potential bug on wal_files service on low activity standby with 0 wal file

[l00ptr]

The check_wal_files function returns some errors when using the wal_files service. It also returns a wrong number of wal file on one of our low activity standby:

$ ~/bin/check_pgactivity  --service wal_files

Use of uninitialized value in string gt at ./bin/check_pgactivity line 8790.
Use of uninitialized value $first_seg in string gt at ./bin/check_pgactivity line 8790.
Use of uninitialized value $seg_kept in numeric gt (>) at ./bin/check_pgactivity line 8802.
Use of uninitialized value in string ne at ./bin/check_pgactivity line 8825.
Use of uninitialized value $ratio in multiplication (*) at ./bin/check_pgactivity line 795.
Use of uninitialized value $ratio in multiplication (*) at ./bin/check_pgactivity line 795.

POSTGRES_WAL_FILES OK: 1 WAL files | total_wal=1 recycled_wal=0 tli=0 written_wal=1 kept_wal=0

I think it's because the following query return an empty resultset :

DEBUG: host service:main is version 9.0.23/90023
DEBUG: Query: 
        SELECT s.f,
          greatest(
            1 + current_setting('checkpoint_segments')::float4 *
              (2 + current_setting('checkpoint_completion_target')::float4),
            1 + current_setting('wal_keep_segments')::float4 +
              2 * current_setting('checkpoint_segments')::float4
          ),
          CASE WHEN pg_is_in_recovery()
            THEN NULL
            ELSE pg_current_xlog_location()
          END,
          current_setting('wal_keep_segments')::integer,
          substring(s.f from 1 for 8) AS tli
        FROM pg_ls_dir('pg_xlog') AS s(f)
        WHERE f ~ '^[0-9A-F]{24}$'
        ORDER BY
          (pg_stat_file('pg_xlog/'||s.f)).modification DESC,
          f DESC
DEBUG: Env. service:  main 
DEBUG: Query rc: 0
DEBUG: Query result: $VAR1 = [];

dprinting the rs var on line https://github.com/OPMDG/check_pgactivity/blob/master/check_pgactivity#L8946 shows this :

rs: $VAR1 = [
          []
        ];

So in think that's why we iterate (at least once) throught this empty array on L8951 and get the result 1 WAL files | total_wal=1

[l00ptr]

after searching a little bit more, this issue seems related to the those 3 lines of code just after @rs = @{ query_ver( $hosts[0], %queries ) }; :

    $first_seg = $rs[0][0];
    $max_segs  = $rs[0][1]; #segments to keep including kept segments
    $tli = hex($rs[0][4]);

Here is a isolated test case to reproduce this error :

➜  ~ cat hydrat.pl 

sub machin() {
    return [];
}
@rs = @{ machin() };
my $var_machin = $rs[0][1];

foreach my $row (@rs) {
    print "oups";
}

#print( machin() );
➜  ~ perl hydrat.pl                                       
oups%