19.07: net-snmp: snmpd not listening after reboot

Maintainer: @stintel
Environment: ath79, openwrt-19.07 branch (works fine in master!)

Description: snmpd responds perfectly well on first boot, but after reboot, it is non-responsive until it is restarted. snmp requests time out, no logging. Process is running, conf file is generated in /var/run/snmpd.conf as normal

Only hint is that the agentXSocket file is NOT created! (This is how I first came to this, my subagent wasn't working)

root@OpenWrt:/# pgrep -lfa snmpd
1248 /usr/sbin/snmpd -Lf /dev/null -f
root@OpenWrt:/# cat /var/run/snmpd.conf 
agentaddress UDP:161,UDP6:161
master agentx
agentXSocket /var/run/agentx.sock
sysLocation office
sysContact bofh@example.com
sysName HeartOfGold
com2sec ro default public
com2sec rw localhost private
com2sec6 ro default public
com2sec6 rw localhost private
group public v1 ro
group public v2c ro
group public usm ro
group private v1 rw
group private v2c rw
group private usm rw
view all included .1 
access public "" any noauth exact all none none
access private "" any noauth exact all all all
exec  filedescriptors /bin/cat /proc/sys/fs/file-nr
engineIDType 3
engineIDNic eth0
root@OpenWrt:/# ls -l /var/run/ag*
ls: /var/run/agent*: No such file or directory

After restart, works just fine, and agentx file is created and snmp requests are answered

root@OpenWrt:/# /etc/init.d/snmpd restart
root@OpenWrt:/# ls -l /var/run/ag*
srwxr-xr-x    1 root     root             0 Oct 20 10:27 /var/run/agentx.sock
root@OpenWrt:/# 

Openwrt build info. (Tried various points to very early on 19.07 branch, but that was still with my own extra apps, this is a perfectly clean one)

root@OpenWrt:/# cat /etc/openwrt_release 
DISTRIB_ID='OpenWrt'
DISTRIB_RELEASE='19.07-SNAPSHOT'
DISTRIB_REVISION='r11221-7dd822983b'
DISTRIB_TARGET='ath79/generic'
DISTRIB_ARCH='mips_24kc'
DISTRIB_DESCRIPTION='OpenWrt 19.07-SNAPSHOT r11221-7dd822983b'
DISTRIB_TAINTS='no-all'
root@OpenWrt:/# 

Diffconfig on clean 19.07 branch

$ ./scripts/diffconfig.sh 
CONFIG_TARGET_ath79=y
CONFIG_TARGET_ath79_generic=y
CONFIG_TARGET_MULTI_PROFILE=y
CONFIG_TARGET_DEVICE_ath79_generic_DEVICE_etactica_eg200=y
CONFIG_DEVEL=y
CONFIG_DOWNLOAD_FOLDER="/home/karlp/src/dl"
CONFIG_PACKAGE_libnetsnmp=y
CONFIG_PACKAGE_libpci=y
CONFIG_PACKAGE_snmpd=y