Eryk Sun added the comment:
In issue 20844 I suggested opening the file in binary mode, i.e. change the
call to _Py_wfopen(filename, L"rb") in Modules/main.c. That would also entail
documenting that PyRun_SimpleFileExFlags requires a FILE pointer that's opened
in binary mode. After making this change, there's no problem parsing
"encoding-problem-cp1252.py":
>python --version
Python 3.6.0a4+
>python encoding-problem-cp1252.py
ok
When fp_setreadl is called while parsing "encoding-problem-cp1252.py", 47 bytes
in the FILE buffer have been read -- up to the end of the coding spec. Let's
verify this in the debugger:
0:000> bp python35_d!fp_setreadl
0:000> g
Breakpoint 0 hit
python35_d!fp_setreadl:
00000000`662bee00 4889542410 mov qword ptr [rsp+10h],rdx
ss:000000d7`6cfeead8=000000d76cfeeaf8
0:000> ;as /x fp @@(((python35_d!tok_state *)@rcx)->fp)
0:000> ;as /x ptr @@(((ucrtbased!__crt_stdio_stream_data *)${fp})->_ptr)
0:000> ;as /x base @@(((ucrtbased!__crt_stdio_stream_data *)${fp})->_base)
0:000> ?? ${ptr} - ${base}
int64 0n47
ftell() should return 47, but instead it returns -1. You can see this by
opening the file in Python 2 on Windows, which uses FILE streams:
>>> f = open('encoding-problem-cp1252.py')
>>> f.read(47)
'#!/usr/bin/env python\n# -*- coding: cp1252 -*-\n'
>>> f.tell()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 0] Error
ftell starts by getting the file position from the OS and then subtracts the
unread bytes in the buffer. The buffer has already undergone CRLF => LF
translation, so ftell makes an assumption that the file uses CRLF line endings
and thus subtracts 2 bytes for each unread LF. In this case the buffer happens
to have 48 unread LFs, so ftell returns -1, with the only actual error being a
fundamentally flawed design in the CRT's text mode.
----------
nosy: +eryksun
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue27797>
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