opt: expand short hostnames in -x when PDSH_DEFAULT_DOMAIN is set

[BiloxiGeek]

When using -g to target a netgroup that returns FQDNs, exclusions specified with short hostnames via -x silently fail because the names don't match entries in the working collection.

This patch checks for a PDSH_DEFAULT_DOMAIN environment variable. If set, any short hostname (containing no '.') passed to -x is also tried as a FQDN (hostname.domain) against the working collection, so exclusions work correctly in mixed short/FQDN environments.

Also adds documentation for PDSH_DEFAULT_DOMAIN to the man page.

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Codecov Report

❌ Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 72.49%. Comparing base (398ca48) to head (21bdd8d).
⚠️ Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
src/pdsh/opt.c	40.00%	6 Missing ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #181      +/-   ##
==========================================
- Coverage   72.57%   72.49%   -0.09%     
==========================================
  Files          25       25              
  Lines        4088     4097       +9     
==========================================
+ Hits         2967     2970       +3     
- Misses       1121     1127       +6     

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[grondo]

This seems pretty reasonable. Note that -x also supports regex matching, but I can see how that would be cumbersome for this use case.

If PDSH_DEFAULT_DOMAIN is set, should the domain also be automatically added to hostnames in -w (if no domain is already there?) If the environment variable is only meant to apply to excluded hosts then maybe using PDSH_EXCLUDE_DEFAULT_DOMAIN would be a more appropriate name.

Also, tests for this work should be added under tests/ if you don't mind. Thanks!

[BiloxiGeek]

Actually I hadn't known -x could do regex, that might be a better solution since it's already cooked in. I must have missed that when I was first learning how to use pdsh and never thought to revisit the man page about it. I'd say you can reject this as a fix for a non-existent problem. The idea I came up with still has merits though in that the regex is great for hostname matching but the default domain environment gets rid of the regex string for the more overall domain name matching. If you want to roll this into the code independently of this PR please do so but I can deal with the regex way for now.

Adding the default domain to hosts in -w would make sense if DNS doesn't resolve the short name. All the systems I deal with have a good search domain set in resolve.conf so using short hostnames in -w hasn't been a problem. Maybe change the -x so it will do a direct match to whatever hostnames are already in -w or from the group in -g and if there's no match from those then try adding the default domain. Basically let -x use dns and the list already populated by -w or -g.