PostgresNode::pid is improved

[dmitry-lipetsk]

• We do multiple attempts to read pid file.
• We process a case when we see that node is stopped between test and read.
• We process a case when pid-file is empty.

[dmitry-lipetsk]

Example of problem with PostgresNode.pid:
https://app.travis-ci.com/github/postgrespro/testgres/jobs/630950715

____________________ TestgresTests.test_pg_ctl_wait_option _____________________
[gw3] linux -- Python 3.12.3 /tmp/testgres_venv/bin/python
self = <testgres.tests.test_simple.TestgresTests object at 0x7fbc7ed0e900>
    def test_pg_ctl_wait_option(self):
>       with get_new_node() as node:
node       = PostgresNode(name='testgres-3103c943-9757-416e-b7c5-55899be2539e', port=None, base_dir='/tmp/tgsn_05zpeoer')
self       = <testgres.tests.test_simple.TestgresTests object at 0x7fbc7ed0e900>
tests/test_simple.py:431: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
testgres/node.py:197: in __exit__
    self.cleanup(attempts)
        attempts   = 3
        c1         = True
        c2         = False
        got_exception = False
        self       = PostgresNode(name='testgres-3103c943-9757-416e-b7c5-55899be2539e', port=None, base_dir='/tmp/tgsn_05zpeoer')
        traceback  = None
        type       = None
        value      = None
testgres/node.py:1040: in cleanup
    self._try_shutdown(max_attempts)
        full       = False
        max_attempts = 3
        self       = PostgresNode(name='testgres-3103c943-9757-416e-b7c5-55899be2539e', port=None, base_dir='/tmp/tgsn_05zpeoer')
testgres/node.py:342: in _try_shutdown
    node_pid = self.pid
        attempts   = 0
        max_attempts = 3
        self       = PostgresNode(name='testgres-3103c943-9757-416e-b7c5-55899be2539e', port=None, base_dir='/tmp/tgsn_05zpeoer')
        with_force = False
testgres/node.py:213: in pid
    lines = self.os_ops.readlines(pid_file)
        pid_file   = '/tmp/tgsn_05zpeoer/data/postmaster.pid'
        self       = PostgresNode(name='testgres-3103c943-9757-416e-b7c5-55899be2539e', port=None, base_dir='/tmp/tgsn_05zpeoer')
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
self = <testgres.testgres.operations.local_ops.LocalOperations object at 0x7fbc7edc9640>
filename = '/tmp/tgsn_05zpeoer/data/postmaster.pid', num_lines = 0
binary = False, encoding = None
    def readlines(self, filename, num_lines=0, binary=False, encoding=None):
        """
        Read lines from a local file.
        If num_lines is greater than 0, only the last num_lines lines will be read.
        """
        assert num_lines >= 0
        mode = 'rb' if binary else 'r'
        if num_lines == 0:
>           with open(filename, mode, encoding=encoding) as file:  # open in binary mode
E           FileNotFoundError: [Errno 2] No such file or directory: '/tmp/tgsn_05zpeoer/data/postmaster.pid'
binary     = False
encoding   = None
filename   = '/tmp/tgsn_05zpeoer/data/postmaster.pid'
mode       = 'r'
num_lines  = 0
self       = <testgres.testgres.operations.local_ops.LocalOperations object at 0x7fbc7edc9640>
testgres/operations/local_ops.py:341: FileNotFoundError

[dmitry-lipetsk]

The current implementation of PostgresNode::pid() is bad.

We have to check and use the information from pg_ctl:

dima@ubuntu-work-01:/home$ pg_ctl status
pg_ctl: server is running (PID: 2864920)
/usr/local/pgsql/bin/postgres