Assuming the following config file:

dbusername  = mysqluser
dbpassword  = mysqlpassword
backupdir   = /usr/local/mysql_backup
hotcopy     = no
sqldump     = yes
compress    = yes

@defaultsfile@ is set to "@--defaults-extra-file=/etc/mysql/debian.cnf@" and the dynamically generated content in @workcnf/$mycnf@ is disregarded.

This patch against HEAD resolves that behaviour by assuming the following order of precedence:

@dbusername@/@dbpassword@

@$user@

@$configfile@ (default)

--- a/handlers/mysql.in 2010-04-30 13:45:14.802811874 +0100
+++ b/handlers/mysql.in 2010-04-30 14:55:23.784512949 +0100
@@ -86,7 +86,7 @@

 defaultsfile=""

-if [ "$dbusername" != "" -a "$dbpassword" != "" ]
+if [ -n "$dbusername" -a -n "$dbpassword" ]
 then
    if [ $usevserver = yes ]
    then
@@ -140,13 +140,8 @@
 EOF
    umask $oldmask
    defaultsfile="--defaults-extra-file=$mycnf"
-fi
-
-# if a user is not set, use $configfile, otherwise use $mycnf
-if [ "$user" == "" ]; then
-   user=root;
-   defaultsfile="--defaults-extra-file=$configfile"
-else
+# if a user is set use $configfile
+elif [ -n "$user" ]; then
    userset=true;
    if [ $usevserver = yes ]
    then
@@ -169,6 +164,10 @@

    defaultsfile="--defaults-extra-file=$userhome/.my.cnf"
    debug "using $defaultsfile"
+# otherwise use $mycnf
+else
+   user=root;
+   defaultsfile="--defaults-extra-file=$configfile"
 fi

 #######################################################################

(from redmine: created on 2010-04-29, closed on 2010-06-24)