[bug #62950] [gropdf] revise 'download' resolution process

[Deri James]

Follow-up Comment #2, bug #62950 (project groff):

I think this is a bit more than a wish. The current gropdf behaviour (choose
last) causes build to fail if site or system download file contains "stale"
entries for the groff system fonts. Staleness can occur if ghostscript is
updated without making amendments to the download files (or re-running
BuildFoundries).

If the priority order is simply changed to "choose first" you run into the
problem that if a user downloads groff as a package, and the site-font
contains stale entries for the standard groff fonts, jobs which previously
completed successfully would now fail because the first matching entry is the
stale entry in site-font. Whereas with the current "choose last" the
(possibly) valid system entry would be used, i.e. a code change which would
result in groff no longer working on a previously functioning system. Ralph's
suggestion (an array of hashes, one for each download file) would certainly
work, but I think there is a slightly easier method. 

The goal is to find the first valid entry, i.e. the font file entry points to
a file which exists and is readable, plus inform user if they have tried to
use a font which points to a file which does not exist. Since this is a binary
issue, whether the file pointed to is readable or not, I create two hashes
from the various download files, valid entries and invalid entries. When
gropdf is required to embed a font, and it is not found in the valid entries,
one of two messages are output. If it is present in the invalid hash a message
identifying the font and particular download file is issued, otherwise the
existing message concerning possible corrupt download file.

I have documented the method I have adopted for gropdf in case someone wishes
to do something similar to grops.


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