Parse out Ghostscript warnings in identfication

[ageacademia]

This bug is partly identified by this Ghostscript thread, which details that GS may additionally output warning messages when identify is run on the associated file. This results in parsing errors, as the output is now multi-line and the file's attributes are read.

The following occurs when MiniMagick attempts to access those attributes:

# using the attached file from the original Ghostscript thread
wget https://bugs.ghostscript.com/attachment.cgi?id=18348 -O attachment.pdf

1:(main)> t = MiniMagick::Image.open("attachment.pdf")
=> #<MiniMagick::Image:0x000000000c63b918 ... >
2:(main)> t.dimensions
MiniMagick::Invalid: image data can't be read
from <omitted>/mini_magick.rb:37:in `rescue in cheap_info'
Caused by ArgumentError: invalid value for Integer(): "Error:"
from <omitted>/mini_magick.rb:27:in `Integer'
3:(main)> t.info("raw: %m %w %h %b")
=> "   **** Error: stream operator isn't terminated by valid EOL.\n               Output may be incorrect.\n   **** Error: stream operator isn't terminated by valid EOL.\n               Output may be incorrect.\nPDF 596 842 63214B"

The fix is to identify if the warning messages appear, and if so, match
the attributes via strict regular expression.

t = MiniMagick::Image.open("attachment.pdf")
=> #<MiniMagick::Image:0x000000000ce25f98 ... >
2:(main)> t.dimensions
=> [596, 842]

[janko]

Thanks, the change looks good to me 👍

On a general note, I wish Ghostscript was dumping these into stderr instead, it looks like something that's intended for stderr.

[ageacademia]

Thanks for your message. I would imagine there is option to pass into GS that could suppress warning messages (it feels like a bit of a band-aid to parse this at the application level). But I didn't want to get too much into the weeds of customized GS settings.

At least, this approach offers no loss of information: I think if someone really needed to check PDF-compliance, they should be able to inspect the raw value t.info("raw: %m %w %h %b") and look at it stdout themselves.