Image fails to open file after seek-read-seek

[mmiermans]

The combination of doing a seek and read (in this order) causes MagickReadImageFile to consistently return 0. The following code requires that there is an image wand.png. It has three test cases: the first two run successfully and the third one fails after doing a seek-read-seek.

from wand.image import Image

fn = 'wand.png'

# This works as expected
fp = open(fn)
try:
  fp.read(1)
  fp.seek(0)
  with Image(file=fp) as img:
    print '%s could be opened with read-seek' % img.format
finally:
  fp.close()

# This is a workaround that fixes the issue, but I have no idea why a flush 
# would change a file that has not been written to. 
fp = open(fn)
try:
  fp.seek(0)
  fp.read(1)
  fp.flush()
  fp.seek(0)
  with Image(file=fp) as img:
    print '%s could be opened with seek-read-flush-seek' % img.format
finally:
  fp.close()

# This fails
fp = open(fn)
try:
  fp.seek(0)
  fp.read(1)
  fp.seek(0)
  with Image(file=fp) as img:
    print '%s could be opened with seek-read-seek' % img.format
finally:
  fp.close()

Here is the output that I get:

PNG could be opened with read-seek
PNG could be opened with seek-read-flush-seek
Traceback (most recent call last):
  File "./test.py", line 34, in <module>
    with Image(file=fp) as img:
  File "/usr/local/lib/python2.7/dist-packages/wand/image.py", line 1979, in __init__
    self.read(file=file, resolution=resolution)
  File "/usr/local/lib/python2.7/dist-packages/wand/image.py", line 2048, in read
    self.raise_exception()
  File "/usr/local/lib/python2.7/dist-packages/wand/resource.py", line 222, in raise_exception
    raise e
wand.exceptions.MissingDelegateError: no decode delegate for this image format `/tmp/magick-ZEtTP5H0' @ error/constitute.c/ReadImage/544

I've traced the problem back to the call to MagickReadImageFile returning 0 in wand/image.py:2029. Different images and a new virtualenv made no change. I am on the latest stable version of Wand (0.3.9) and on:

>>> print(sys.version)
2.7.8 (default, Oct 20 2014, 15:05:19) 
[GCC 4.9.1]

At the moment I am just really curious what is going on internally. Thanks for doing an awesome job!

---

Edit: My guess is that the problem originates from magick_wand.c:6837.

[dahlia]

I just noticed that it is not reproducible since Python 3.2. It seems probably because builtin open() became an alias of io.open() since Python 3.

[dahlia]

Also the workaround using flush() doesn’t work for me.

[emcconville]

This is an oddity from Python 2. Be sure to open the file in binary mode with open(fn, 'rb') or even open(fn, 'r+b').