crates/clawhdf5/src/lib.rs

@@ -47,6 +47,7 @@ pub mod lazy;
 #[cfg(feature = "mmap")]
 pub mod mmap_file;
 pub mod reader;
+mod swmr;
 pub mod types;
 pub mod vlen;
 pub mod writer;
@@ -57,6 +58,7 @@ pub use lazy::{LazyDataset, LazyFile, LazyGroup};
 #[cfg(feature = "mmap")]
 pub use mmap_file::{MmapDataset, MmapFile, MmapGroup};
 pub use reader::{Dataset, File, Group, SharedStorage, VdsResolver};
+pub use swmr::{FileStorage, SWMR_READ_ATTEMPTS};
 pub use types::{AttrValue, DType};
 pub use vlen::VlenValue;
 pub use writer::FileBuilder;

crates/clawhdf5/src/reader.rs

@@ -115,6 +115,10 @@ struct FileData {
     /// parser reads asks for it, and the patched/overlay checks and range
     /// conversions behind it cost a local metadata walk a few percent.
     contiguous: Option<WholeView>,
+    /// A file a SWMR writer may still be appending to
+    /// ([`File::open_swmr`]): reads are bounded by the storage's current
+    /// length, not by `end`, and nothing is ever read as one slice.
+    live: bool,
 }
 
 /// A borrow of the HDF5 data held by a [`FileData`]'s own `backing` or
@@ -138,7 +142,7 @@ impl FileData {
     /// bytes past the recorded end of file are not read, as in libhdf5.
     fn new(mut backing: Backing) -> Result<(Self, Superblock), Error> {
         if let Backing::Storage(storage) = backing {
-            return Self::new_storage(storage);
+            return Self::new_storage(storage, false);
         }
         let whole = backing.whole_file().unwrap_or_default();
         let (user_block, hdf5) = signature::split_user_block(whole)?;
@@ -173,14 +177,15 @@ impl FileData {
             overlay: Vec::new(),
             image_error,
             contiguous: None,
+            live: false,
         };
         data.contiguous = data.find_contiguous();
         Ok((data, superblock))
     }
 
     /// [`Self::new`] for a [`Storage`] backend: the same checks, through
-    /// reads of the storage.
-    fn new_storage(storage: SharedStorage) -> Result<(Self, Superblock), Error> {
+    /// reads of the storage. A `live` file is never read as one slice.
+    fn new_storage(storage: SharedStorage, live: bool) -> Result<(Self, Superblock), Error> {
         let file_len = storage.len();
         let base = signature::find_signature_in(&*storage)?;
         let mut data = Self {
@@ -191,6 +196,7 @@ impl FileData {
             overlay: Vec::new(),
             image_error: None,
             contiguous: None,
+            live,
         };
         // Worked out again below, once the end of file and any cache image
         // are known.
@@ -230,6 +236,9 @@ impl FileData {
 
     /// [`Self::contiguous`], worked out from `backing` and `patched`.
     fn find_contiguous(&self) -> Option<WholeView> {
+        if self.live {
+            return None;
+        }
         let bytes = self.compute_contiguous()?;
         Some(WholeView {
             ptr: bytes.as_ptr(),
@@ -295,6 +304,12 @@ impl Storage for FileData {
         if let Some(all) = self.contiguous() {
             return all.read_at(offset, len);
         }
+        if self.live {
+            // No end fixed at open: the storage's reads end where the file
+            // ends now.
+            let bytes = cut_to(self.remote()?.read_at(self.base + offset, len)?, len);
+            return Ok(self.with_overlay(offset, bytes));
+        }
         let size = self.end - self.base;
         let len = usize::try_from(size.saturating_sub(offset)).map_or(len, |avail| avail.min(len));
         if len == 0 {
@@ -305,6 +320,11 @@ impl Storage for FileData {
     }
 
     fn len(&self) -> u64 {
+        if self.live
+            && let Backing::Storage(s) = &self.backing
+        {
+            return s.len().saturating_sub(self.base);
+        }
         self.end - self.base
     }
 
@@ -312,7 +332,7 @@ impl Storage for FileData {
         if let Some(all) = self.contiguous() {
             return all.read_ranges(ranges);
         }
-        let size = self.end - self.base;
+        let size = Storage::len(self);
         let shifted: Vec<Range<u64>> = ranges
             .iter()
             .map(|r| {
@@ -375,6 +395,9 @@ pub struct File {
     /// Resolves external Virtual Dataset source files instead of
     /// `base_dir` (see [`File::set_vds_resolver`]).
     vds_resolver: Option<VdsResolver>,
+    /// Attempts per operation on a live file, and the retries made (see
+    /// [`File::open_swmr`]).
+    swmr: crate::swmr::Retries,
 }
 
 impl File {
@@ -394,6 +417,7 @@ impl File {
                 chunk_cache: ChunkCache::new(),
                 base_dir,
                 vds_resolver: None,
+                swmr: crate::swmr::Retries::default(),
             })
         }
         #[cfg(not(feature = "mmap"))]
@@ -428,6 +452,7 @@ impl File {
             chunk_cache: ChunkCache::new(),
             base_dir: None,
             vds_resolver: None,
+            swmr: crate::swmr::Retries::default(),
         })
     }
 
@@ -463,9 +488,130 @@ impl File {
             chunk_cache: ChunkCache::new(),
             base_dir: None,
             vds_resolver: None,
+            swmr: crate::swmr::Retries::default(),
         })
     }
 
+    /// Open a file that a libhdf5 SWMR writer (h5py `f.swmr_mode = True`)
+    /// may still be appending to, as libhdf5's SWMR reader does
+    /// (`H5F_ACC_SWMR_READ`, h5py `File(path, "r", swmr=True)`).
+    ///
+    /// The file is read with positioned reads ([`FileStorage`](crate::FileStorage)),
+    /// never mapped, and reads are bounded by the file's length at the
+    /// time of each read rather than a length fixed at open. The chunk cache
+    /// is not used, so every read reads the chunk index and chunks as they
+    /// are now. A [`Dataset`] handle keeps the extent it was opened (or
+    /// last [refreshed](Dataset::refresh)) with, as in libhdf5: call
+    /// [`Dataset::refresh`] to see the writer's appends.
+    ///
+    /// An operation that fails with an error a concurrent write can cause
+    /// (a checksum mismatch, a short read, a bad signature or version byte,
+    /// a chunk or chunk index that does not decode: every format error but
+    /// a wrong path or selection, an unsupported feature or a bad
+    /// argument) is run again from the start, up to
+    /// [`swmr_read_attempts`](Self::swmr_read_attempts) times (100 by
+    /// default, libhdf5's default for SWMR readers), with a pause of 1 µs
+    /// doubling up to 10 ms between attempts. Data is only returned from an
+    /// attempt in which every structure read verified, so a torn read is
+    /// an error (after the last attempt), never data.
+    ///
+    /// A file whose superblock says it is open for writing without SWMR is
+    /// refused with [`Error::Locked`], as libhdf5 refuses it: such a writer
+    /// does not order its writes for readers. Any other file opens (one
+    /// whose writer has closed it reads like [`File::open`]).
+    pub fn open_swmr<P: AsRef<std::path::Path>>(path: P) -> Result<Self, Error> {
+        let storage = crate::swmr::FileStorage::open(path.as_ref()).map_err(Error::Io)?;
+        let mut f = Self::open_storage_swmr(Arc::new(storage))?;
+        f.base_dir = path.as_ref().parent().map(|p| p.to_path_buf());
+        Ok(f)
+    }
+
+    /// [`File::open_swmr`] over any [`Storage`] whose [`Storage::len`]
+    /// follows the file as it grows. A storage that caches blocks, or pins
+    /// the file's length at open (`clawhdf5-remote`'s `BlockCache` and
+    /// `HttpStorage`), does not show the writer's appends.
+    pub fn open_storage_swmr(storage: SharedStorage) -> Result<Self, Error> {
+        let swmr = crate::swmr::Retries::default();
+        let (data, superblock) = swmr.retry(|| FileData::new_storage(storage.clone(), true))?;
+        if superblock.version >= 3 && superblock.is_write_access() && !superblock.is_swmr_write() {
+            return Err(Error::Locked(
+                "the file is open for writing without SWMR (libhdf5: \"file is already open \
+                 for write\"); a SWMR reader needs a SWMR writer"
+                    .into(),
+            ));
+        }
+        Ok(Self {
+            data,
+            superblock,
+            chunk_cache: ChunkCache::new(),
+            base_dir: None,
+            vds_resolver: None,
+            swmr,
+        })
+    }
+
+    /// Whether the file was opened with [`File::open_swmr`] or
+    /// [`File::open_storage_swmr`].
+    pub fn is_swmr_read(&self) -> bool {
+        self.data.live
+    }
+
+    /// How many times an operation on a SWMR-read file is tried (see
+    /// [`File::open_swmr`]); 100 unless set. Files not opened for SWMR
+    /// reading try every operation once.
+    pub fn swmr_read_attempts(&self) -> u32 {
+        self.swmr.attempts
+    }
+
+    /// Set how many times an operation on a SWMR-read file is tried before
+    /// its error is returned (libhdf5's `H5Pset_metadata_read_attempts`); at
+    /// least 1.
+    pub fn set_swmr_read_attempts(&mut self, attempts: u32) {
+        self.swmr.attempts = attempts.max(1);
+    }
+
+    /// How many times an operation on this SWMR-read file has been run
+    /// again because a concurrent write made it fail (the counterpart of
+    /// libhdf5's `H5Fget_metadata_read_retry_info`), counting the attempts
+    /// made while opening it. Always 0 for other files.
+    pub fn swmr_retries(&self) -> u64 {
+        self.swmr.retries()
+    }
+
+    /// Whether a SWMR writer has the file open now: the superblock is read
+    /// again and its SWMR-write flag returned. libhdf5 clears the flag when
+    /// the writer closes the file, so a reader can stop following it then
+    /// (and one last [`Dataset::refresh`] sees the final extents).
+    pub fn swmr_writer_active(&self) -> Result<bool, Error> {
+        self.retry(|| {
+            let sb = Superblock::parse_in(&self.data, 0)?;
+            Ok(sb.version >= 3 && sb.is_swmr_write())
+        })
+    }
+
+    /// Run `op`, again on errors a SWMR writer can cause when the file is
+    /// live (see [`File::open_swmr`]); once otherwise.
+    fn retry<T>(&self, op: impl FnMut() -> Result<T, Error>) -> Result<T, Error> {
+        if self.data.live {
+            self.swmr.retry(op)
+        } else {
+            let mut op = op;
+            op()
+        }
+    }
+
+    /// The chunk cache reads of this file go through: the file's own, or
+    /// for a live file a fresh one per read, since a cached chunk index
+    /// would hide the writer's new chunks and a cached edge chunk would
+    /// read as fill where the writer has written since.
+    fn with_chunk_cache<T>(&self, op: impl FnOnce(&ChunkCache) -> T) -> T {
+        if self.data.live {
+            op(&ChunkCache::new())
+        } else {
+            op(&self.chunk_cache)
+        }
+    }
+
     /// Resolve external Virtual Dataset source files (their names as the
     /// mappings store them) with `resolver`, instead of reading them from
     /// the directory of the file (for [`File::open`]) or refusing them (for
@@ -488,20 +634,23 @@ impl File {
     ///
     /// The path uses `/` separators (e.g., `"group1/values"`).
     pub fn dataset(&self, path: &str) -> Result<Dataset<'_>, Error> {
-        let addr = with_bytes!(self.data.meta()?, |d| group_v2::resolve_path_any_in(
-            d,
-            &self.superblock,
-            path
-        ))?;
-        let hdr = self.parse_header(addr)?;
-        if !has_message(&hdr, MessageType::DataLayout) {
-            return Err(Error::NotADataset(path.to_string()));
-        }
-        Dataset {
-            file: self,
-            header: hdr,
-        }
-        .check_open()
+        self.retry(|| {
+            let addr = with_bytes!(self.data.meta()?, |d| group_v2::resolve_path_any_in(
+                d,
+                &self.superblock,
+                path
+            ))?;
+            let hdr = self.parse_header(addr)?;
+            if !has_message(&hdr, MessageType::DataLayout) {
+                return Err(Error::NotADataset(path.to_string()));
+            }
+            Dataset {
+                file: self,
+                address: addr,
+                header: hdr,
+            }
+            .check_open()
+        })
     }
 
     /// A `Dataset` handle for the object header at `address` (an address
@@ -511,15 +660,18 @@ impl File {
     /// are scanned); keep the address instead to open the same dataset
     /// repeatedly.
     pub fn dataset_at(&self, address: u64) -> Result<Dataset<'_>, Error> {
-        let hdr = self.parse_header(address)?;
-        if !has_message(&hdr, MessageType::DataLayout) {
-            return Err(Error::NotADataset(format!("object at address {address}")));
-        }
-        Dataset {
-            file: self,
-            header: hdr,
-        }
-        .check_open()
+        self.retry(|| {
+            let hdr = self.parse_header(address)?;
+            if !has_message(&hdr, MessageType::DataLayout) {
+                return Err(Error::NotADataset(format!("object at address {address}")));
+            }
+            Dataset {
+                file: self,
+                address,
+                header: hdr,
+            }
+            .check_open()
+        })
     }
 
     /// A `Group` handle for the object header at `address` (from
@@ -538,11 +690,11 @@ impl File {
     /// The path uses `/` separators (e.g., `"sensors"`).
     /// Use `"/"` or `""` for the root group.
     pub fn group(&self, path: &str) -> Result<Group<'_>, Error> {
-        let addr = with_bytes!(self.data.meta()?, |d| group_v2::resolve_path_any_in(
-            d,
-            &self.superblock,
-            path
-        ))?;
+        let addr = self.retry(|| {
+            Ok(with_bytes!(self.data.meta()?, |d| {
+                group_v2::resolve_path_any_in(d, &self.superblock, path)
+            })?)
+        })?;
         Ok(Group {
             file: self,
             address: addr,
@@ -803,15 +955,19 @@ impl<'f> Group<'f> {
 
     /// Get a dataset within this group by name.
     pub fn dataset(&self, name: &str) -> Result<Dataset<'f>, Error> {
-        let hdr = self.file.parse_header(self.child_address(name)?)?;
-        if !has_message(&hdr, MessageType::DataLayout) {
-            return Err(Error::NotADataset(name.to_string()));
-        }
-        Dataset {
-            file: self.file,
-            header: hdr,
-        }
-        .check_open()
+        self.file.retry(|| {
+            let address = self.child_address(name)?;
+            let hdr = self.file.parse_header(address)?;
+            if !has_message(&hdr, MessageType::DataLayout) {
+                return Err(Error::NotADataset(name.to_string()));
+            }
+            Dataset {
+                file: self.file,
+                address,
+                header: hdr,
+            }
+            .check_open()
+        })
     }
 
     /// Get a subgroup within this group by name.
@@ -842,13 +998,15 @@ impl<'f> Group<'f> {
     /// name index rather than by listing the group (see
     /// [`group_v2::resolve_child`]).
     fn child_address(&self, name: &str) -> Result<u64, Error> {
-        with_bytes!(self.file.data.meta()?, |d| group_v2::resolve_child_in(
-            d,
-            &self.file.superblock,
-            self.address,
-            name
-        ))
-        .map_err(Error::Format)
+        self.file.retry(|| {
+            with_bytes!(self.file.data.meta()?, |d| group_v2::resolve_child_in(
+                d,
+                &self.file.superblock,
+                self.address,
+                name
+            ))
+            .map_err(Error::Format)
+        })
     }
 
     /// This group's children that can be opened, as `(name, object header
@@ -869,10 +1027,12 @@ impl<'f> Group<'f> {
     /// [`group_v2::resolve_group_children`]); dangling, external and
     /// user-defined links are left out.
     fn children(&self) -> Result<Vec<GroupEntry>, Error> {
-        with_bytes!(self.file.data.meta()?, |d| {
-            group_v2::resolve_group_children_in(d, &self.file.superblock, self.address)
+        self.file.retry(|| {
+            with_bytes!(self.file.data.meta()?, |d| {
+                group_v2::resolve_group_children_in(d, &self.file.superblock, self.address)
+            })
+            .map_err(Error::Format)
         })
-        .map_err(Error::Format)
     }
 }
 
@@ -884,6 +1044,8 @@ impl<'f> Group<'f> {
 #[derive(Debug)]
 pub struct Dataset<'f> {
     file: &'f File,
+    /// Address of the object header, for [`Dataset::refresh`].
+    address: u64,
     header: ObjectHeader,
 }
 
@@ -906,6 +1068,31 @@ impl<'f> Dataset<'f> {
         Ok(self)
     }
 
+    /// Read the dataset's object header again, as libhdf5's `H5Drefresh`
+    /// (h5py `Dataset.refresh()`) does: afterwards [`shape`](Self::shape)
+    /// and every read use the dataset's extent as it is in the file now.
+    /// For a file a SWMR writer is appending to ([`File::open_swmr`]) this
+    /// is how a reader sees the appends; a transient failure is retried
+    /// (see there). On error the handle keeps its old header.
+    pub fn refresh(&mut self) -> Result<(), Error> {
+        let file = self.file;
+        let address = self.address;
+        let fresh = file.retry(|| {
+            let hdr = file.parse_header(address)?;
+            if !has_message(&hdr, MessageType::DataLayout) {
+                return Err(Error::NotADataset(format!("object at address {address}")));
+            }
+            Dataset {
+                file,
+                address,
+                header: hdr,
+            }
+            .check_open()
+        })?;
+        self.header = fresh.header;
+        Ok(())
+    }
+
     /// Returns the shape (dimensions) of the dataset.
     pub fn shape(&self) -> Result<Vec<u64>, Error> {
         let ds = self.dataspace()?;
@@ -1096,6 +1283,15 @@ impl<'f> Dataset<'f> {
         if matches!(selection, clawhdf5_format::selection::Selection::All) {
             return self.read_raw();
         }
+        self.file.retry(|| self.read_selection_once(selection))
+    }
+
+    /// [`read_selection`](Self::read_selection) of a selection other than
+    /// `All`, once.
+    fn read_selection_once(
+        &self,
+        selection: &clawhdf5_format::selection::Selection,
+    ) -> Result<Vec<u8>, Error> {
         let dt = self.datatype()?;
         let ds = self.dataspace()?;
         let dl = self.data_layout()?;
@@ -1182,6 +1378,17 @@ impl<'f> Dataset<'f> {
         if matches!(selection, clawhdf5_format::selection::Selection::All) {
             return full();
         }
+        self.file
+            .retry(|| self.read_typed_selection_once(selection, convert))
+    }
+
+    /// [`read_typed_selection`](Self::read_typed_selection) of a selection
+    /// other than `All`, once.
+    fn read_typed_selection_once<T: data_read::NativeElement>(
+        &self,
+        selection: &clawhdf5_format::selection::Selection,
+        convert: fn(&[u8], &Datatype) -> Result<Vec<T>, FormatError>,
+    ) -> Result<Vec<T>, Error> {
         let dt = self.datatype()?;
         if T::is_native(&dt) && self.file.data.contiguous().is_some() {
             if let Ok(Some(raw)) = self.read_raw_ref() {
@@ -1587,20 +1794,28 @@ impl<'f> Dataset<'f> {
         }
         let ds = self.dataspace()?;
         let pipeline = self.filter_pipeline()?;
-        Ok(data_read::read_chunked_native_in::<T, _>(
-            &self.header.messages,
-            &self.file.data,
-            &dl,
-            &ds,
-            &dt,
-            pipeline.as_ref(),
-            self.file.offset_size(),
-            self.file.length_size(),
-            Some(&self.file.chunk_cache),
-        )?)
+        self.file.retry(|| {
+            self.file.with_chunk_cache(|cache| {
+                Ok(data_read::read_chunked_native_in::<T, _>(
+                    &self.header.messages,
+                    &self.file.data,
+                    &dl,
+                    &ds,
+                    &dt,
+                    pipeline.as_ref(),
+                    self.file.offset_size(),
+                    self.file.length_size(),
+                    Some(cache),
+                )?)
+            })
+        })
     }
 
     fn read_raw(&self) -> Result<Vec<u8>, Error> {
+        self.file.retry(|| self.read_raw_once())
+    }
+
+    fn read_raw_once(&self) -> Result<Vec<u8>, Error> {
         let dt = self.datatype()?;
         let ds = self.dataspace()?;
         let dl = self.data_layout()?;
@@ -1622,16 +1837,18 @@ impl<'f> Dataset<'f> {
             self.file.offset_size(),
             self.file.length_size(),
             || {
-                Ok(data_read::read_raw_data_cached_in(
-                    &self.file.data,
-                    &dl,
-                    &ds,
-                    &dt,
-                    pipeline.as_ref(),
-                    self.file.offset_size(),
-                    self.file.length_size(),
-                    &self.file.chunk_cache,
-                )?)
+                self.file.with_chunk_cache(|cache| {
+                    Ok(data_read::read_raw_data_cached_in(
+                        &self.file.data,
+                        &dl,
+                        &ds,
+                        &dt,
+                        pipeline.as_ref(),
+                        self.file.offset_size(),
+                        self.file.length_size(),
+                        cache,
+                    )?)
+                })
             },
         )
     }

crates/clawhdf5/src/swmr.rs

@@ -0,0 +1,298 @@
+//! Reading files a SWMR writer is still appending to (see
+//! `docs/design/swmr.md` and [`File::open_swmr`](crate::File::open_swmr)):
+//! a [`FileStorage`] whose length grows with the file, and the bounded
+//! retries of operations a concurrent write can make fail.
+
+use std::borrow::Cow;
+use std::cell::Cell;
+use std::path::Path;
+use std::sync::atomic::{AtomicU64, Ordering};
+use std::time::Duration;
+
+use clawhdf5_format::error::FormatError;
+use clawhdf5_format::storage::Storage;
+
+use crate::error::Error;
+
+/// How many times a live file ([`File::open_swmr`](crate::File::open_swmr))
+/// tries an operation that fails with an error a concurrent write can cause
+/// before returning the error: 100, libhdf5's default number of metadata
+/// read attempts for SWMR access (`H5Pset_metadata_read_attempts`).
+pub const SWMR_READ_ATTEMPTS: u32 = 100;
+
+/// Longest pause between two attempts.
+const MAX_PAUSE: Duration = Duration::from_millis(10);
+
+/// A local file read with positioned reads (`pread` on Unix, `seek_read` on
+/// Windows), never mapped, whose [`Storage::len`] is the file's length at
+/// the time of the call: a [`Storage`] for a file that another process is
+/// appending to. A read past the end is short, as the trait allows.
+#[derive(Debug)]
+pub struct FileStorage {
+    file: std::fs::File,
+    #[cfg(not(any(unix, windows)))]
+    lock: std::sync::Mutex<()>,
+}
+
+impl FileStorage {
+    /// Open the file at `path` for reading.
+    pub fn open<P: AsRef<Path>>(path: P) -> std::io::Result<Self> {
+        Ok(Self::new(std::fs::File::open(path)?))
+    }
+
+    /// A storage over an open file.
+    pub fn new(file: std::fs::File) -> Self {
+        Self {
+            file,
+            #[cfg(not(any(unix, windows)))]
+            lock: std::sync::Mutex::new(()),
+        }
+    }
+
+    /// Up to `buf.len()` bytes at `offset`; fewer only at the end of file.
+    fn read_into(&self, offset: u64, buf: &mut [u8]) -> std::io::Result<usize> {
+        let mut got = 0;
+        while got < buf.len() {
+            match self.read_once(offset + got as u64, &mut buf[got..]) {
+                Ok(0) => break,
+                Ok(n) => got += n,
+                Err(e) if e.kind() == std::io::ErrorKind::Interrupted => {}
+                Err(e) => return Err(e),
+            }
+        }
+        Ok(got)
+    }
+
+    #[cfg(unix)]
+    fn read_once(&self, offset: u64, buf: &mut [u8]) -> std::io::Result<usize> {
+        std::os::unix::fs::FileExt::read_at(&self.file, buf, offset)
+    }
+
+    #[cfg(windows)]
+    fn read_once(&self, offset: u64, buf: &mut [u8]) -> std::io::Result<usize> {
+        std::os::windows::fs::FileExt::seek_read(&self.file, buf, offset)
+    }
+
+    #[cfg(not(any(unix, windows)))]
+    fn read_once(&self, offset: u64, buf: &mut [u8]) -> std::io::Result<usize> {
+        use std::io::{Read, Seek, SeekFrom};
+        let _guard = self.lock.lock().unwrap_or_else(|p| p.into_inner());
+        let mut f = &self.file;
+        f.seek(SeekFrom::Start(offset))?;
+        f.read(buf)
+    }
+}
+
+impl Storage for FileStorage {
+    fn read_at(&self, offset: u64, len: usize) -> Result<Cow<'_, [u8]>, FormatError> {
+        // Never allocate more than the file holds, whatever a (possibly
+        // hostile) size field asked for.
+        let avail = self.len().saturating_sub(offset);
+        let len = usize::try_from(avail).map_or(len, |a| a.min(len));
+        let mut buf = vec![0u8; len];
+        let got = self
+            .read_into(offset, &mut buf)
+            .map_err(|e| FormatError::Storage(format!("read of {len} bytes at {offset}: {e}")))?;
+        buf.truncate(got);
+        Ok(Cow::Owned(buf))
+    }
+
+    fn len(&self) -> u64 {
+        self.file.metadata().map_or(0, |m| m.len())
+    }
+}
+
+/// Whether `e` can be caused by reading a structure while a SWMR writer
+/// rewrites or has not yet finished writing it — a checksum mismatch, a
+/// short read, a bad signature, a chunk index or chunk that does not
+/// decode, ... (see [`is_transient_format`]) — so that the operation is
+/// worth running again.
+pub(crate) fn is_transient(e: &Error) -> bool {
+    match e {
+        Error::Format(f) => is_transient_format(f),
+        Error::Io(io) => io.kind() == std::io::ErrorKind::UnexpectedEof,
+        _ => false,
+    }
+}
+
+/// A read that raced a write can garble any field of a structure, and the
+/// parsers report that as whatever check fails first (the checksum, a
+/// signature, a version byte, a size), so every format error counts except
+/// those that bytes read later cannot change: a name or selection the
+/// caller got wrong, a feature this reader does not support, an argument
+/// that does not fit.
+fn is_transient_format(e: &FormatError) -> bool {
+    use FormatError as F;
+    !matches!(
+        e,
+        F::PathNotFound(_)
+            | F::SelectionOutOfBounds(_)
+            | F::UnsupportedFilter(_)
+            | F::ExternalDataFilesUnsupported
+            | F::ExternalLinkUnsupported { .. }
+            | F::ContiguousStorageRequired(_)
+            | F::TypeMismatch { .. }
+            | F::DataSizeMismatch { .. }
+            | F::SerializationError(_)
+            | F::CompressionError(_)
+            | F::DuplicateDatasetName(_)
+            | F::InvalidLinkName
+    )
+}
+
+thread_local! {
+    /// Set while this thread runs an operation under [`retry`], so an
+    /// operation made of retried operations retries as a whole, not each
+    /// part up to the limit.
+    static RETRYING: Cell<bool> = const { Cell::new(false) };
+}
+
+/// A live file's retry policy: attempts per operation, and a count of the
+/// retries made.
+#[derive(Debug)]
+pub(crate) struct Retries {
+    pub(crate) attempts: u32,
+    retried: AtomicU64,
+}
+
+impl Default for Retries {
+    fn default() -> Self {
+        Self {
+            attempts: SWMR_READ_ATTEMPTS,
+            retried: AtomicU64::new(0),
+        }
+    }
+}
+
+impl Retries {
+    /// [`retry`] with this policy, counting the retries.
+    pub(crate) fn retry<T>(&self, op: impl FnMut() -> Result<T, Error>) -> Result<T, Error> {
+        retry(self.attempts, &self.retried, op)
+    }
+
+    /// Retries made so far.
+    pub(crate) fn retries(&self) -> u64 {
+        self.retried.load(Ordering::Relaxed)
+    }
+}
+
+/// Run `op` up to `attempts` times while it fails with a transient error
+/// ([`is_transient`]), pausing 1 µs, 2 µs, 4 µs, … up to 10 ms between
+/// attempts, and adding each retry to `retried`; return its first success
+/// or last error. Inside another `retry` on the same thread, `op` runs
+/// once.
+fn retry<T>(
+    attempts: u32,
+    retried: &AtomicU64,
+    mut op: impl FnMut() -> Result<T, Error>,
+) -> Result<T, Error> {
+    if RETRYING.with(Cell::get) {
+        return op();
+    }
+    struct Reset;
+    impl Drop for Reset {
+        fn drop(&mut self) {
+            RETRYING.with(|r| r.set(false));
+        }
+    }
+    RETRYING.with(|r| r.set(true));
+    let _reset = Reset;
+    let mut pause = Duration::from_micros(1);
+    let mut attempt = 1;
+    loop {
+        match op() {
+            Err(e) if attempt < attempts && is_transient(&e) => {
+                retried.fetch_add(1, Ordering::Relaxed);
+                std::thread::sleep(pause);
+                pause = (pause * 2).min(MAX_PAUSE);
+                attempt += 1;
+            }
+            result => return result,
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn file_storage_reads_what_the_file_holds_now() {
+        let dir = tempfile::tempdir().unwrap();
+        let path = dir.path().join("grow.bin");
+        std::fs::write(&path, b"hello").unwrap();
+        let s = FileStorage::open(&path).unwrap();
+        assert_eq!(s.len(), 5);
+        assert_eq!(&*s.read_at(1, 3).unwrap(), b"ell");
+        assert_eq!(&*s.read_at(3, 10).unwrap(), b"lo");
+        assert!(s.read_at(9, 4).unwrap().is_empty());
+        // The file grows after the storage was opened.
+        use std::io::Write;
+        std::fs::OpenOptions::new()
+            .append(true)
+            .open(&path)
+            .unwrap()
+            .write_all(b", world")
+            .unwrap();
+        assert_eq!(s.len(), 12);
+        assert_eq!(&*s.read_at(3, 100).unwrap(), b"lo, world");
+    }
+
+    #[test]
+    fn retry_runs_again_only_for_transient_errors() {
+        let n = AtomicU64::new(0);
+        let mut calls = 0;
+        let r: Result<u32, Error> = retry(5, &n, || {
+            calls += 1;
+            if calls < 3 {
+                Err(Error::Format(FormatError::ChecksumMismatch {
+                    expected: 1,
+                    computed: 2,
+                }))
+            } else {
+                Ok(7)
+            }
+        });
+        assert_eq!(r.unwrap(), 7);
+        assert_eq!(calls, 3);
+        assert_eq!(n.load(Ordering::Relaxed), 2);
+
+        // Gives up after `attempts`.
+        let mut calls = 0;
+        let r: Result<(), Error> = retry(4, &n, || {
+            calls += 1;
+            Err(Error::Format(FormatError::UnexpectedEof {
+                expected: 8,
+                available: 0,
+            }))
+        });
+        assert!(r.is_err());
+        assert_eq!(calls, 4);
+        assert_eq!(n.load(Ordering::Relaxed), 5);
+
+        // A permanent error is returned at once.
+        let mut calls = 0;
+        let r: Result<(), Error> = retry(4, &n, || {
+            calls += 1;
+            Err(Error::Format(FormatError::UnsupportedFilter(999)))
+        });
+        assert!(r.is_err());
+        assert_eq!(calls, 1);
+        assert_eq!(n.load(Ordering::Relaxed), 5);
+
+        // Nested: the inner operation runs once per outer attempt.
+        let mut inner = 0;
+        let mut outer = 0;
+        let r: Result<(), Error> = retry(3, &n, || {
+            outer += 1;
+            retry(3, &n, || {
+                inner += 1;
+                Err(Error::Format(FormatError::SignatureNotFound))
+            })
+        });
+        assert!(r.is_err());
+        assert_eq!((outer, inner), (3, 3));
+        // The flag is reset afterwards.
+        assert!(!RETRYING.with(Cell::get));
+    }
+}

crates/clawhdf5/tests/swmr_interop.rs

@@ -6,10 +6,13 @@
 //! The live tests need python3 with h5py; they are skipped without it,
 //! unless `CLAWHDF5_REQUIRE_INTEROP=1`.
 
+use std::io::{BufRead, BufReader, Read};
 use std::path::{Path, PathBuf};
-use std::process::Command;
+use std::process::{Command, Stdio};
+use std::sync::Arc;
+use std::sync::atomic::{AtomicBool, AtomicU32, Ordering};
 
-use clawhdf5::File;
+use clawhdf5::{File, Selection};
 
 fn python() -> String {
     std::env::var("CLAWHDF5_PYTHON").unwrap_or_else(|_| "python3".to_string())
@@ -77,6 +80,18 @@ fn a_copy_taken_mid_write_reads_past_its_recorded_end_of_file() {
     check_mid_write_copy(&File::open(&path).unwrap());
     check_mid_write_copy(&File::open_buffered(&path).unwrap());
     check_mid_write_copy(&File::from_bytes(std::fs::read(&path).unwrap()).unwrap());
+    let want_a: Vec<i64> = (1..=148).collect();
+    let want_b: Vec<f64> = (0..148)
+        .flat_map(|i| (0..4).map(move |j| (10 * i + j + 1) as f64))
+        .collect();
+    let mm = clawhdf5::MmapFile::open(&path).unwrap();
+    assert_eq!(mm.dataset("a").unwrap().read_i64().unwrap(), want_a);
+    assert_eq!(mm.dataset("b").unwrap().read_f64().unwrap(), want_b);
+    let lazy = clawhdf5::LazyFile::open_mmap(&path).unwrap();
+    assert_eq!(lazy.dataset("a").unwrap().read_i64().unwrap(), want_a);
+    assert_eq!(lazy.dataset("b").unwrap().read_f64().unwrap(), want_b);
+    let storage = File::open_storage(Arc::new(std::fs::read(&path).unwrap())).unwrap();
+    check_mid_write_copy(&storage);
 }
 
 #[test]
@@ -97,7 +112,10 @@ print("ok")
 "#,
         p = path.display()
     );
-    let out = Command::new(python()).args(["-c", &script]).output().unwrap();
+    let out = Command::new(python())
+        .args(["-c", &script])
+        .output()
+        .unwrap();
     assert!(
         out.status.success(),
         "h5py failed:\n{}",
@@ -105,3 +123,459 @@ print("ok")
     );
     check_mid_write_copy(&File::open(&path).unwrap());
 }
+
+#[test]
+fn open_swmr_reads_the_mid_write_copy() {
+    let f = File::open_swmr(fixture("swmr_mid_write.h5")).unwrap();
+    assert!(f.is_swmr_read());
+    assert_eq!(f.swmr_read_attempts(), clawhdf5::SWMR_READ_ATTEMPTS);
+    // The copy still has the SWMR-write flag: as far as it says, its writer
+    // is still writing.
+    assert!(f.swmr_writer_active().unwrap());
+    check_mid_write_copy(&f);
+    let mut a = f.dataset("a").unwrap();
+    a.refresh().unwrap();
+    assert_eq!(a.shape().unwrap(), vec![148]);
+    assert_eq!(f.swmr_retries(), 0);
+    assert!(
+        !File::open(fixture("swmr_mid_write.h5"))
+            .unwrap()
+            .is_swmr_read()
+    );
+}
+
+/// The mid-write copy with its superblock's flags set to `flags` (and the
+/// superblock checksum updated).
+fn mid_write_copy_with_flags(flags: u8) -> Vec<u8> {
+    let mut bytes = std::fs::read(fixture("swmr_mid_write.h5")).unwrap();
+    // Superblock v3, 8-byte offsets: 12 bytes, 4 addresses, checksum.
+    bytes[11] = flags;
+    let sum = clawhdf5_format::checksum::jenkins_lookup3(&bytes[..44]);
+    bytes[44..48].copy_from_slice(&sum.to_le_bytes());
+    bytes
+}
+
+#[test]
+fn open_swmr_refuses_a_file_open_for_writing_without_swmr() {
+    // libhdf5 refuses it too: "file is already open for write".
+    let bytes = mid_write_copy_with_flags(0x01);
+    let err = File::open_storage_swmr(Arc::new(bytes)).unwrap_err();
+    assert!(matches!(err, clawhdf5::Error::Locked(_)), "{err}");
+    // Closed (flags clear): opens, and no writer is active.
+    let f = File::open_storage_swmr(Arc::new(mid_write_copy_with_flags(0))).unwrap();
+    assert!(!f.swmr_writer_active().unwrap());
+}
+
+/// A storage whose next `torn` reads come back garbled, as a read racing a
+/// rewrite of the structure can see them: the middle byte changed, or with
+/// `invert` every byte (signatures included).
+struct Torn {
+    bytes: Vec<u8>,
+    torn: AtomicU32,
+    invert: AtomicBool,
+}
+
+impl clawhdf5::Storage for Torn {
+    fn read_at(
+        &self,
+        offset: u64,
+        len: usize,
+    ) -> Result<std::borrow::Cow<'_, [u8]>, clawhdf5_format::error::FormatError> {
+        let got = self.bytes.as_slice().read_at(offset, len)?;
+        let tear = self
+            .torn
+            .fetch_update(Ordering::SeqCst, Ordering::SeqCst, |n| n.checked_sub(1))
+            .is_ok();
+        if tear && !got.is_empty() {
+            let mut v = got.into_owned();
+            if self.invert.load(Ordering::SeqCst) {
+                v.iter_mut().for_each(|b| *b = !*b);
+            } else {
+                let mid = v.len() / 2;
+                v[mid] ^= 0x5a;
+            }
+            return Ok(v.into());
+        }
+        Ok(got)
+    }
+
+    fn len(&self) -> u64 {
+        self.bytes.len() as u64
+    }
+}
+
+#[test]
+fn a_torn_metadata_read_is_retried_and_never_returned() {
+    use clawhdf5::Storage as _;
+    let torn = Arc::new(Torn {
+        bytes: std::fs::read(fixture("swmr_mid_write.h5")).unwrap(),
+        torn: AtomicU32::new(0),
+        invert: AtomicBool::new(false),
+    });
+    let f = File::open_storage_swmr(torn.clone()).unwrap();
+    assert_eq!(f.swmr_retries(), 0);
+    assert!(torn.len() > 0);
+
+    // The next 3 reads (object headers) come back garbled: the lookup
+    // fails its checks, is run again, and returns the right dataset. The
+    // same with every byte garbled, signatures included.
+    for invert in [false, true] {
+        torn.invert.store(invert, Ordering::SeqCst);
+        let before = f.swmr_retries();
+        torn.torn.store(3, Ordering::SeqCst);
+        let a = f.dataset("a").unwrap();
+        assert!(f.swmr_retries() > before, "invert {invert}");
+        assert_eq!(a.read_i64().unwrap(), (1..=148).collect::<Vec<i64>>());
+        // Two garbled reads: the prefix of the root group's header (whose
+        // garbled byte may go unused, as it is read again whole) and the
+        // whole header, which fails its checksum.
+        let before = f.swmr_retries();
+        torn.torn.store(2, Ordering::SeqCst);
+        let b = f.dataset("b").unwrap();
+        assert_eq!(b.read_f64().unwrap().len(), 148 * 4);
+        assert!(f.swmr_retries() > before, "invert {invert}");
+    }
+    torn.invert.store(false, Ordering::SeqCst);
+
+    // With one attempt the error is returned instead (both reads of the
+    // header garbled, as above).
+    let mut f = File::open_storage_swmr(torn.clone()).unwrap();
+    f.set_swmr_read_attempts(1);
+    torn.torn.store(2, Ordering::SeqCst);
+    assert!(f.dataset("a").is_err());
+    assert_eq!(f.swmr_retries(), 0);
+    torn.torn.store(0, Ordering::SeqCst);
+
+    // A refresh that keeps failing gives up after the attempts, and the
+    // handle keeps its extent.
+    f.set_swmr_read_attempts(5);
+    let mut b = f.dataset("b").unwrap();
+    torn.torn.store(1000, Ordering::SeqCst);
+    assert!(b.refresh().is_err());
+    assert_eq!(f.swmr_retries(), 4);
+    torn.torn.store(0, Ordering::SeqCst);
+    assert_eq!(b.shape().unwrap(), vec![148, 4]);
+    b.refresh().unwrap();
+    assert_eq!(b.read_f64().unwrap().len(), 148 * 4);
+
+    // A file not opened for SWMR reading does not retry.
+    let plain = File::open_storage(torn.clone()).unwrap();
+    torn.torn.store(2, Ordering::SeqCst);
+    assert!(plain.dataset("a").is_err());
+    assert_eq!(plain.swmr_retries(), 0);
+}
+
+// ---------------------------------------------------------------------------
+// A live file: an h5py SWMR writer appends while clawhdf5 and h5py read.
+// ---------------------------------------------------------------------------
+
+/// The h5py SWMR writer. It creates three chunked datasets whose values are
+/// a function of their position, so a reader can check every value it
+/// reads without knowing when it was written:
+///
+/// - `a`: int64 `(n,)`, chunks of 100, no filter — `a[i] = i + 1`
+///   (Extensible Array index);
+/// - `b`: float64 `(n, 4)`, chunks of 16 x 4, gzip — `b[i, j] = 10 i + j + 1`
+///   (Extensible Array index);
+/// - `c`: int32 `(r, k)`, both unlimited, chunks of 8 x 8, no filter —
+///   `c[i, j] = 1000 i + j + 1` (version-2 B-tree index).
+///
+/// It switches to SWMR mode, prints `ready`, then for `steps` steps appends a
+/// random number of rows to each dataset (and every 7th step a column to
+/// `c`), resizing before writing as SWMR writers must, flushing each
+/// dataset and pausing 2 ms, and finally closes the file and writes the
+/// final contents to `<path>.<name>.bin`. `CLAWHDF5_SWMR_STEPS` sets the
+/// number of steps (default 2500).
+const WRITER: &str = r#"
+import sys, time, random, h5py, numpy as np
+path, steps = sys.argv[1], int(sys.argv[2])
+rng = random.Random(1234)
+f = h5py.File(path, "w", libver="latest")
+a = f.create_dataset("a", shape=(0,), maxshape=(None,), chunks=(100,), dtype="i8")
+b = f.create_dataset("b", shape=(0, 4), maxshape=(None, 4), chunks=(16, 4), dtype="f8",
+                     compression="gzip")
+c = f.create_dataset("c", shape=(0, 1), maxshape=(None, None), chunks=(8, 8), dtype="i4")
+f.swmr_mode = True
+print("ready", flush=True)
+na = nb = rc = 0
+kc = 1
+for step in range(steps):
+    k = rng.randint(1, 60)
+    a.resize((na + k,)); a[na:na + k] = np.arange(na, na + k) + 1; na += k
+    a.flush()
+    k = rng.randint(1, 20)
+    rows = np.arange(nb, nb + k)[:, None] * 10 + np.arange(4) + 1
+    b.resize((nb + k, 4)); b[nb:nb + k] = rows; nb += k
+    b.flush()
+    if step % 7 == 6:
+        c.resize((rc, kc + 1))
+        if rc:
+            c[:, kc] = np.arange(rc) * 1000 + kc + 1
+        kc += 1
+    k = rng.randint(1, 3)
+    c.resize((rc + k, kc))
+    c[rc:rc + k] = np.arange(rc, rc + k)[:, None] * 1000 + np.arange(kc) + 1
+    rc += k
+    c.flush()
+    time.sleep(0.002)
+f.close()
+with h5py.File(path, "r") as f:
+    for name in "abc":
+        open(f"{path}.{name}.bin", "wb").write(f[name][()].tobytes())
+        print(name, *f[name].shape, flush=True)
+"#;
+
+/// h5py's own SWMR reader, run beside ours as the reference: the same
+/// checks, until `<path>.stop` exists. Prints its iteration count.
+const H5PY_READER: &str = r#"
+import os, sys, h5py, numpy as np
+path = sys.argv[1]
+f = h5py.File(path, "r", swmr=True)
+ds = {n: f[n] for n in "abc"}
+last = {n: (0,) * ds[n].ndim for n in "abc"}
+its = 0
+while True:
+    stop = os.path.exists(path + ".stop")
+    for n, d in ds.items():
+        d.refresh()
+        shape = d.shape
+        assert all(s >= l for s, l in zip(shape, last[n])), (n, shape, last[n])
+        last[n] = shape
+        v = d[()]
+        if n == "a":
+            want = np.arange(shape[0]) + 1
+        elif n == "b":
+            want = np.arange(shape[0])[:, None] * 10 + np.arange(4) + 1
+        else:
+            want = np.arange(shape[0])[:, None] * 1000 + np.arange(shape[1]) + 1
+        bad = np.argwhere(v != want)
+        assert len(bad) == 0, (n, shape, bad[:5], v[tuple(bad[0])], want[tuple(bad[0])])
+    its += 1
+    if stop:
+        break
+print("iterations", its, *last["a"], *last["b"], *last["c"], flush=True)
+"#;
+
+fn want_a(n: u64) -> Vec<i64> {
+    (1..=n as i64).collect()
+}
+
+fn want_b(rows: std::ops::Range<u64>) -> Vec<f64> {
+    rows.flat_map(|i| (0..4).map(move |j| (10 * i + j + 1) as f64))
+        .collect()
+}
+
+fn want_c(rows: std::ops::Range<u64>, cols: u64) -> Vec<i32> {
+    rows.flat_map(|i| (0..cols).map(move |j| (1000 * i + j + 1) as i32))
+        .collect()
+}
+
+/// One pass of the Rust reader: refresh every dataset, check its extent
+/// did not shrink, and check what it reads (the last rows of each, and all
+/// of them every `full_every` passes).
+fn check_pass(
+    datasets: &mut [clawhdf5::Dataset<'_>; 3],
+    last: &mut [Vec<u64>; 3],
+    pass: u64,
+    full_every: u64,
+) {
+    for (k, d) in datasets.iter_mut().enumerate() {
+        d.refresh().unwrap_or_else(|e| panic!("refresh {k}: {e}"));
+        let shape = d.shape().unwrap();
+        assert!(
+            shape.iter().zip(&last[k]).all(|(s, l)| s >= l),
+            "dataset {k} shrank: {shape:?} after {:?}",
+            last[k]
+        );
+        last[k] = shape;
+    }
+    let full = pass.is_multiple_of(full_every);
+    let na = last[0][0];
+    let from = if full { 0 } else { na.saturating_sub(200) };
+    let got = if full {
+        datasets[0].read_i64()
+    } else {
+        datasets[0].read_i64_selection(&Selection::slice(std::slice::from_ref(&(from..na))))
+    }
+    .unwrap_or_else(|e| panic!("read a: {e}"));
+    let want: Vec<i64> = want_a(na).split_off(from as usize);
+    assert!(got == want, "a {from}..{na}: {:?}", first_diff(&got, &want));
+
+    let nb = last[1][0];
+    let from = if full { 0 } else { nb.saturating_sub(50) };
+    let sel = Selection::slice(&[from..nb, 0..4]);
+    let got = datasets[1]
+        .read_f64_selection(&sel)
+        .unwrap_or_else(|e| panic!("read b: {e}"));
+    let want = want_b(from..nb);
+    assert!(got == want, "b {from}..{nb}: {:?}", first_diff(&got, &want));
+
+    let (rc, kc) = (last[2][0], last[2][1]);
+    let from = if full { 0 } else { rc.saturating_sub(9) };
+    let got = if full {
+        datasets[2].read_i32()
+    } else {
+        datasets[2].read_i32_selection(&Selection::slice(&[from..rc, 0..kc]))
+    }
+    .unwrap_or_else(|e| panic!("read c: {e}"));
+    let want = want_c(from..rc, kc);
+    assert!(
+        got == want,
+        "c {from}..{rc} x {kc}: {:?}",
+        first_diff(&got, &want)
+    );
+}
+
+fn first_diff<T: PartialEq + std::fmt::Debug>(got: &[T], want: &[T]) -> String {
+    if got.len() != want.len() {
+        return format!("{} values, want {}", got.len(), want.len());
+    }
+    let i = got.iter().zip(want).position(|(g, w)| g != w).unwrap();
+    format!("first difference at {i}: {:?}, want {:?}", got[i], want[i])
+}
+
+#[test]
+fn a_live_file_reads_consistently_while_an_h5py_swmr_writer_appends() {
+    skip_if_no_python!();
+    let steps: u32 = std::env::var("CLAWHDF5_SWMR_STEPS")
+        .ok()
+        .and_then(|s| s.parse().ok())
+        .unwrap_or(2500);
+    let dir = tempfile::tempdir_in(env!("CARGO_TARGET_TMPDIR")).unwrap();
+    let path = dir.path().join("live.h5");
+
+    let mut writer = Command::new(python())
+        .args(["-c", WRITER, path.to_str().unwrap(), &steps.to_string()])
+        .stdout(Stdio::piped())
+        .spawn()
+        .unwrap();
+    let mut out = BufReader::new(writer.stdout.take().unwrap());
+    let mut line = String::new();
+    out.read_line(&mut line).unwrap();
+    assert_eq!(line.trim(), "ready", "writer did not start");
+
+    // h5py's SWMR reader on the same file, as the reference.
+    let h5py_reader = Command::new(python())
+        .args(["-c", H5PY_READER, path.to_str().unwrap()])
+        .stdout(Stdio::piped())
+        .stderr(Stdio::piped())
+        .spawn()
+        .unwrap();
+
+    let file = File::open_swmr(&path).unwrap();
+    assert!(file.is_swmr_read());
+    assert!(file.swmr_writer_active().unwrap());
+    // Two reader threads share the file, each with its own handles; each
+    // makes passes until it has seen the writer exit, then one more.
+    let writer_done = AtomicBool::new(false);
+    let reader = || {
+        let mut datasets = ["a", "b", "c"].map(|n| file.dataset(n).unwrap());
+        let mut last = [vec![0], vec![0, 4], vec![0, 1]];
+        let (mut passes, mut grew) = (0u64, 0u64);
+        loop {
+            let done = writer_done.load(Ordering::Acquire);
+            let before = last[0][0];
+            check_pass(&mut datasets, &mut last, passes, 50);
+            passes += 1;
+            grew += u64::from(last[0][0] > before);
+            if done {
+                break;
+            }
+        }
+        // The writer has closed the file: one more pass reads the final
+        // extents, all of them.
+        check_pass(&mut datasets, &mut last, 0, 50);
+        (passes, grew, last)
+    };
+    let (status, results) = std::thread::scope(|s| {
+        let readers = [s.spawn(reader), s.spawn(reader)];
+        let status = writer.wait().unwrap();
+        writer_done.store(true, Ordering::Release);
+        (status, readers.map(|r| r.join().unwrap()))
+    });
+    assert!(status.success(), "writer failed");
+    assert!(!file.swmr_writer_active().unwrap());
+    let last = results[0].2.clone();
+    assert_eq!(results[1].2, last);
+    let passes: Vec<u64> = results.iter().map(|r| r.0).collect();
+    let grew: u64 = results.iter().map(|r| r.1).min().unwrap();
+
+    std::fs::write(path.with_extension("h5.stop"), b"").unwrap();
+    let reference = h5py_reader.wait_with_output().unwrap();
+    assert!(
+        reference.status.success(),
+        "h5py's SWMR reader failed:\n{}",
+        String::from_utf8_lossy(&reference.stderr)
+    );
+    let reference = String::from_utf8_lossy(&reference.stdout).into_owned();
+
+    // The writer's final contents, as h5py reads the closed file.
+    let mut finals = String::new();
+    out.read_to_string(&mut finals).unwrap();
+    let shapes: std::collections::HashMap<&str, Vec<u64>> = finals
+        .lines()
+        .map(|l| {
+            let mut w = l.split_whitespace();
+            let name = w.next().unwrap();
+            (name, w.map(|v| v.parse().unwrap()).collect())
+        })
+        .collect();
+    assert_eq!(shapes["a"], last[0]);
+    assert_eq!(shapes["b"], last[1]);
+    assert_eq!(shapes["c"], last[2]);
+    let bin = |name: &str| std::fs::read(format!("{}.{name}.bin", path.display())).unwrap();
+    let le = |b: &[u8], n: usize| -> Vec<[u8; 8]> {
+        b.chunks(n)
+            .map(|c| {
+                let mut v = [0u8; 8];
+                v[..n].copy_from_slice(c);
+                v
+            })
+            .collect()
+    };
+    // The live handle and a fresh non-SWMR open of the closed file both
+    // read exactly what h5py reads.
+    for f in [&file, &File::open(&path).unwrap()] {
+        let a: Vec<[u8; 8]> = f
+            .dataset("a")
+            .unwrap()
+            .read_i64()
+            .unwrap()
+            .iter()
+            .map(|v| v.to_le_bytes())
+            .collect();
+        assert_eq!(a, le(&bin("a"), 8));
+        let b: Vec<[u8; 8]> = f
+            .dataset("b")
+            .unwrap()
+            .read_f64()
+            .unwrap()
+            .iter()
+            .map(|v| v.to_le_bytes())
+            .collect();
+        assert_eq!(b, le(&bin("b"), 8));
+        let c: Vec<[u8; 8]> = f
+            .dataset("c")
+            .unwrap()
+            .read_i32()
+            .unwrap()
+            .iter()
+            .map(|v| {
+                let mut x = [0u8; 8];
+                x[..4].copy_from_slice(&v.to_le_bytes());
+                x
+            })
+            .collect();
+        assert_eq!(c, le(&bin("c"), 4));
+    }
+    eprintln!(
+        "SWMR: {steps} writer steps; clawhdf5 readers made {passes:?} passes (each saw `a` \
+         grow at least {grew} times), {} retries; h5py reader: {}",
+        file.swmr_retries(),
+        reference.trim()
+    );
+    assert!(
+        grew >= 2,
+        "a reader never saw the file grow ({passes:?} passes)"
+    );
+}