在Jenkins上执行python脚本时，尝试访问串行设备时，为什么会出现“权限被拒绝”错误？

Question

If I execute the following script as user 'build' in the command line it works:

#!/usr/bin/python

import serial
import pwd
import os
import grp

user = pwd.getpwuid(os.getuid()).pw_name
print user

groups = [g.gr_name for g in grp.getgrall() if user in g.gr_mem]
gid = pwd.getpwnam(user).pw_gid
groups.append(grp.getgrgid(gid).gr_name)
print groups

print oct(os.stat("/dev/ttyUSB0").st_mode & 0777)

ser = serial.Serial('/dev/ttyUSB0', 115200, timeout=1)
print ser.portstr       # check which port was really used
ser.write("hello")      # write a string
ser.close()

The output is

$ python test-serial.py
build
['dialout', 'build']
0660
/dev/ttyUSB0

If I execute this script in a Jenkins job I get

[EnvInject] - Loading node environment variables.
Building remotely on build-node in workspace /home/build/workspace/test-serial-python
[test-serial-python] $ /bin/sh -xe /tmp/hudson7262474284926512955.sh
+ /usr/bin/python /home/build/test-serial.py
build
['dialout', 'build']
0660
Traceback (most recent call last):
  File "/home/build/test-serial.py", line 17, in <module>
    ser = serial.Serial('/dev/ttyUSB0', 115200, timeout=1)
  File "/usr/lib/python2.7/dist-packages/serial/serialutil.py", line 261, in __init__
    self.open()
  File "/usr/lib/python2.7/dist-packages/serial/serialposix.py", line 278, in open
    raise SerialException("could not open port %s: %s" % (self._port, msg))
serial.serialutil.SerialException: could not open port /dev/ttyUSB0: [Errno 13] Permission denied: '/dev/ttyUSB0'
Build step 'Execute shell' marked build as failure
Finished: FAILURE

If I change the file permissions with sudo chmod 666 /dev/ttyUSB0 the Jenkins job succeeds

[EnvInject] - Loading node environment variables.
Building remotely on build-node in workspace /home/build/workspace/test-serial-python
[test-serial-python] $ /bin/sh -xe /tmp/hudson9120277699281127818.sh
+ /usr/bin/python /home/build/test-serial.py
build
['dialout', 'build']
0666
/dev/ttyUSB0
Finished: SUCCESS

What could be the problem here?

Answer 1

The solutions was a plain 'Did you try to turn it off and on again?'. The build succeeded after restarting the Jenkins slave.

The reason for this is that I added the user 'build' to the group 'dialout' and after a logout/login it worked on the command line. But I forgot about reconnecting the Jenkins slave.

The output of the Python script showed the correct group membership (because it is read from a database) but this was not activated before reconnecting the Jenkins slave.