[ python-Bugs-678250 ] test_mmap failling on AIX

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Bugs item #678250, was opened at 2003-01-31 11:44
Message generated for change (Comment added) made by mdr0
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Category: Extension Modules
Group: None
Status: Open
Resolution: None
Priority: 5
Submitted By: Neal Norwitz (nnorwitz)
Assigned to: Nobody/Anonymous (nobody)
Summary: test_mmap failling on AIX

Initial Comment:
test_mmap is failing on a flush while trying to do:
  Copy-on-write memory map data not written correctly

The problem is that the mmap is opened with 
ACCESS_COPY.  This translates to MAP_PRIVATE.  
On AIX, the msync man page says:  "When the 
MS_SYNC and MAP_PRIVATE flags both are used, the 
msync subroutine returns an errno value of EINVAL."

I'm not sure what the correct fix should be.

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Comment By: Mark D. Roth (mdr0)
Date: 2004-03-10 11:14

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I'm running into this problem under both AIX 4.3.3 and 5.1.
 Is this something that's going to affect python if I put it
into production, or is it "safe" to ignore it?


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Comment By: Neal Norwitz (nnorwitz)
Date: 2004-01-07 16:06

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run the test with the -v flag:  ./python
./Lib/test/regrtest.py -v test_mmap

I think only everything should be fine up to copy-on-write
tests.  Here's what a good run looks like:

[neal at epoch c3]$ ./python ./Lib/test/regrtest.py -v test_mmap
test_mmap
<type 'mmap.mmap'>
  Position of foo: 1.0 pages
  Length of file: 2.0 pages
  Contents of byte 0: '\x00'
  Contents of first 3 bytes: '\x00\x00\x00'
                                                           
                    
  Modifying file's content...
  Contents of byte 0: '3'
  Contents of first 3 bytes: '3\x00\x00'
  Contents of second page: '\x00foobar\x00'
  Regex match on mmap (page start, length of match): 1.0 6
  Seek to zeroth byte
  Seek to 42nd byte
  Seek to last byte
  Try to seek to negative position...
  Try to seek beyond end of mmap...
  Try to seek to negative position...
  Attempting resize()
  Creating 10 byte test data file.
  Opening mmap with access=ACCESS_READ
  Ensuring that readonly mmap can't be slice assigned.
  Ensuring that readonly mmap can't be item assigned.
  Ensuring that readonly mmap can't be write() to.
  Ensuring that readonly mmap can't be write_byte() to.
  Ensuring that readonly mmap can't be resized.
  Opening mmap with size too big
  Opening mmap with access=ACCESS_WRITE
  Modifying write-through memory map.
  Opening mmap with access=ACCESS_COPY
  Modifying copy-on-write memory map.
  Ensuring copy-on-write maps cannot be resized.
  Ensuring invalid access parameter raises exception.
 Test passed
1 test OK.


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Comment By: Richard Wheeler (wheelrl)
Date: 2004-01-07 15:57

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I am getting the same error on AIX 5.2 with Python 2.3.3 
release.  What can I do to get around the test error and 
verify the mmap is working?

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Comment By: Tim Peters (tim_one)
Date: 2003-05-06 15:00

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Hmm.  I suspect the flush docs() are too strong (does flush 
really promise to materialize bytes *on disk*?  it doesn't for 
other Python file objects, you also need os.fsync() for that).

Your point is well taken, though, and whatever flush() does 
normally do, it's not going to do it for a copy-on-write mmap.  
So fine by me if we declare that attempting to flush() a copy-
on-write mmap raises an exception.

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Comment By: Martin v. Löwis (loewis)
Date: 2003-05-03 05:09

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The documentation for flush says

"Flushes changes made to the in-memory copy of a file back
to disk."

But it doesn't do that, and we all agree it shouldn't do
that. So I would claim that it is an error to use .flush on
an mmap object that was opened in ACCESS_COPY. 

This is like trying to write to a file that was opened for
reading only: one *could* declare that the write just does
nothing, but it helps the developer more if you get an
exception, because the code is likely wrong (i.e. not
following the likely intentions of the author).

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Comment By: Tim Peters (tim_one)
Date: 2003-04-28 14:55

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Sorry, I've had nothing to do with mmap beyond fixing bugs.  
The "access" feature was due to Jay Miller, although I believe 
I checked in his patch.

Martin, I don't understand why you think it's reasonable for 
flush to complain here:  the mmap is open for writing, so 
what's surprising about expecting to be able to flush after a 
write?  Simply that there's no associated file, due to copy-on-
write?  Then user code would have to be acutely aware of how 
an mmap'ed object was opened, just to avoid nuisance 
complaints when they flush after writing.

So that's a third alternative:  alter the implementation to make 
mmap.flush() do nothing when an mmap object was opened 
as copy-on-write.

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Comment By: Martin v. Löwis (loewis)
Date: 2003-03-04 01:00

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I think the test is somewhat bogus: It tries to check that
modification to an ACCESS_COPY doesn't modify the underlying
file, but assumes that .flush becomes a no-op, even though
an exception is more reasonable (IMO; errors should never
pass silently).

So I see two options: Declare that .flush() always raises an
exception (and modify implementations that don't produce an
exception accordingly), or declare that aspect to be
system-dependent, and modify the test (and the
documentation) to expect and ignore an exception.

Assigning to Tim, as he incorporated that feature into mmap.

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