Strange behavior of prepend! with subarrays

[bkamins]

The following code produces unexpected result:

x = [1, 2, 3]
y = sub(x, :)
prepend!(x, y)

and first three entries of x are garbage in result. The behavior is inconsistent with the following code:

x = [1, 2, 3]
y = x
prepend!(x, y)

The reason is that prepend! checks if a===items and otherwise assumes that items is unchanged when array a is grown at the beginning.

[TotalVerb]

I don't know how easy it would be to completely fix this. Memory can be aliased in a lot of different ways.

[bkamins]

But now we have undefined behavior of prepend! when memory is aliased (as in two examples above). The simple fix I see is to copy items before resizing of a but this looses performance.

[timholy]

I haven't spent (and ATM can't spend) any real time thinking about this, but it occurs that operations like push! and prepend! should perhaps throw an error on a view---views are not resizable, since the container is immutable.

[TotalVerb]

Those operations already do throw errors on views, since they're not defined for them. The issue here is prepending to an array the elements of some view that is based on that array itself.

[tkelman]

should be able to write a band aid SubArray specialized method that checks if the parent array is the one being prepended to, and does a copy if so?

[martinholters]

I guess there are many more possibilities for hidden aliasing. An easy one:

a=collect(1:10);
a[2:6]=sub(a,1:5);
all(a[1:6].==1) # gives true

Similarly, other view-of-another-array types would be affected by the same problem. If it wasn't for the special treatment of Arrays by reshape (using internal data sharing), ReshapedArray would also be a hot candidate. My fear is that without a good strategy, we will need lots of band aid. For the prepend! case, an additional check a===parent(items) would do; maybe this could similarly be included in other functions that mutate an Array based on another AbstractArray.

EDIT: Not a case likely to occur in practice, but

a=collect(1:10);
prepend!(a, reshape(reshape(sub(a, 1:4), 2, 2), 4))

nicely exposes the problem with a ReshapedArray as items, so specializing on SubArray would not be sufficient.

[laborg]

Ref: #50824