[issue46858] mmap constructor resets the file pointer on Windows

[Eryk Sun]

Eryk Sun  added the comment:

The resize() method also modifies the file pointer. Instead of fixing that 
oversight, I think it should directly set the file's FileEndOfFileInfo and 
FileAllocationInfo. For example:

// resize the file
if (!SetFileInformationByHandle(
self->file_handle, FileEndOfFileInfo,
&max_size, sizeof(max_size)) ||
  !SetFileInformationByHandle(
self->file_handle, FileAllocationInfo,
&max_size, sizeof(max_size)))
{
// resizing failed. try to remap the file
file_resize_error = GetLastError();
max_size.QuadPart = self->size;
new_size = self->size;
}

This is cheaper in terms of system calls. The existing implementation makes 
four system calls: one to set the file pointer in SetFilePointerEx() and three 
in SetEndOfFile(), which queries the file pointer, sets the end-of-file info, 
and sets the allocation info. 

Note that this approach doesn't modify the file pointer in any case. This may 
be surprising if the file size shrinks to less than the existing file pointer. 
But os.ftruncate() behaves the same way, as does the resize() method in Linux.

--
nosy: +eryksun
versions:  -Python 3.7, Python 3.8

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[issue46858] mmap constructor resets the file pointer on Windows

[benrg]

New submission from benrg :

On Windows, `mmap.mmap(f.fileno(), ...)` has the undocumented side effect of 
setting f's file pointer to 0.

The responsible code in mmapmodule is this:

/* Win9x appears to need us seeked to zero */
lseek(fileno, 0, SEEK_SET);

Win9x is no longer supported, and I'm quite sure that NT doesn't have whatever 
problem they were trying to fix. I think this code should be deleted, and a 
regression test added to verify that mmap leaves the file pointer alone on all 
platforms.

(mmap also maintains its own file pointer, the `pos` field of `mmap_object`, 
which is initially set to zero. This issue is about the kernel file pointer, 
not mmap's pointer.)

--
components: IO, Library (Lib), Windows
messages: 414039
nosy: benrg, paul.moore, steve.dower, tim.golden, zach.ware
priority: normal
severity: normal
status: open
title: mmap constructor resets the file pointer on Windows
type: behavior
versions: Python 3.10, Python 3.11, Python 3.7, Python 3.8, Python 3.9

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