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struct: fix infinite loop in grow_buffer() for small sizes - #471
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The growth step in grow_buffer() was `n += ((n >> 1) + 7u) & ~7u`
which adds 0 when new_size is small (<= 15), since (n>>1)+7 lands in
[3,14] and rounding down to an 8-byte boundary yields 0. The loop then
spins forever.
This is hit when a buffer is first grown with a zero-length size (which
allocates a chunk but leaves capacity at 0), and then a subsequent
put() needs to grow from that 0 capacity. e.g.:
b = struct.buffer()
b.put("*", "") # grow to 0 -> data != NULL, capacity stays 0
b.put("*", "x") # grow from 0 -> infinite loop
Use +8u (not +7u) so the growth step is always >= 8 bytes and the loop
always makes progress.
Fixes `ucode -lstruct -p 'b=struct.buffer(); b.put("*",""); b.put("*","x")'`
hanging.
Signed-off-by: Jo-Philipp Wich <jo@mein.io>
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The growth step in grow_buffer() was
n += ((n >> 1) + 7u) & ~7uwhich adds 0 when new_size is small (<= 15), since (n>>1)+7 lands in [3,14] and rounding down to an 8-byte boundary yields 0. The loop then spins forever.This is hit when a buffer is first grown with a zero-length size (which allocates a chunk but leaves capacity at 0), and then a subsequent put() needs to grow from that 0 capacity. e.g.:
b = struct.buffer()
b.put("", "") # grow to 0 -> data != NULL, capacity stays 0
b.put("", "x") # grow from 0 -> infinite loop
Use +8u (not +7u) so the growth step is always >= 8 bytes and the loop always makes progress.
Fixes
ucode -lstruct -p 'b=struct.buffer(); b.put("*",""); b.put("*","x")'hanging.