/dev/ttyACM0 File does not exist while Postfix pipe

When I run script as regular user manually, everything is OK. But if an email is received and piped into Go script, I can not open serial port because file does not exist.

postfix   1239  1025  0 13:20 ? 00:00:00 pipe -n watchParadox -t unix flags=F user=watch argv=/usr/local/bin/watch -paradox
watch     1240  1239  0 13:20 ? 00:00:00 /usr/local/bin/watch -paradox

Script is running under watch user who has been added to dialout group, postfix user, just to be sure, is in dialout also.

In my script I ran ls -la command to find out which files do truly exist:

drwxr-xr-x   6 root root  380 Feb 25 13:19 .
dr-xr-xr-x. 18 root root 4096 Feb 22 17:53 ..
lrwxrwxrwx   1 root root   11 Feb 25 13:19 core -> /proc/kcore
lrwxrwxrwx   1 root root   13 Feb 25 13:19 fd -> /proc/self/fd
crw-rw-rw-   1 root root 1, 7 Feb 25 13:19 full
drwxr-xr-x   2 root root    0 Feb 25 13:19 hugepages
lrwxrwxrwx   1 root root   28 Feb 25 13:19 log -> /run/systemd/journal/dev-log
drwxrwxrwt   2 root root   40 Feb 25 13:19 mqueue
crw-rw-rw-   1 root root 1, 3 Feb 25 13:19 null
lrwxrwxrwx   1 root root    8 Feb 25 13:19 ptmx -> pts/ptmx
drwxr-xr-x   2 root root    0 Feb 25 13:19 pts
crw-rw-rw-   1 root root 1, 8 Feb 25 13:19 random
drwxrwxrwt   2 root root   40 Feb 25 13:19 shm
lrwxrwxrwx   1 root root   15 Feb 25 13:19 stderr -> /proc/self/fd/2
lrwxrwxrwx   1 root root   15 Feb 25 13:19 stdin -> /proc/self/fd/0
lrwxrwxrwx   1 root root   15 Feb 25 13:19 stdout -> /proc/self/fd/1
crw-rw-rw-   1 root root 5, 0 Feb 25 13:19 tty
crw-rw-rw-   1 root root 1, 9 Feb 25 13:19 urandom 

I am certain following file exists but is not shown when postfix pipe is executed.

crw-rw---- 1 root dialout 166, 0 25. úno 13.19 ttyACM0

I tried chmod 777 on the file but no luck in that department. Opening file via https://github.com/tarm/serial library which uses Go function:

os.OpenFile("ttyACM0", syscall.O_RDWR|syscall.O_NOCTTY|syscall.O_NONBLOCK, 0666)

results with

> ttyACM0: no such file or directory

The problem is not the path because I tried relative (using chdir) and absolute with same exact outcome.

I even disabled SELinux which is not what I want to do but in search of solution I try anything.

Code is OK because manual execution of the script passes through properly. I think there is something wrong with Linux settings.

The correct path the ttyACM0 is /dev/ttyACM0. You define a udev rules to set the file mode when it attaches.

If that is not reachable from your program, it might be the case that your program runs chrooted().

It seems like it was truly Linux problem and some underlying fabrics of it which I do not understand. I was running Fedora25 but the problem does not occur on Debian8.