Release 4.8.1

Changes from 4.8.0 to 4.8.1

Improvements

• Read-only memory mapping for CTable stores: CTable.open() (and
  FileTableStorage) gain an mmap_mode="r" parameter, mirroring
  blosc2.open(). All members of a read-only store — scalar, list, varlen
  and dictionary columns alike — are then read from mapped pages; for .b2z
  archives, in place at their offsets inside the single mapped container
  file. With several concurrent readers on one file this pays off quickly:
  2.5x/4.4x/4.5x faster wall time for 1/4/8 readers in our benchmark
  (bench/optim_tips/tip_10_mmap_many_readers.py).
• Reduced memory consumption in CTable.extend() when passed an NDArray.

Bug fixes

• Fixed lifetime/use-after-free hazards around zero-copy cframes and
  vlmeta: schunk_from_cframe()/ndarray_from_cframe() with copy=False
  (the default) returned objects pointing into the caller's bytes buffer
  without keeping it alive, so a temporary cframe (e.g.
  ndarray_from_cframe(response.content)) could be reclaimed under the live
  object, corrupting reads. The buffer is now pinned on the returned object.
  Also, vlmeta read paths (__getitem__/__len__/__iter__) now raise
  ReferenceError on an orphaned owner instead of segfaulting, matching the
  write paths.
• BatchArray.delete() / ObjectArray.delete(): negative-step slices
  (e.g. del arr[3:0:-1]) deleted chunks in ascending order, shifting the
  indices of chunks still to be deleted and removing the wrong ones (or
  raising RuntimeError).
• ListArray.extend_arrow(): Arrow chunks were appended to the backend
  without flushing pending cells first, reordering unflushed rows after the
  new ones.
• Shape inference: stack() with a negative axis inserted the new dimension
  one position too early, so a lazyexpr's reported .shape disagreed with
  its computed result; vecdot() also normalized positive axes as if they
  were negative.
• Chunked matmul(): broadcast (size-1) operand batch dims were sliced with
  the result-chunk coordinates, producing empty slices when the broadcast
  dim spans several result chunks.
• DictStore.__setitem__(): overwrite semantics depended on value size —
  embedded keys refused overwrite ("already exists"), while an
  embedded-to-external overwrite double-stored the key and resurrected the
  stale embedded value after a delete. Assignments now behave uniformly
  dict-like, dropping any previous value.
• Explicitly passing cparams=None or dparams=None to NDArray
  constructors crashed; both now mean "defaults".
• Fixed a builtin-shadowing bug that made store detection in
  blosc2.open() recurse ~250 times (silently swallowed) on every
  .b2z/.b2d open; opening a .b2z is now ~10x faster under allocation
  tracing.

Documentation

• Restructured docs (#674), with a new
  Optimization tips
  section, including new tips on grouping related data into a single
  memory-mapped .b2z file and on using mmap_mode="r" with many
  concurrent readers.