Seek and read from closed MemoryIO

[braingram]

ASDF will read from a 'closed' generic_io.MemoryIO (like what is created when reading from an io.BytesIO instance).

Running the following:

import io
import asdf
import numpy

b = io.BytesIO()
arrs = [numpy.arange(10) + i for i in range(10)]
af = asdf.AsdfFile({'arrs': arrs})
af.write_to(b)

b.seek(0)
with asdf.open(b) as raf:
    arefs = raf['arrs']

print(b.tell())
print(arefs[1][:])
print(b.tell())

produces:

2902
[ 1  2  3  4  5  6  7  8  9 10]
1830

showing that the array loading from the 'closed' MemoryIO resulting in a change in the file pointer position.

[braingram]

This also affects opening and reading from regular files:

import asdf
import numpy

fn = 'test.asdf'
arrs = [numpy.arange(10) + i for i in range(10)]
af = asdf.AsdfFile({'arrs': arrs})
af.write_to(fn)

fp = open(fn, 'rb')
with asdf.open(fp) as raf:
    arefs = raf['arrs']

print(fp.tell())
print(arefs[1][:])
print(fp.tell())

2902
[ 1  2  3  4  5  6  7  8  9 10]
1830

[braingram]

Fixed in #1537 both examples above now no longer seek the file pointer and raise an exception:

OSError: Attempt to load block from closed file