Does it work the same for mmap(-1, size, trackfd=True)? I have not found tests for this.

trackfd=True is the default. It's tested in test_resize_past_pos.

It has different behavior. Should trackfd=False have any effect for fd=-1? Should combination fd=-1 and trackfd=False be allowed?

Conceptually, trackfd=False should be the only possible behaviour for fd=-1 -- the fd is not tracked.
However, I don't think it's worth it to include a third value for “trackfd=default”. As it is, the default (True) works as before. Setting False has the documented effect -- disabling resize. This isn't very useful, but I don't think it's worth handling specially.

I'd love to have some idea of why I might want to use this parameter. Right now it only describes the downsides.

On Windows, the internally duplicated handle probably references an open that lacks delete access. It thus prevents deleting the file, even if the mapped section otherwise allows it (e.g. the section is mapped readonly). For example:

>>> f = open('spam.txt')
>>> m = mmap.mmap(f.fileno(), 0, access=mmap.ACCESS_READ)
>>> f.close()
>>> os.remove('spam.txt')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
PermissionError: [WinError 32] The process cannot access the file because it is being used by another process: 'spam.txt'

>>> # I manually closed the internal handle via Process Explorer.
>>> os.remove('spam.txt')
>>> m[:]
b'spam'

That sounds like a reason to at least add the argument for all platforms, which I'm generally in favour of anyway. It can have more appropriate semantics on Windows if needed (i.e. "doesn't hold an extra HANDLE" rather than "FD").

It's probably actually pretty useful to be able to immediately delete the file but keep the mapping open (which will keep the file on disk on Windows at least, so you can't reuse the name while it's in use). And it looks like the mapping doesn't lock out deletes, so I guess it'll work as intended.

I'm not going to hold up this PR for it though. All I'll say is that if we ever do add that option, it should be trackfd=False to "activate" it, for consistency between platforms.

That sounds like a reason to at least add the argument for all platforms, which I'm generally in favour of anyway. It can have more appropriate semantics on Windows if needed (i.e. "doesn't hold an extra HANDLE" rather than "FD").

I think trackfd would be fine on Windows. The fileno parameter is a C file descriptor, not a native OS handle.

It's probably actually pretty useful to be able to immediately delete the file but keep the mapping open (which will keep the file on disk on Windows at least, so you can't reuse the name while it's in use). And it looks like the mapping doesn't lock out deletes, so I guess it'll work as intended.

NTFS supports POSIX delete, in which a deleted file gets renamed to a reserved system directory until all references to the file object have been closed. That includes the internal pointer reference to a file object that's held by the memory manager for the mapped section. The internal file reference doesn't count toward the file's share mode, i.e. a memory-mapped file can be deleted even if the source open didn't share delete access. Actually, I just checked that the delete is allowed nowadays even if the mapped section has write access to the file, so my assumption was wrong that it would only work for a readonly mapping.

You can observe this in Process Explorer. Switch the lower-pane view to DLLs (file- and pagefile-backed memory mappings), and add the name and path columns to the view. You'll see that the backing file gets moved to the "\$Extend\$Deleted" system directory on the volume after the file is 'deleted'.