GeezerVison

Summary

Fixes #160309

codecs.StreamReader.read() silently drops bytes when the stream ends with an incomplete multibyte sequence. The read loop exits on empty
ewdata while �ytebuffer still holds the trailing bytes, and those bytes are never decoded.

Bug demonstration (before fix)

`python

import codecs, io
reader = codecs.getreader('utf-8')(io.BytesIO(b'ab\xe2'))
reader.read()
'ab' # \xe2 is silently dropped — no error raised
`

Every other decoding path raises or applies the error handler:

`python

b'ab\xe2'.decode('utf-8')
UnicodeDecodeError: 'utf-8' codec can't decode byte 0xe2 in position 2: unexpected end of data
codecs.getreader('utf-8')(io.BytesIO(b'ab\xe2'), 'replace').read()
'ab' # should be 'ab\ufffd' — replacement character is also silently dropped
`

Fix

When the underlying stream is exhausted and �ytebuffer is non-empty, decode the leftover bytes immediately before breaking out of the loop. The codec's stateless decode function applies inal=True semantics, so:

• errors='strict' → raises UnicodeDecodeError (correct)
• errors='replace' → emits the replacement character \ufffd (correct)
• errors='ignore' → drops the bytes (correct)

This matches the behaviour of �ytes.decode() and io.TextIOWrapper.

• Issue: codecs.StreamReader.read() silently drops an incomplete character at the end of the stream #160309