Only grab last line of bundle exec which output

[maneyko]

When running pdfkit in a docker container for our configuration, there is no home directory. Bundler < 2 has a warning for this and it goes to STDOUT (see: rubygems/bundler#6067). Here is the example:

irb(main):001:0> `bundle exec which wkhtmltopdf`.chomp
=> "`/home/myuser` is not a directory.\nBundler will use `/tmp/bundler/home/myuser' as your home directory temporarily.\n/usr/bin/wkhtmltopdf"
irb(main):002:0> `bundle exec which wkhtmltopdf`.chomp.lines.last
=> "/usr/bin/wkhtmltopdf"

Now an example where the warning does not come up:

2.4.6 :001 > `bundle exec which wkhtmltopdf`.chomp
 => "/usr/local/bin/wkhtmltopdf"
2.4.6 :002 > `bundle exec which wkhtmltopdf`.chomp.lines.last
 => "/usr/local/bin/wkhtmltopdf"

This PR seems to be very similar to the recently merged #460

Looking forward to your feedback.

[maneyko]

In the above example, if

irb(main):001:0> @default_command_path = `bundle exec which wkhtmltopdf`.chomp
=> "`/home/myuser` is not a directory.\nBundler will use `/tmp/bundler/home/myuser' as your home directory temporarily.\n/usr/bin/wkhtmltopdf"

Then @default_command_path will remain unchanged after lib/pdfkit/configuration.rb#L32 since it is not nil and not empty. This results in unexpected behavior as @default_command_path is a multiline string with warning messages. As a result, the wkhtmltopdf executable cannot be found by pdfkit, as this error comes up elsewhere downstream, even though it just needs to be parsed from this output more accurately.

Based on the context here, it seems that there is never a scenario where we would want multiline output from bundle exec which wkhtmltopdf, so if anything it adds as a safeguard against these kinds of situations.

[serene]

Can we add a test for this?

[maneyko]

@serene
Done. Thanks.