[SciPy-dev] Problem in linalg functions with overwrite check

[Travis Oliphant]

I've found a potential problem in the linear algebra functions related
to the overwrite_a parameter.

Many routines alter the overwrite_a parameter after conversion of the
input object to an array.  The check is not very robust however when the
input is an object that uses an array as it's main core (like a Matrix
object).

For example. Code looks something like this

a1 = asarray(a)   # Convert a to an array object

overwrite_a = overwrite_a or (a1 is not a)

So, if a was not already an array then overwrite_a is modified to
overwrite the a1 array (which is presumed to be a copy anyway).   

The problem with this strategy is that if a is an object that has an
__array__ method.  The asarray function will call it to obtain the
object a as an array.  Often, this call does not return a new copy of
the data but just a reference to the array forming the underlying
storage of a.  So, while a and a1 are different objects and so a1 is not
a, they use the same storage space, so overwriting a1 changes the object
a leading to strange, unexpected behavior.

I got caught while passing Matrix objects to the linalg.det routine and
getting different answers everytime the function ran (because the
underlying matrix was being changed everytime).  

I'm asking for suggestions on how to fix this.  

We could for example, try to call the __array__ method on our own and
then if it succeeds never alter the overwrite_a parameter.  Or more
elegantly perhaps we could try to write some function
share_storage(a1,a) --- maybe partly in C  that tries to determine
whether or not a1 and a actually share the same storage.

Comments welcome. 

-Travis Oliphant