[pexepect] Add a small sleep between pexect calls

[JDevlieghere]

Summary

When we upgraded pexpect to a more recent version, we experienced issues where pexpect was sending commands faster to LLDB could process. The problem was especially apparent on CI systems, where the high load caused LLDB to be inevitable more constrained. Usually the issue would go away when running the test by itself and would reappear when running multiple tests concurrently.

When we looked at this, the pexecpt implementation already contains a small timeout. Apparently that sometimes wasn't sufficient for LLDB.

Rather than adding timeouts in the affected tests, I would propose to just do it in lldbpexpect.py. It's definitely not the best solution, but it does solve the flakiness issues I'm investigating.

[JDevlieghere]

To give a little more context: the particular issue I was looking at (TestCommandScriptImmediateOutput) occurs with sendline. We send a bunch of commands and always check for the prompt. I can reproduce the error when piping the test commands directly to LLDB.

[JDevlieghere]

Apparently Pavel already investigated and fixed this in r357459. We also discussed a better way to make this work by checking the echo of the command passed to sendline.

lldb/packages/Python/lldbsuite/test/lldbpexpect.py

 from __future__ import print_function
 from __future__ import absolute_import
 
 # System modules
 import os
 import sys
+import time
 
 # Third-party modules
 import six
 
 # LLDB Modules
 import lldb
 from .lldbtest import *
 from . import lldbutil
class PExpectTest(TestBase):
 
         def expect(self, patterns=None, timeout=None, exact=None):
             if patterns is None:
                 return None
             if timeout is None:
                 timeout = self.timeout
             if exact is None:
                 exact = False
+            time.sleep(0.1) # Give LLDB time to process
             if exact:
                 return self.child.expect_exact(patterns, timeout=timeout)
             else:
                 return self.child.expect(patterns, timeout=timeout)
 
         def expectall(self, patterns=None, timeout=None, exact=None):
             if patterns is None:
                 return None