we've recently updated Imagemagick in NixOS from version 6.9.7-6 to
6.9.8-4, and this change causes the Ikiwiki test suite to fail in
t/img.t, like so:

	#   Failed test at t/img.t line 119.
	#          got: 'no image'
	#     expected: '10x10'

	#   Failed test at t/img.t line 129.
	#          got: 'no image'
	#     expected: '12x12'

	#   Failed test at t/img.t line 130.
	#          got: 'no image'
	#     expected: '16x2'

	#   Failed test at t/img.t line 134.
	#          got: 'no image'
	#     expected: '8x8'

	#   Failed test at t/img.t line 135.
	#          got: 'no image'
	#     expected: '4x4'

	#   Failed test at t/img.t line 136.
	#          got: 'no image'
	#     expected: '6x6'

	#   Failed test at t/img.t line 138.
	#          got: 'no image'
	#     expected: '11x11'

	#   Failed test at t/img.t line 139.
	#          got: 'no image'
	#     expected: '12x12'

	#   Failed test at t/img.t line 140.
	#          got: 'no image'
	#     expected: '13x13'
	# Looks like you failed 9 tests of 62.
	t/img.t ........................
	Dubious, test returned 9 (wstat 2304, 0x900)
	Failed 9/62 subtests

Is this is a known problem and is there maybe a fix for this issue?

> This was not a known bug before your report. It looks as though every
> time we use `Image::Magick->Read(":foo.png")`, which is (or was)
> ImageMagick's syntax for opening a file of unknown type without
> interpreting a prefix containing `:` as a special directive instead
> of part of the filename, it fails.
>
> Please try re-running the test with better diagnostics using
> [commit 4ace7dbb7](http://source.ikiwiki.branchable.com/?p=source.git;a=commitdiff;h=4ace7dbb7)
> and report what it says. --[[smcv]]

>> I see the same issue on Fedora, with ImageMagic 6.9.9-19:
>> 
>>     #   Failed test at t/img.t line 119.
>>     #          got: 'no image: Exception 435: unable to open image `:t/tmp/out/imgconversions/10x-redsquare.png': No such file or directory @ error/blob.c/OpenBlob/2701'
>>     #     expected: '10x10'
>>     [...]
>> 
>> So it seems, that an empty coder prefix is not accepted anymore. To me it seems that [this commit](https://github.com/ImageMagick/ImageMagick/commit/4bc9b6b) changed the behavior. Unfortunately, the commit message doens't tell us about the reasons behind. The commit is included from version 6.9.8-3 on.

>>> This should now be [[fixed|done]] in git and in the next release.
>>> The test failure does not indicate a loss of functionality, unless
>>> you are using uncommon image formats enabled with
>>> `img_allowed_formats: [everything]`, which is a potential security
>>> vulnerability because it exposes the attack surface of all
>>> ImageMagick decoder modules. --[[smcv]]
