Quota warning executable can not log to /dev/stdout

[Michael Härtl]

I'm running dovecot 2.2.27 in a alpine based docker container and want 
to log to /dev/stdout. I therefore added this setting:

     log_path = /dev/stdout

Logging works fine so far.

Now I've added a quota-warning script. With the log_path setting above 
the script creates warnings like:

Oct 28 10:27:01 quota-warning: Error: Can't open log file /dev/stdout: 
No such device or address
Oct 28 10:27:01 quota-warning: Fatal: master: service(quota-warning): 
child 460 returned error 75


When I change log_path to something like /tmp/dovecot.log everything 
works fine.

Dovecot itself is started via s6 with a run script like this:

#!/bin/sh
exec /usr/sbin/dovecot -F -c /etc/dovecot/dovecot.conf


What could cause this issue and how could I fix it?



For the record here's my doveconf -n output:


# 2.2.27 (c0f36b0): /etc/dovecot/dovecot.conf
# OS: Linux 4.4.27-x86_64-jb1 x86_64  ext4
auth_mechanisms = plain login
auth_verbose = yes
first_valid_uid = 100
log_path = /dev/stdout
mail_gid = dovecot
mail_home = /var/mail/domains/%d/%n
mail_location = maildir:/var/mail/domains/%d/%n
mail_max_userip_connections = 160
mail_plugins = " quota"
mail_privileged_group = dovecot
mail_uid = vmail
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope 
encoded-character vacation subaddress comparator-i;ascii-numeric 
relational regex imap4flags copy include variables body enotify 
environment mailbox date index ihave duplicate mime foreverypart extracttext
namespace inbox {
   inbox = yes
   location =
   mailbox Drafts {
     auto = subscribe
     special_use = \Drafts
   }
   mailbox Junk {
     auto = subscribe
     special_use = \Junk
   }
   mailbox Sent {
     auto = subscribe
     special_use = \Sent
   }
   mailbox "Sent Messages" {
     special_use = \Sent
   }
   mailbox Trash {
     auto = subscribe
     special_use = \Trash
   }
   prefix =
   separator = .
}
passdb {
   args = /etc/dovecot/dovecot-mysql.conf.ext
   driver = sql
}
plugin {
   quota = maildir:User quota
   quota_exceeded_message = Storage quota for user %u exceeded (Benutzer 
%u hat sein Speichervolumen ueberschritten)
   quota_grace = 10%%
   quota_rule2 = Trash:storage=+100M
   quota_status_nouser = DUNNO
   quota_status_overquota = 552 5.2.2 Mailbox is full
   quota_status_success = DUNNO
   quota_warning = storage=85%% quota-warning 80 %u %d
}
protocols = imap lmtp sieve
service auth {
   unix_listener /var/spool/postfix/private/auth {
     group = postfix
     mode = 0666
     user = postfix
   }
   unix_listener auth-userdb {
     mode = 0777
   }
}
service lmtp {
   unix_listener /var/spool/postfix/private/dovecot-lmtp {
     group = postfix
     mode = 0600
     user = postfix
   }
}
service quota-status {
   client_limit = 1
   executable = quota-status -p postfix
   inet_listener {
     port = 10001
   }
}
service quota-warning {
   executable = script /usr/local/bin/quota-warning.sh
   group = dovecot
   unix_listener quota-warning {
     mode = 0666
     user = dovecot
   }
   user = vmail
}
ssl_cert = </certs/fullchain.pem
ssl_key =  # hidden, use -P to show it
ssl_protocols = !SSLv2 !SSLv3
userdb {
   args = /etc/dovecot/dovecot-mysql.conf.ext
   driver = sql
}
protocol imap {
   mail_plugins = " quota imap_quota"
}
protocol lmtp {
   mail_plugins = " quota sieve"
}