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Title: mmap.flush does not check for errors on windows
Type: behavior Stage: resolved
Components: Library (Lib) Versions: Python 3.8
Status: closed Resolution: fixed
Dependencies: Superseder:
Assigned To: Nosy List: berker.peksag, ishimoto, ocean-city, paul.moore, pitrou, schmir, serhiy.storchaka, steve.dower, tim.golden, zach.ware
Priority: normal Keywords: patch

Created on 2008-02-15 13:20 by schmir, last changed 2022-04-11 14:56 by admin. This issue is now closed.

File name Uploaded Description Edit
mmap-flush.txt schmir, 2008-03-10 08:19 make flush return None
Pull Requests
URL Status Linked Edit
PR 8692 merged berker.peksag, 2018-08-06 18:55
Messages (11)
msg62427 - (view) Author: Ralf Schmitt (schmir) Date: 2008-02-15 13:20
mmap.flush returns the result of the call to FlushViewOfFile as an
integer, and does not check for errors. On unix it does check for
errors. The function should return None and raise an exception if an
error occurs...
This bug can lead to data loss...

Here's the current version of that function:

static PyObject *
mmap_flush_method(mmap_object *self, PyObject *args)
	Py_ssize_t offset = 0;
	Py_ssize_t size = self->size;
	if (!PyArg_ParseTuple(args, "|nn:flush", &offset, &size))
		return NULL;
	if ((size_t)(offset + size) > self->size) {
		PyErr_SetString(PyExc_ValueError, "flush values out of range");
		return NULL;
	return PyInt_FromLong((long) FlushViewOfFile(self->data+offset, size));
#elif defined(UNIX)
	/* XXX semantics of return value? */
	/* XXX flags for msync? */
	if (-1 == msync(self->data + offset, size, MS_SYNC)) {
		return NULL;
	return PyInt_FromLong(0);
	PyErr_SetString(PyExc_ValueError, "flush not supported on this system");
	return NULL;
msg63436 - (view) Author: Ralf Schmitt (schmir) Date: 2008-03-10 08:19
attached is a patch. I hope it is ok to change the semantics a bit. 
They were very strange:
- on unix it raises an exception on errors. otherwise it always returns 0.
- on windows it returns non-zero on success, and returns zero on errors.

Now, flush returns None or raises an Exception.
msg65549 - (view) Author: Ralf Schmitt (schmir) Date: 2008-04-16 13:21
now this insanity is even documented with svn revision 62359
msg65550 - (view) Author: Antoine Pitrou (pitrou) * (Python committer) Date: 2008-04-16 16:02
Perhaps it would be better if the patch came with unit tests. I agree
that the situation right now is insane.

PS : I have no power to commit or even accept a patch :)
msg65551 - (view) Author: Ralf Schmitt (schmir) Date: 2008-04-16 16:26
I thought about this too, but I don't know of a way to provoke an error.
(Other than passing in illegal values, but the code tries to catch
those. And btw, raises an Exception on windows :)

One could currently pass in a negative value for size (this isn't caught
in the checks). e.g.

m.flush(500, -500) gives an 'error: [Errno 22] Invalid argument' on linux.

but then you don't want to rely on another bug for testing purposes.
msg138227 - (view) Author: Hirokazu Yamamoto (ocean-city) * (Python committer) Date: 2011-06-13 03:00
This issue seems to be reproduced in following way.

1. Attach USB flash drive. (On my machine, it was attached as E drive)

2. Run python interactive shell and run following commands.
   (Confirmed on Python2.6)

  > import mmap
  > f = open("e:/temp.tmp", "w+b")
  > f.write("foo")
  > f.flush()
  > m = mmap.mmap(f.fileno(), 3)
  > m[:] = "xxx"

3. Detach USB flash drive violently here! (Without safety mechanism
   to detach USB flash drive, just plug it off) Note that
   python shell is still running.

4. Enter following command in python shell.
  > m.flush()
   It returns *0*. (Means failure)
msg242715 - (view) Author: Mark Lawrence (BreamoreBoy) * Date: 2015-05-07 14:37
I think we should be properly handling errors.  If people agree I'll provide a new patch to cover code and doc changes, but I've no idea how to provide any form of unit test for the change.
msg323563 - (view) Author: Serhiy Storchaka (serhiy.storchaka) * (Python committer) Date: 2018-08-15 11:03
If we break compatibility in any case, what if return None instead of 0? The function always returning 0 looks strange. The function always returning None is common.
msg323571 - (view) Author: Steve Dower (steve.dower) * (Python committer) Date: 2018-08-15 15:40
I agree. We should definitely document it as a change, but I don't think there's anything useful you can do in the case of a flush failing anyway, so it seems unlikely that anyone could depend on the return value.

Returning None for success and exception on error sounds like a good change. Technically this is no longer returning zero, as previously documented, so if anyone wants to get *really* pedantic, we're not returning the "failed" result for success ;)
msg323898 - (view) Author: Berker Peksag (berker.peksag) * (Python committer) Date: 2018-08-22 18:21
New changeset e7d4b2f205c711d056bea73a0093e2e2b200544b by Berker Peksag in branch 'master':
bpo-2122: Make mmap.flush() behave same on all platforms (GH-8692)
msg323899 - (view) Author: Berker Peksag (berker.peksag) * (Python committer) Date: 2018-08-22 18:22
Thanks for the suggestions and for the reviews!
Date User Action Args
2022-04-11 14:56:30adminsetgithub: 46375
2018-08-22 18:22:51berker.peksagsetstatus: open -> closed
priority: high -> normal
messages: + msg323899

assignee: brian.curtin ->
resolution: fixed
stage: patch review -> resolved
2018-08-22 18:21:16berker.peksagsetnosy: + berker.peksag
messages: + msg323898
2018-08-15 15:40:45steve.dowersetmessages: + msg323571
2018-08-15 11:03:44serhiy.storchakasetnosy: + serhiy.storchaka

messages: + msg323563
versions: + Python 3.8, - Python 2.7, Python 3.4, Python 3.5
2018-08-06 21:05:29BreamoreBoysetnosy: - BreamoreBoy
2018-08-06 18:55:51berker.peksagsetpull_requests: + pull_request8187
2015-05-07 14:45:54brian.curtinsetnosy: - brian.curtin
2015-05-07 14:37:18BreamoreBoysetnosy: + paul.moore, BreamoreBoy, zach.ware, steve.dower

messages: + msg242715
versions: + Python 3.4, Python 3.5, - Python 3.2, Python 3.3
2012-07-25 03:18:45ishimotosetnosy: + ishimoto
2011-06-13 11:17:41pitrousetstage: test needed -> patch review
2011-06-13 03:00:19ocean-citysetmessages: + msg138227
2011-06-12 18:50:10terry.reedysetversions: + Python 3.3, - Python 3.1
2010-09-03 23:48:03brian.curtinsetassignee: brian.curtin
2010-09-03 23:23:15pitrousetnosy: + tim.golden, brian.curtin

versions: - Python 2.6
2010-05-11 20:56:40terry.reedysetversions: + Python 2.7, Python 3.2
2009-05-16 19:42:55pitrousetnosy: + ocean-city
2009-05-16 19:37:56ajaksu2setpriority: high
stage: test needed
versions: + Python 3.1, - Python 2.5, Python 3.0
2008-04-16 16:26:17schmirsetmessages: + msg65551
2008-04-16 16:02:58pitrousetnosy: + pitrou
messages: + msg65550
2008-04-16 13:21:02schmirsetmessages: + msg65549
2008-04-15 05:50:28loewissetkeywords: + patch
2008-03-10 08:19:23schmirsetfiles: + mmap-flush.txt
messages: + msg63436
2008-02-15 13:20:02schmircreate