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[Christian Ehrhardt  <1827286@xxxxxxxxxxxxxxxxxx>]

Debugging per [1], did not show anything new.
There just is no activity on the actual access of /home all I see is the trigger for /home registering at startup.

Startup:
ubuntu@xenial-autofs-client:/$ sudo automount -f -v --debug
Starting automounter version 5.1.1, master map /etc/auto.master
using kernel protocol version 5.02
lookup_nss_read_master: reading master file /etc/auto.master
parse_init: parse(sun): init gathered global options: (null)
lookup_read_master: lookup(file): read entry +dir:/etc/auto.master.d
lookup_nss_read_master: reading master dir /etc/auto.master.d
lookup_read_master: lookup(dir): scandir: /etc/auto.master.d
lookup_read_master: lookup(file): read entry /-
master_do_mount: mounting /-
automount_path_to_fifo: fifo name /var/run/autofs.fifo--
lookup_nss_read_map: reading map file /etc/auto.nfs
parse_init: parse(sun): init gathered global options: nosymlink,fstype=nfs,nfsvers=3,rw,hard,intr,rsize=8192,wsize=8192
mounted direct on /home with timeouts disabled
do_mount_autofs_direct: mounted trigger /home
st_ready: st_ready(): state = 0 path /-

It is the open syscall itself that fails:
0.000029 open("/home", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = -1 ELOOP (Too many levels of symbolic links) <0.000012>

So we must have set up the kernel/trigger to be broken - no userspace component active at the time (although it might have set it up wrong before).
But since I haven't found fixes in autofs maybe it is fixed in the kernel then?

Yes, here I found a fix.
Installing linux-virtual-hwe-16.04 (since I was in a VM to test) resolved the issue.
With the same setup I can now mount /home just fine again on Xenial.

On a bare metal system you likely want linux-generic-hwe-16.04 instead.

I'm adding a kernel task to potentially identify and backport the fix to the 4.4 kernel, but since after so much time it isn't in the stable releases chances are that it is hard to backport.
Until then you can get this issue resolved with the -hwe kernels.

[1]:
https://help.ubuntu.com/community/Autofs#Debugging_Auto_Mount_Problems

** Also affects: linux (Ubuntu)
   Importance: Undecided
       Status: New

** Also affects: linux (Ubuntu Xenial)
   Importance: Undecided
       Status: New

** Also affects: autofs5 (Ubuntu Xenial)
   Importance: Undecided
       Status: New

** Also affects: linux (Ubuntu Bionic)
   Importance: Undecided
       Status: New

** Also affects: autofs5 (Ubuntu Bionic)
   Importance: Undecided
       Status: New

** Changed in: autofs5 (Ubuntu Bionic)
       Status: New => Invalid

** Changed in: autofs5 (Ubuntu Xenial)
       Status: New => Invalid

** Changed in: autofs5 (Ubuntu)
       Status: Confirmed => Invalid

** Changed in: linux (Ubuntu)
       Status: New => Fix Released

** Changed in: linux (Ubuntu Xenial)
       Status: New => Confirmed

** Changed in: linux (Ubuntu Bionic)
       Status: New => Fix Released

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https://bugs.launchpad.net/bugs/1827286

Title:
  autofs  - "Too many levels of symbolic links" after apt upgrade

Status in autofs5 package in Ubuntu:
  Invalid
Status in linux package in Ubuntu:
  Fix Released
Status in autofs5 source package in Xenial:
  Invalid
Status in linux source package in Xenial:
  Confirmed
Status in autofs5 source package in Bionic:
  Invalid
Status in linux source package in Bionic:
  Fix Released

Bug description:
  Description:    Ubuntu 16.04.6 LTS
  Release:        16.04

  I moved to autofs this week as a workaround for Bug #1577575 failing
  to mount NFS entries at boot. This worked file until I ran 'apt
  upgrade' today.

  Now trying to access /vol/home mount results in the following error:

  root@chi:~# ls -la /vol/home/
  ls: cannot access '/vol/home/': Too many levels of symbolic links
  root@chi:~# 

  Oddly enough, the other two autofs entries work fine (output trimmed):
  root@chi:~# ls -la /vol/media
  total 4
  ...
  root@chi:~# 
  root@chi:~# ls -al /vol/temp/
  total 24
  ...
  root@chi:~# 

  
  I have a scratch VM I can break tonight by performing upgrades individually and testing after each.  Will provide that detail when available.

  
  Here is /var/log/apt/history.log

   Start-Date: 2019-05-01  17:37:00
  Commandline: apt upgrade
  Upgrade: libdns-export162:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), libisccfg140:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), ureadahead:amd64 (0.100.0-19, 0.100.0-19.1), libldap-2.4-2:amd64 (2.4.42+dfsg-2ubuntu3.4, 2.4.42+dfsg-2ubuntu3.5), libirs141:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), bind9-host:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), dnsutils:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), libisc160:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), passwd:amd64 (1:4.2-3.1ubuntu5.3, 1:4.2-3.1ubuntu5.4), bind9utils:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), libisc-export160:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), liblwres141:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), login:amd64 (1:4.2-3.1ubuntu5.3, 1:4.2-3.1ubuntu5.4), iproute2:amd64 (4.3.0-1ubuntu3.16.04.4, 4.3.0-1ubuntu3.16.04.5), bind9:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), libdns162:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), unattended-upgrades:amd64 (0.90ubuntu0.10, 1.1ubuntu1.18.04.7~16.04.2), libisccc140:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), libbind9-140:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), uidmap:amd64 (1:4.2-3.1ubuntu5.3, 1:4.2-3.1ubuntu5.4), bind9-doc:amd64 (1:9.10.3.dfsg.P4-8ubuntu1.12, 1:9.10.3.dfsg.P4-8ubuntu1.14), tzdata:amd64 (2018i-0ubuntu0.16.04, 2019a-0ubuntu0.16.04)
  End-Date: 2019-05-01  17:37:24

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