utils/ssh: Remove _check_env()

Replace _check_env() by lazily initialized global var.

devlib/utils/ssh.py

@@ -67,9 +67,17 @@ from devlib.connection import (ConnectionBase, ParamikoBackgroundCommand, PopenB
 DEFAULT_SSH_SUDO_COMMAND = "sudo -k -p ' ' -S -- sh -c {}"
 
 
-ssh = None
-scp = None
-sshpass = None
+# Lazy init of some globals
+def __getattr__(attr):
+    if attr in {'ssh', 'scp', 'sshpass'}:
+        path = which(attr)
+        if path:
+            globals()[attr] = path
+            return path
+        else:
+            raise HostError(f'OpenSSH must be installed on the host: could not find {attr} command')
+    else:
+        raise AttributeError(f"Module '{__name__}' has no attribute '{attr}'")
 
 
 logger = logging.getLogger('ssh')
@@ -176,7 +184,6 @@ def telnet_get_shell(host,
                   port=None,
                   timeout=10,
                   original_prompt=None):
-    _check_env()
     start_time = time.time()
     while True:
         conn = TelnetPxssh(original_prompt=original_prompt)
@@ -792,7 +799,6 @@ class TelnetConnection(SshConnectionBase):
             strict_host_check=strict_host_check,
         )
 
-        _check_env()
         self.options = self._get_default_options()
 
         self.lock = threading.Lock()
@@ -1607,16 +1613,6 @@ def _give_password(password, command):
     return (pass_string + command, redacted_string + command)
 
 
-def _check_env():
-    global ssh, scp, sshpass  # pylint: disable=global-statement
-    if not ssh:
-        ssh = which('ssh')
-        scp = which('scp')
-        sshpass = which('sshpass')
-    if not (ssh and scp):
-        raise HostError('OpenSSH must be installed on the host.')
-
-
 def process_backspaces(text):
     chars = []
     for c in text: