Unexpected short name and net_adm:localhost() differences

[essen]

Describe the bug The local host name used for the node when -sname is set does not always match what net_adm:localhost() returns, when net_adm:localhost() returns a short name.

To Reproduce In all cases /etc/hostname contains zen.

1 When /etc/hosts contains:

127.0.0.1       localhost
::1             localhost
127.0.0.1       zen.localdomain zen
::1             zen.localdomain zen

I get

% erl -sname abc
Erlang/OTP 26 [erts-14.1.1] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Eshell V14.1.1 (press Ctrl+G to abort, type help(). for help)
(abc@localhost)1> net_adm:localhost().
"zen"
(abc@localhost)2> 

2 When /etc/hosts contains:

127.0.0.1       zen.localdomain zen
::1             zen.localdomain zen
127.0.0.1       localhost
::1             localhost

I get the correct, but unconvenient:

% erl -sname abc     
Erlang/OTP 26 [erts-14.1.1] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Eshell V14.1.1 (press Ctrl+G to abort, type help(). for help)
(abc@zen)1> net_adm:localhost().
"zen.localdomain"
(abc@zen)2> 

3 When /etc/hosts contains:

127.0.0.1       localhost
::1             localhost
127.0.0.1       zen
::1             zen

I get:

% erl -sname abc
Erlang/OTP 26 [erts-14.1.1] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Eshell V14.1.1 (press Ctrl+G to abort, type help(). for help)
(abc@localhost)1> net_adm:localhost().
"zen"
(abc@localhost)2> 

4 Finally, only in the following case where /etc/hosts contains:

127.0.0.1       zen
::1             zen
127.0.0.1       localhost
::1             localhost

I get the same name in both places:

% erl -sname abc
Erlang/OTP 26 [erts-14.1.1] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Eshell V14.1.1 (press Ctrl+G to abort, type help(). for help)
(abc@zen)1> net_adm:localhost().
"zen"
(abc@zen)2> 

5 Bonus, when /etc/hosts contains:

127.0.0.1       zen zen.localdomain
::1             zen zen.localdomain
127.0.0.1       localhost
::1             localhost

I also get the correct:

% erl -sname abc
Erlang/OTP 26 [erts-14.1.1] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Eshell V14.1.1 (press Ctrl+G to abort, type help(). for help)
(abc@zen)1> net_adm:localhost().
"zen"
(abc@zen)2> 

Expected behavior I expect -sname to use zen instead of localhost when net_adm:localhost() returns "zen". I expect -sname to use localhost only when net_adm:localhost() also returns "localhost".

I do not necessarily expect the hosts to match when net_adm:localhost() returns "zen.localdomain". That one is understandable since -sname uses a short name. In that case I expect -sname to be set to zen (which it already is as seen in the second snippet). It would be more convenient if net_adm:localhost() would return the same value though, when the short name is also configured and/or reachable.

The local host resolution algorithms should agree when possible.

Affected versions At least OTP 26 and 27.

Additional context This breaks slave as that module uses remsh only if the host isn't local and because the hosts don't match it thinks it's not local. I know this module is deprecated.