This was tested on systems with these version combinations:

• (standard Ubuntu 22.04): sshfs=3.7.1, fuse=3.10.5, kerneliface=7.31
• manually installed sshfs/fuse: sshfs=3.7.3, fuse=3.14.0, kerneliface=7.38

Initial setup:

• create two subdirectories somewhere, "server" and "client". I did this in $HOME/tmp
• create a file "test1.txt" in server, owned by current user and main group of user
• create file uidfile with only line CURUSER:CURUID, in my case johann:1000
• create file gidfile with only line MAINGRP:GRPID in my case johann:1000

Now:

• run: sshfs -d localhost:/home/johann/tmp/server /home/johann/tmp/client -o idmap=file,uidfile=uidfile,gidfile=gidfile
• Option -d makes it easy to see if the command exits or keeps running ...
• THIS WORKS!

Modification 1:

• replace the groupname/id in gidfile with another group, even one the user is in (so the combination is legal, but not the primary group of the user). Run the same command again:
• DOES NOT WORK, exits with exit code 1 without any error message!

Modification 2:

• same as Modification 1, but run with added parameter nomap=ignore
• This works, does not exit!

Modification 3:

• same as initial setup
• but update uidfile to contain the wrong user id
• again, if nomap is ignore, this works, otherwise it does NOT work and exits without error message.

Obvervation:

• it seems that whenever the primary group of the user does not get mapped correctly AND no nomap parameter is specified or nomap=error, then the command exits without error message and with exit code 1
• so the real bug here is that no error message is written when some problem with uids or gids from the files is encountered
• This makes it hard to figure out initially what is going on at all (in the standard situation, without the -d flag, one expects the sshfs command to immediately return, but nothing is mounted)
• Even after one becomes aware that the problem might be related to the uid/gid files, it is very hard to figure out which of those is to blame and which setting or missing setting is responsible for the problem