Hi!
This patch is needed for the Galera add-on to MariaDB, which runs some 
scripts like for example wsrep_sst_rsync that needs access to additional 
binaries in PATH.

I tested the patch with (and without) below snippets to the 
mysql-service in my config.scm and successfully connected to a 
MariaDB/Galera cluster.

I ran these commands to test:
guix pull --url=/home/user1/src/guix --profile=/tmp/guix.master 
--disable-authentication --allow-downgrades ; 
GUIX_PROFILE="/tmp/guix.master" ; . "$GUIX_PROFILE/etc/profile" ; guix 
system reconfigure config.scm --fallback --allow-downgrades

------------------------------------------------------------------

(extra-environment #~(list (string-append "PATH=/usr/bin:/bin:" #$rsync 
"/bin:" #$coreutils "/bin:" #$gawk "/bin:" #$grep "/bin:" #$mariadb 
"/bin:" #$iproute "/sbin:" 
"/run/setuid-programs:/run/current-system/profile/bin:/run/current-system/profile/sbin" 
) (string-append "SHELL=" #$bash) "USER=mysql" 
"SSL_CERT_FILE=/etc/ssl/certs/ca-certificates.crt" 
"SSL_CERT_DIR=/run/current-system/profile/etc/ssl/certs"))


(extra-content #~(string-append "log_error=/var/lib/mysql/log_error.log
# 
https://www.percona.com/blog/2017/07/26/what-is-innodb_autoinc_lock_mode-and-why-should-i-care/
binlog_format=ROW
default-storage-engine=innodb
innodb_autoinc_lock_mode=2

# Galera Provider Configuration
wsrep_on=ON
wsrep_provider=" #$galera "/lib/libgalera_smm.so

# Galera Cluster Configuration
wsrep_cluster_name=\"test_cluster\"
wsrep_cluster_address=\"gcomm://redacted,redacted\"
# according to 
https://galeracluster.com/library/documentation/mysql-wsrep-options.html
# leaving it empty starts a new cluster, so you should immediately 
reconfigure again after doing this.
#wsrep_cluster_address=\"gcomm://\"

# Galera Synchronization Configuration
wsrep_sst_method=rsync

# Galera Node Configuration
wsrep_node_address=\"redacted\"
wsrep_node_name=\"librem13v3guixsd\""))
                                ))

------------------------------------------------------------------

Please someone also review [bug#47517] [PATCH] gnu: nginx: Enable stream 
module

which adds support for tcp loadbalancing that can be used to scale a 
MariaDB/Galera cluster.

Best regards,
David