ThreadTest sometimes fails

Found in: 1.18
CategoryFixed: fixed in 744 for release 1.19

There may be some thread safety issues with PlashGlibc. ThreadTest sometimes fails:

======================================================================
ERROR: tests_thread_test_py.ThreadTest.test_parallel
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/thread_test.py", line 89, in test_parallel
    self._run_test("test_parallel")
  File "tests/thread_test.py", line 83, in _run_test
    plash.process_test.check_exit_status(status)
  File "/work/plash/python/plash/process_test.py", line 41, in check_exit_status
    raise ProcessExitError("Status %i" % status)
ProcessExitError: Status 139

This occurred once:

*** glibc detected *** /tmp/tmpXCQ_U0/test-case: double free or corruption (out): 0x00002aaaac004ee0 ***
fstat(fd, &st): No such file or directory
fd >= 0: Function not implemented
E....
======================================================================
ERROR: tests_thread_test_py.ThreadTest.test_parallel
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/thread_test.py", line 89, in test_parallel
    self._run_test("test_parallel")
  File "tests/thread_test.py", line 83, in _run_test
    plash.process_test.check_exit_status(status)
  File "/work/plash/python/plash/process_test.py", line 39, in check_exit_status
    os.WEXITSTATUS(status))
ProcessExitError: Process exited with status 1

That said, the non-PlashGlibc version of this test has failed occasionally, so there may be a bug in the test case too:

fd >= 0: File exists
F...
======================================================================
FAIL: tests_thread_test_py.ThreadTest.test_parallel_native
----------------------------------------------------------------------
Traceback (most recent call last):
  File "tests/thread_test.py", line 65, in test_parallel_native
    assert rc == 0, rc
AssertionError: 1

Problems found:

thread-test.c increments a counter in a non-thread-safe way. This explains failures such as printing unlink(socket_filename): No such file or directory. It explains why the non-PlashGlibc version of the test fails.
- Wrap use of the counter in a lock.
dup2() and close() call plash_init() without the lock held. This can cause two threads to clash at start-up time. This explains why failure doesn't appear to be made more likely by running the tests for longer (by increasing the number of loop iterations). Running thread-test.c more times should increase the chance of failure.
dup2() and close() also inspect the FD table without the lock held.
fork() and execve() do not use the lock, although they are not covered by the thread test case.
The req_and_reply() helper function and its variants in libc-comms.c also call plash_init() without the lock held.

Tasks:

Fix dup2() and close()
Fix fork()
Fix execve()
Fix req_and_reply() and variants