[Bug 1825194] [NEW] resolvconf is racy, which leads to broken resolv.conf in parallel calls

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It has been found that simultaneous calls to resolvconf can lead to
inconsistent content in resolv.conf. For instance, no nameservers while
NetworkManager has one in its record (see LP: #1824395):

$ cat /run/resolvconf/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
# DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN

$ cat /run/resolvconf/interface/NetworkManager
nameserver 192.168.1.6
nameserver 192.168.1.2

This can happen easily when calling "netplan apply", which can re-start
both networkd and NM. resolvconf is called at that point by the
"systemd-networkd-resolvconf-update.service" service, and also directly
by NetworkManager, which leads to the situation described above. This is
not surprising as there is nothing preventing different instances of
resolvconf to access the same files. This sort of situation can be
reproduced by running in a loop commands like:

$ printf "\n" | sudo resolvconf -a NetworkManager & printf "\n" | sudo resolvconf -a networkd
$ printf "nameserver 80.58.61.250\n" | sudo resolvconf -a NetworkManager & printf "\n" | sudo resolvconf -a networkd

Eventually, this leads to a resolv.conf that is not consistent with the
last run command.

** Affects: resolvconf (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: patch
-- 
resolvconf is racy, which leads to broken resolv.conf in parallel calls
https://bugs.launchpad.net/bugs/1825194
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