Stop RebufferedBytesIO from hanging at EOF (fixes #988) - #1140

Problem

RebufferedBytesIO.read (and write) poll the underlying substream in a loop. They treated two distinct outcomes identically:

• None — a non-blocking stream that has no data yet
• b"" — the stream has reached end of file

Both hit if not newdata: sleep(0); continue, so a substream that signals EOF by returning empty bytes caused construct to spin forever instead of stopping. This is the hang reported in #988.

from construct.lib import RebufferedBytesIO
import io

rb = RebufferedBytesIO(io.BytesIO(b"abc"))
rb.read(3)   # b"abc"
rb.read(1)   # hangs forever before this fix

Fix

Distinguish the two cases:

• newdata is None → still keep polling (sleep(0); continue), preserving non-blocking behaviour.
• empty bytes → break out of the loop.

After breaking, read falls through to its existing could not read enough bytes IOError, and write stops trying to backfill. The same fix is applied to both methods (the write loop had the same pattern and could also spin).

Tests

Added test_rebuffered_reading_past_eof which asserts an over-read now raises IOError rather than hanging. Existing test_bitstream.py tests still pass.