dataset: string dataset reading and offset updates to vlen strings

CHANGELOG.md

@@ -1,5 +1,49 @@
 # Changelog
 
+## 0.4.1
+
+### Added
+
+- `H5Dataset::read_strings` and `read_strings_lossy` decode a whole string
+  dataset in one call, at whatever width the file declares (issue #5, requested
+  by @janosh). Reading
+  a fixed-string column previously meant `read_raw_bytes` plus hand-written
+  slicing, and the width is a per-dataset property, so the caller had to
+  hard-code one and get it wrong on the next file. Both string datatypes go
+  through the same call, the padding rule decides where each value ends, and
+  the character set is enforced — `read_strings` fails on bytes that do not
+  decode, `read_strings_lossy` substitutes U+FFFD for producers that mislabel
+  the character set.
+
+- `H5Dataset::write_vlen_strings_slice` replaces variable-length strings at an
+  arbitrary offset (issue #6, requested by @janosh). Vlen datasets could only
+  be created whole or
+  appended to, so correcting one entry meant rewriting the dataset. The new
+  strings go into one global-heap collection and their references are written
+  over the range; elements still held in the append buffer are patched there,
+  so the flush at close does not write the pre-update reference back over
+  them.
+
+- Superseded global heap objects are now freed. Overwriting a vlen element
+  reads the reference it replaces first and removes the object it names, as
+  libhdf5 does (`H5T__vlen_disk_write` → `H5T__vlen_disk_delete` →
+  `H5HG_remove`): the collection is rewritten at its existing size with the
+  recovered bytes given to its free-space marker, and one left with no objects
+  returns its block to the allocator. Without this, each update stranded a
+  4096-byte collection (`H5HG_MINALLOC`), so a column updated in a loop grew
+  the file without bound. Under SWMR the release is suppressed, since a reader
+  may still be following those references — the same rule a relocated chunk's
+  old block follows.
+
+### Fixed
+
+- A vlen sequence of 4 GiB or more is now refused instead of silently
+  truncated. Every vlen write site cast the byte length to the 32-bit on-disk
+  length field with `as u32`, so the heap object stored all the bytes while
+  the reference recorded the wrapped length — reads returned the low-32-bits
+  prefix with no error. The conversion now has one owner (`vlen_seq_len`)
+  that errors.
+
 ## 0.4.0
 
 ### Changed

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "rust-hdf5"
 description = "Pure Rust HDF5 library with full read/write and SWMR support"
-version = "0.4.0"
+version = "0.4.1"
 edition = "2021"
 rust-version = "1.89"
 license = "MIT"