utils/ssh: Implement work around for issue in pexpect

As detailed in https://github.com/pexpect/pexpect/issues/552 when
sending a command with exactly the length of the prompt less than the
window width, an extra prompt is introduced into the returned data
causing the matching to fail and only return part of the commands
output. To workaround this check the length of the command to be
submitted and if of the specific length add an additional whitespace
character to the end of the command to prevent this behaviour.

devlib/utils/ssh.py

@@ -41,7 +41,8 @@ from pexpect import EOF, TIMEOUT, spawn
 # pylint: disable=redefined-builtin,wrong-import-position
 from devlib.exception import (HostError, TargetStableError, TargetNotRespondingError,
                               TimeoutError, TargetTransientError)
-from devlib.utils.misc import which, strip_bash_colors, check_output, sanitize_cmd_template
+from devlib.utils.misc import (which, strip_bash_colors, check_output,
+                               sanitize_cmd_template, memoized)
 from devlib.utils.types import boolean
 
 
@@ -253,7 +254,7 @@ class SshConnection(object):
         # simulate impatiently hitting ^C until command prompt appears
         logger.debug('Sending ^C')
         for _ in range(self.max_cancel_attempts):
-            self.conn.sendline(chr(3))
+            self._sendline(chr(3))
             if self.conn.prompt(0.1):
                 return True
         return False
@@ -267,15 +268,15 @@ class SshConnection(object):
             command = self.sudo_cmd.format(quote(command))
             if log:
                 logger.debug(command)
-            self.conn.sendline(command)
+            self._sendline(command)
             if self.password:
                 index = self.conn.expect_exact([self.password_prompt, TIMEOUT], timeout=0.5)
                 if index == 0:
-                    self.conn.sendline(self.password)
+                    self._sendline(self.password)
         else:  # not as_root
             if log:
                 logger.debug(command)
-            self.conn.sendline(command)
+            self._sendline(command)
         timed_out = self._wait_for_prompt(timeout)
         # the regex removes line breaks potential introduced when writing
         # command to shell.
@@ -321,6 +322,21 @@ class SshConnection(object):
         except TimeoutError as e:
             raise TimeoutError(command_redacted, e.output)
 
+    def _sendline(self, command):
+        # Workaround for https://github.com/pexpect/pexpect/issues/552
+        if len(command) == self._get_window_size()[1] - self._get_prompt_length():
+            command += ' '
+        self.conn.sendline(command)
+
+    @memoized
+    def _get_prompt_length(self):
+        self.conn.sendline()
+        self.conn.prompt()
+        return len(self.conn.after)
+
+    @memoized
+    def _get_window_size(self):
+        return self.conn.getwinsize()
 
 class TelnetConnection(SshConnection):