When _initilize_memcached() is called the first time, it sets the 'ismemcached'-key and returns 1.

When _initilize_memcached() is called the second time, the 'ismemcached'-key already exists and '' is returned (this default to False in Perl).
Koha thinks that memcached is not in use and decides to use some other caching mechanism instead.

Cache::Memcached::Fast returns undef on server error:
https://metacpan.org/pod/Cache::Memcached::Fast#add

Koha MUST listen to undefined, instead of False to define whether or not memcached is properly working.


Also added simple helpful diagnostics to help identify and log when this memcached server connection problem arises.


Here is a small test run to verify this behaviour:
root@kohadev:~# echo "fresh memcached start"
fresh memcached start
root@kohadev:~# perl l.pl
127.0.0.1:4545 1.4.14 (Ubuntu)
def  : '1'
getkey: 'text'
root@kohadev:~# echo "val already cached"
val already cached
root@kohadev:~# perl l.pl
127.0.0.1:4545 1.4.14 (Ubuntu)
def  : ''
getkey: 'text'
root@kohadev:~# echo "memcached killed and not responding"
memcached killed and not responding
root@kohadev:~# perl l.pl
undef
getkey: ''
root@kohadev:~# echo "The return value is undefined"
The return value is undefined
root@kohadev:~#



Here is the code snippet used to test the Memcached-server in the previous example output:

kivilahtio@koha_ci_1 /h/k/Koha> cat t.pl
use Cache::Memcached::Fast;
use Modern::Perl;

my $memd = new Cache::Memcached::Fast({
    servers => [ '/lxcBindMount/tmp/memcached.sock' ],
    namespace => 'koha_ci_1:',
    connect_timeout => 0.2,
    io_timeout => 0.5,
    close_on_error => 1,
    max_failures => 3,
    failure_timeout => 2,
    ketama_points => 150,
    nowait => 1,
    hash_namespace => 1,
});


# Get server versions.
my $versions = $memd->server_versions;
while (my ($server, $version) = each %$versions) {
    print "$server $version\n";
}

# Store scalars.
my $rv = $memd->add('skey', 'text');
print "undef\n" if not defined $rv;
print "def  : '$rv'\n" if defined $rv;
print "getkey: '".$memd->get('skey')."'\n";