GeezerVison

…ecompression

Frames produced by the one-shot compression APIs record the decompressed
size in the frame header.  When the input starts at such a frame header,
read that size with ZSTD_getFrameContentSize() and allocate the output
buffer at its exact size, instead of growing it progressively and
shrinking it on finish.  An exactly filled buffer is then returned
without a copy.

This is only a sizing hint: decompression does not rely on it, so a
hand-crafted header recording a wrong size produces exactly the same
results and exceptions as before.  Two guards bound the allocation: the
recorded size is trusted only up to 1 GiB, and recorded sizes claiming
more than a 32768x expansion of the available input -- more than the
format can produce -- are ignored.

Co-Authored-By: Claude Opus 5 <[email protected]>

On top of the pre-allocation in the previous commit:

- Pre-size only when the first input of a frame holds the whole zstd1
  frame, and check the recorded size against that frame's compressed
  size instead of all of the input, so that neither a frame fed in
  pieces nor the data following a frame leads to a pre-allocation.
  The 1 GiB limit stays.
- Check the frame's window size against window_log_max before
  pre-sizing: libzstd's single-pass mode, which a pre-sized output
  enables, doesn't check it.
- zstd.decompress() feeds each frame whole, sized with get_frame_size(),
  so every frame of multi-frame input is pre-sized, not only the first.

Co-authored-by: Maik Schulze <[email protected]>
Co-Authored-By: Claude Opus 5.5 <[email protected]>

… feeding

test_output_allocated_once_each_frame passed even without the
whole-frame feeding in zstd.decompress(): its 2 MiB frame compresses to
459 bytes, so it fits whole into the 1 KiB that _decompress_all() feeds
a later frame first.  Use 2 MiB of zeros followed by 4 KiB of random
bytes instead: the frame compresses to more than 1 KiB, and most of its
output is decoded from its first KiB, which grows the output buffer
unless the frame is fed whole.

Co-Authored-By: Claude Opus 5.5 <[email protected]>