[Python-bugs-list] [Bug #128210] Threads using same stream blow up (Windows)
noreply@sourceforge.net
noreply@sourceforge.net
Thu, 11 Jan 2001 19:48:19 -0800
Bug #128210, was updated on 2001-Jan-09 13:10
Here is a current snapshot of the bug.
Project: Python
Category: Windows
Status: Closed
Resolution: Wont Fix
Bug Group: 3rd Party
Priority: 1
Submitted by: tim_one
Assigned to : tim_one
Summary: Threads using same stream blow up (Windows)
Details: Blows up under released Windows 2.0 and CVS Pythons (so it's not
due to anything new):
import thread
def read(f):
import time
time.sleep(.01)
n = 0
while n < 1000000:
x = f.readline()
n += len(x)
print "r",
print "read " + `n`
m.release()
m = thread.allocate_lock()
f = open("ga", "w+")
print "opened"
m.acquire()
thread.start_new_thread(read, (f,))
n = 0
x = "x" * 113 + "\n"
while n < 1000000:
f.write(x)
print "w",
n += len(x)
m.acquire()
print "done"
Typical run:
C:\Python20>\code\python\dist\src\pcbuild\python temp.py
opened
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w
w w w w w w w w w w w w w w w w w w w w w w w w w w w w w w r w r
w r w r w r w r w r w r w r w r w r w r w r w r w r w r w r w r w
r r w r w r w r w r w r
and then it dies in msvcrt.dll with a bad pointer. Also dies under the
debugger (yay!) ... always dies like so:
+ We (Python) call the MS fwrite, from fileobject.c file_write.
+ MS fwrite succeeds with its _lock_str(stream) call.
+ MS fwrite then calls MS _fwrite_lk.
+ MS _fwrite_lk calls memcpy, which blows up for a non-obvious reason.
Looks like the stream's _cnt member has gone mildly negative, which
_fwrite_lk casts to unsigned and so treats like a giant positive count, and
so memcpy eventually runs off the end of the process address space.
Only thing I can conclude from this is that MS's internal stream-locking
implementation is buggy. At least on W98SE. Other flavors of Windows?
Other platforms?
Note that I don't claim the program above is *sensible*, just that it
shouldn't blow up. Alas, short of adding a separate mutex in Python file
objects-- or writing our own stdio --I don't believe I can fix this.
Follow-Ups:
Date: 2001-Jan-11 19:48
By: tim_one
Comment:
Since this appears to be unique to Windows, appears to be a bug in Windows
stdio, has got to be exceedingly rare in practice, and unreasonably
difficult to worm around in the Python implementation, closing this as
WontFix.
-------------------------------------------------------
Date: 2001-Jan-11 06:59
By: gvanrossum
Comment:
I knew it wasn't sensible (I did read what you wrote). I just don't like
having bug reports that we can't do something about without a lot of
hackery. I've seen stdio implementations before that were totally naive
about mixing reads and writes, even in a single-threaded program.
That said, I can't get this to crash on Linux.
-------------------------------------------------------
Date: 2001-Jan-10 10:33
By: tim_one
Comment:
Guido, I've said multiple times (and in this very bug report!) that the
program isn't sensible. That isn't the point. Having multiple threads
even just reading from the same stream without explicit user locks isn't
sensible either, and you can't have it both ways: on Python-Dev you said
the point to using FLOCKFILE in get_line was to prevent core dumps in case
the user was doing multiple reads by *accident*.
That's fine: I agree, and a file open for update is likely just as prone
to mixed read/write "accidents".
As far as the std goes, input following output should have intervening
fflush, fseek, fsetpos or rewind; while output following input should--
unless input hit EOF --have intervening fseek, fsetpos or rewind (not
fflush, though! fflush immediately after input is undefined).
Adding flags to the file object doesn't seem to help much, because the
combo of e.g. "I'm doing a read now" and "I'm setting the 'reading' flag"
needs to be atomic in a threaded world else it's unreliable (and then
you're back to Python-level locks to repair that).
Does this blow up on Linux for you? That's more interesting to me. I've
implemented stdio in the past, and I think what's happening here is quite
likely what it looks like: a bug in MS's file-locking (the _cnt member
just doesn't go negative if the FILE* struct is properly locked during
manipulations!). If that's true, this is surely rare enough that we can
wait for MS to fix it. But in the meantime, it is a way to get Python to
coredump on Windows.
-------------------------------------------------------
Date: 2001-Jan-10 06:48
By: gvanrossum
Comment:
Mixing reads and writes on the same stream is always a bad idea. I believe
that the stdio docs prescribe that you must use fflush() whenever switching
between reading and writing. We could enforce this by adding two flags,
"reading" and "writing" to the file object.
(It's possible that using fseek() is also allowed to change directions.)
I'm not sure tht this is worth fixing though -- no sensible programmer will
do this.
-------------------------------------------------------
Date: 2001-Jan-10 00:37
By: tim_one
Comment:
Here's a standalone C program with the same symptom:
#include <process.h>
#include <stdio.h>
static FILE* fp;
void
reader(void* irrelevant)
{
char buf[100];
for (;;) {
char* p = fgets(buf, sizeof buf, fp);
if (p) {
putchar('r');
}
}
}
void
main()
{
int i;
char string[100];
for (i = 0; i < sizeof(string) - 1; ++i) {
string[i] = 'x';
}
string[sizeof(string) - 1] = '\n';
fp = fopen("whatever", "w+");
_beginthread(reader, 0, NULL);
for (;;) {
fwrite(string, 1, sizeof(string), fp);
putchar('w');
}
}
-------------------------------------------------------
Date: 2001-Jan-09 14:41
By: tim_one
Comment:
Adding info communicated via email:
+ Also blows up under Windows 2000.
+ Yes, C doesn't define what happens here even if it weren't threaded (you
can't mix reads and writes willy-nilly (without e.g. seeking before
reading) and expect something sensible). The program is not sensible. But
even silly programs shouldn't *blow up* in Python.
-------------------------------------------------------
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http://sourceforge.net/bugs/?func=detailbug&bug_id=128210&group_id=5470