[Bug 215540] ntpd fails to start when ran via ssh with pseudo-terminal and terminals/connection are closed before child opens new fds

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=215540

            Bug ID: 215540
           Summary: ntpd fails to start when ran via ssh with
                    pseudo-terminal and terminals/connection are closed
                    before child opens new fds
           Product: Base System
           Version: 11.0-RELEASE
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: freebsd-bugs at FreeBSD.org
          Reporter: dereks at lifeofadishwasher.com

ntpd fails to start if terminals are closed before ntpd's child process can
open new fds.  Testing with HAVE_CLOSEFROM in config.h doesn't have this issue.

Before restrarting ntpd from A:

  $ ntpq -pn
       remote           refid      st t when poll reach   delay   offset 
jitter
 
==============================================================================
   north-america.p .POOL.          16 p    -   64    0    0.000    0.000  
0.002
  *50.22.155.163   209.51.161.238   2 u    1   64    1   87.964   -2.998  
2.572
  ...

Using something like the following to restart ntp on an array of machines:

    for m in A B C; do ssh -t "$m" 'sudo service ntpd restart' ; done

Later from host A:

  ...
  Dec 23 18:40:57 <ntp.notice> A ntpd[80966]: ntpd 4.2.8p9-a (1): Starting
  ...

  $ ntpq -pn
  ntpq: read: Connection refused
  ...

ntpd fails to finish starting up and silently fails.  The same is true from B
and C.

Running the same without ssh's -t or sleeping before the connection closes
allows ntpd child to startup.  This assumes the user can run sudo without a
password:

  for m in A B C; do ssh "$m" 'sudo service ntpd restart' ; done

or

  for m in A B C; do ssh -t "$m" 'sudo service ntpd restart; sleep 1' ; done

Testing ntpd from the relang/11.0 branch with '#define HAVE_CLOSEFROM 1' in
config.h appears to solve the problem.
This will cause libntp/ntp_worker.c close_all_beyond() to use closefrom(2) that
appears to not suffer the same issue as the two other ifdef's.

However, the issue appears to be contrib/ntp/ doesn't have a test to see if the
system has closefrom(2) so it appears we have to add it manually?

This was initially found when restarting ntpd on a legion of machines via
ansible.  I'd suspect other config management tools that use ssh with
pseudo-terminals would suffer the same issue.  The above  is a raw example of
what something like ansible might perform.

This is also a problem in 9.3 and 10.x

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