Linux discovery finds placeholder /dev/ttyS* devices

[PaulStoffregen]

When run on Ubuntu 20.04, serial-discovery is finding the /dev/ttyS* placeholder ports.

Looks like the go-serial library is supposed to check for this, but it seems to not be working.
https://github.com/bugst/go-serial/blob/f344e5f764a882b93ba5e38988551cbb2186e948/serial_unix.go#L285

./serial-discovery 
LIST
{
  "eventType": "command_error",
  "message": "First command must be HELLO, but got 'LIST'",
  "error": true
}
HELLO 1 "manual run"
{
  "eventType": "hello",
  "message": "OK",
  "protocolVersion": 1
}
START
{
  "eventType": "start",
  "message": "OK"
}
LIST
{
  "eventType": "list",
  "ports": [
    {
      "address": "/dev/ttyS8",
      "label": "/dev/ttyS8",
      "protocol": "serial",
      "protocolLabel": "Serial Port",
      "properties": {}
    },
    {
      "address": "/dev/ttyS9",
      "label": "/dev/ttyS9",
      "protocol": "serial",
      "protocolLabel": "Serial Port",
      "properties": {}
    },
    {
      "address": "/dev/ttyS0",
      "label": "/dev/ttyS0",
      "protocol": "serial",
      "protocolLabel": "Serial Port",
      "properties": {}
    },
    {
      "address": "/dev/ttyS2",
      "label": "/dev/ttyS2",
      "protocol": "serial",
      "protocolLabel": "Serial Port",
      "properties": {}
    },
    {
      "address": "/dev/ttyS3",
      "label": "/dev/ttyS3",
      "protocol": "serial",
      "protocolLabel": "Serial Port",
      "properties": {}
    },
    {
      "address": "/dev/ttyS7",
      "label": "/dev/ttyS7",
      "protocol": "serial",
      "protocolLabel": "Serial Port",
      "properties": {}
    },
    {
      "address": "/dev/ttyS6",
      "label": "/dev/ttyS6",
      "protocol": "serial",
      "protocolLabel": "Serial Port",
      "properties": {}
    },
    {
      "address": "/dev/ttyACM0",
      "label": "/dev/ttyACM0",
      "protocol": "serial",
      "protocolLabel": "Serial Port (USB)",
      "properties": {
        "pid": "0x0483",
        "serialNumber": "5393790",
        "vid": "0x16c0"
      }
    },
    {
      "address": "/dev/ttyS1",
      "label": "/dev/ttyS1",
      "protocol": "serial",
      "protocolLabel": "Serial Port",
      "properties": {}
    },
    {
      "address": "/dev/ttyS4",
      "label": "/dev/ttyS4",
      "protocol": "serial",
      "protocolLabel": "Serial Port",
      "properties": {}
    },
    {
      "address": "/dev/ttyS5",
      "label": "/dev/ttyS5",
      "protocol": "serial",
      "protocolLabel": "Serial Port",
      "properties": {}
    }
  ]
}

[cmaglie]

@PaulStoffregen are the ttySxx accessible from your user?
From some tests on my local machine I see that if the port is not accessible the enumerator will (wrongly) list it:

$ cat /dev/ttyS0 
cat: /dev/ttyS0: Permission denied
$ go run portlist/portlist.go 
Port: /dev/ttyACM0
   USB ID      : 2341:0057
   USB serial  : 
Port: /dev/ttyS0
Port: /dev/ttyS1
Port: /dev/ttyS10
Port: /dev/ttyS11
Port: /dev/ttyS12
Port: /dev/ttyS13
Port: /dev/ttyS14
Port: /dev/ttyS15
Port: /dev/ttyS16
Port: /dev/ttyS17
Port: /dev/ttyS18
Port: /dev/ttyS19
Port: /dev/ttyS2
Port: /dev/ttyS20
Port: /dev/ttyS21
Port: /dev/ttyS22
Port: /dev/ttyS23
Port: /dev/ttyS24
Port: /dev/ttyS25
Port: /dev/ttyS26
Port: /dev/ttyS27
Port: /dev/ttyS28
Port: /dev/ttyS29
Port: /dev/ttyS3
Port: /dev/ttyS30
Port: /dev/ttyS31
Port: /dev/ttyS4
Port: /dev/ttyS5
Port: /dev/ttyS6
Port: /dev/ttyS7
Port: /dev/ttyS8
Port: /dev/ttyS9

but if I give the correct permissions to the user everything is fine again:

$ cat /dev/ttyACM0 
^C
$ go run portlist/portlist.go 
Port: /dev/ttyACM0
   USB ID      : 2341:0057
   USB serial  : 

can you check if this is your case?
I've prepared a fix here bugst/go-serial#135

[PaulStoffregen]

Yes, confirmed.

[cmaglie]

This has been fixed with #38, and is already in production.

I'm working on another fix in the go-serial library, that allows checking if a serial port is fake or not without opening it (following your suggestion on libudev here) but it's mid-term because it requires a more involved change in the go-serial library.