
            "../secrets.asdf",
            "data/../../secrets.asdf",
            "file:///etc/passwd",
            "https://example.invalid/x.asdf",
        ] {
            assert!(
                external_relative_path(bad).is_err(),
                "{bad:?} should be rejected as an external source"
            );
        }
    }

    #[test]
    fn a_memory_backed_file_resolves_no_external_sources() {
        let file = build(b"whatever", Compression::None, None);
        let r = Reader::from_bytes(file);
        let r = r.unwrap();
        assert!(r.path().is_none());
        // There is no directory to resolve against, so this must fail rather
        // than guess at the working directory.
        assert!(r.external_block("other.asdf").is_err());
    }

    #[test]
    fn an_external_source_is_read_from_the_neighbouring_file() {
        let dir = std::env::temp_dir().join(format!("asdf-exploded-{}", std::process::id()));
        std::fs::create_dir_all(&dir).unwrap();

        // The data file: one block holding four little-endian int32s.
        let payload: Vec<u8> = [1i32, 2, 3, 4].iter().flat_map(|v| v.to_le_bytes()).collect();
        let data_file = dir.join("holder0000.asdf");
        std::fs::write(&data_file, build(&payload, Compression::None, None)).unwrap();

        // The referring file: a tree naming it, and no blocks of its own.
        let mut buf = Vec::new();
        buf.extend_from_slice(b"#ASDF 1.0.0\n#ASDF_STANDARD 1.6.0\n");
        buf.extend_from_slice(b"%YAML 1.1\n%TAG ! tag:stsci.edu:asdf/\n--- !core/asdf-1.1.0\n");
        buf.extend_from_slice(
            b"data: !core/ndarray-1.1.0\n  source: holder0000.asdf\n  \
              datatype: int32\n  byteorder: little\n  shape: [4]\n",
        );
        buf.extend_from_slice(b"...\n");
        let referring = dir.join("holder.asdf");
        std::fs::write(&referring, buf).unwrap();

        let r = Reader::open(&referring).unwrap();
        assert_eq!(r.block_count(), 0, "the referring file holds no blocks itself");
        assert_eq!(r.external_block("holder0000.asdf").unwrap(), payload);

        // And inlining follows the reference through to real values.
        let (doc, skipped) = r.tree_inlined().unwrap().unwrap();
        assert!(skipped.is_empty(), "nothing should be left un-inlined: {skipped:?}");
        let root = doc.root().unwrap();
        let array = doc.mapping_get(root, "data").unwrap();
        let values = doc.mapping_get(array, "data").unwrap();
        let items = doc.sequence_items(values).unwrap();
        let read: Vec<&str> = items.iter().map(|i| doc.resolved(*i).as_str().unwrap()).collect();
        assert_eq!(read, ["1", "2", "3", "4"]);
        // `source` is replaced, as it is for an internal block.
        assert!(doc.mapping_get(array, "source").is_none());

        std::fs::remove_dir_all(&dir).ok();
    }

    #[test]
    fn a_missing_external_file_is_reported_not_silently_skipped() {
        let dir =
            std::env::temp_dir().join(format!("asdf-exploded-missing-{}", std::process::id()));
        std::fs::create_dir_all(&dir).unwrap();
        let referring = dir.join("dangling.asdf");
        let mut buf = Vec::new();
        buf.extend_from_slice(b"#ASDF 1.0.0\n#ASDF_STANDARD 1.6.0\n");
        buf.extend_from_slice(b"%YAML 1.1\n%TAG ! tag:stsci.edu:asdf/\n--- !core/asdf-1.1.0\n");
        buf.extend_from_slice(
            b"data: !core/ndarray-1.1.0\n  source: nowhere0000.asdf\n  \
              datatype: int32\n  byteorder: little\n  shape: [4]\n",
        );
        buf.extend_from_slice(b"...\n");
        std::fs::write(&referring, buf).unwrap();

        let r = Reader::open(&referring).unwrap();
        let (_, skipped) = r.tree_inlined().unwrap().unwrap();
        assert_eq!(skipped.len(), 1);
        assert!(skipped[0].contains("nowhere0000.asdf"), "{skipped:?}");

        std::fs::remove_dir_all(&dir).ok();
    }

    #[test]
    fn reads_tree_and_block_data() {
        let payload = b"hello block data".to_vec();
        let file = build(&payload, Compression::None, None);
        let r = Reader::from_bytes(file).unwrap();

        assert_eq!(r.block_count(), 1);
        assert_eq!(&*r.block_data(0).unwrap(), &payload[..]);

        let doc = r.tree().unwrap().unwrap();
        let root = doc.root().unwrap();
        assert!(doc.mapping_get(root, "x").is_some());
    }

    #[test]
    fn uncompressed_data_is_borrowed_not_copied() {
        let file = build(b"borrow me", Compression::None, None);
        let r = Reader::from_bytes(file).unwrap();
        assert!(matches!(r.block_data(0).unwrap(), Cow::Borrowed(_)));
    }

    #[test]
    fn compressed_data_round_trips() {
        let payload = vec![7u8; 4096];
        for c in crate::compression::available() {
            let file = build(&payload, c, None);
            let r = Reader::from_bytes(file).unwrap();
            assert_eq!(r.block_compression(0).unwrap(), c);
            assert_eq!(&*r.block_data(0).unwrap(), &payload[..], "{c:?}");
            // The raw form is the compressed bytes.
            assert!(r.block_raw(0).unwrap().len() < payload.len(), "{c:?}");
        }
    }

    #[test]
    fn valid_checksums_verify() {
        let payload = b"checksum me".to_vec();
        let file = build(&payload, Compression::None, Some(&payload));
        let r = Reader::from_bytes(file).unwrap();
        let (status, _) = r.verify_block_checksum(0).unwrap();
        assert_eq!(status, ChecksumStatus::Valid);
    }

    #[test]
    fn invalid_checksums_are_reported() {
        let payload = b"checksum me".to_vec();
        let file = build(&payload, Compression::None, Some(b"something else"));
        let r = Reader::from_bytes(file).unwrap();
        let (status, computed) = r.verify_block_checksum(0).unwrap();
        assert_eq!(status, ChecksumStatus::Invalid);
        assert_eq!(computed, md5_of(&payload), "the digest of the real data is reported");
    }

    #[test]
    fn an_absent_checksum_is_not_a_failure() {
        let file = build(b"no checksum", Compression::None, None);
        let r = Reader::from_bytes(file).unwrap();
        let (status, _) = r.verify_block_checksum(0).unwrap();
        assert_eq!(status, ChecksumStatus::Absent);
        assert!(!status.is_failure());
    }

    #[cfg(feature = "zlib")]
    #[test]
    fn compressed_checksums_cover_the_stored_bytes() {
        // What the specification means: the digest is over the data as stored.
        let payload = vec![3u8; 2048];
        let stored = Compression::Zlib.compress(&payload).unwrap();
        let file = build(&payload, Compression::Zlib, Some(&stored));
        let r = Reader::from_bytes(file).unwrap();
        assert_eq!(r.verify_block_checksum(0).unwrap().0, ChecksumStatus::Valid);
    }

    /// Python asdf 5.x and earlier checksum the *uncompressed* data for a
    /// compressed block. libasdf detects those writers from `asdf_library`
    /// and verifies against the decompressed bytes instead; so do we.
    #[cfg(feature = "zlib")]
    #[test]
    fn the_python_checksum_bug_is_worked_around() {
        let payload = vec![9u8; 2048];
        let stored = Compression::Zlib.compress(&payload).unwrap();

        let make = |library_version: &str| {
            let mut buf = Vec::new();
            buf.extend_from_slice(b"#ASDF 1.0.0\n#ASDF_STANDARD 1.6.0\n");
            buf.extend_from_slice(b"%YAML 1.1\n%TAG ! tag:stsci.edu:asdf/\n--- !core/asdf-1.1.0\n");
            buf.extend_from_slice(
                format!(
                    "asdf_library: !core/software-1.0.0 {{name: asdf, version: {library_version}}}\n"
                )
                .as_bytes(),
            );
            buf.extend_from_slice(b"...\n");

            let mut header = BlockHeader {
                allocated_size: stored.len() as u64,
                used_size: stored.len() as u64,
                data_size: payload.len() as u64,
                // The bug: digest taken over the *uncompressed* payload.
                checksum: md5_of(&payload),
                ..Default::default()
            };
            header.set_compression("zlib").unwrap();
            header.write(&mut buf);
            buf.extend_from_slice(&stored);
            buf
        };

        // A writer known to be affected: accepted via the workaround.
        let r = Reader::from_bytes(make("4.1.0")).unwrap();
        assert!(r.has_python_checksum_bug());
        assert_eq!(
            r.verify_block_checksum(0).unwrap().0,
            ChecksumStatus::Valid,
            "an affected writer's checksum should verify against the uncompressed data"
        );

        // A writer past the fix: the same file is genuinely invalid.
        let r = Reader::from_bytes(make("6.0.0")).unwrap();
        assert!(!r.has_python_checksum_bug());
        assert_eq!(
            r.verify_block_checksum(0).unwrap().0,
            ChecksumStatus::Invalid,
            "the workaround must not apply to writers that are not affected"
        );
    }

    #[test]
    fn out_of_range_block_indices_error() {
        let file = build(b"one block", Compression::None, None);
        let r = Reader::from_bytes(file).unwrap();
        assert!(r.block(1).is_err());
        assert!(r.block_data(99).is_err());
    }

    #[test]
    fn a_file_without_a_tree_reads_cleanly() {
        let mut buf = Vec::new();
        buf.extend_from_slice(b"#ASDF 1.0.0\n#ASDF_STANDARD 1.6.0\n");
        let header =
            BlockHeader { allocated_size: 4, used_size: 4, data_size: 4, ..Default::default() };
        header.write(&mut buf);
        buf.extend_from_slice(b"data");

        let r = Reader::from_bytes(buf).unwrap();
        assert!(r.tree().unwrap().is_none());
        assert_eq!(&*r.block_data(0).unwrap(), b"data");
    }
}