bpo-5001: More-informative multiprocessing error messages (#3079)

* Make error message more informative

Replace assertions in error-reporting code with more-informative version that doesn't cause confusion over where and what the error is.

* Additional clarification + get travis to check

* Change from SystemError to TypeError

As suggested in PR comment by @pitrou, changing from SystemError; TypeError appears appropriate.

* NEWS file installation; ACKS addition (will do my best to justify it by additional work)

* Making current AssertionErrors in multiprocessing more informative

* Blurb added re multiprocessing managers.py, queues.py cleanup

* Further multiprocessing cleanup - went through pool.py

* Fix two asserts in multiprocessing/util.py

* Most asserts in multiprocessing more informative

* Didn't save right version

* Further work on multiprocessing error messages

* Correct typo

* Correct typo v2

* Blasted colon... serves me right for trying to work on two things at once

* Simplify NEWS entry

* Update 2017-08-18-17-16-38.bpo-5001.gwnthq.rst

* Update 2017-08-18-17-16-38.bpo-5001.gwnthq.rst

OK, never mind.

* Corrected (thanks to pitrou) error messages for notify

* Remove extraneous backslash in docstring.

1 files changed, 9 insertions, 3 deletions

diff --git a/Lib/multiprocessing/forkserver.py b/Lib/multiprocessing/forkserver.py
index 69b842aa939..7a952e2ba69 100644
--- a/Lib/multiprocessing/forkserver.py
+++ b/Lib/multiprocessing/forkserver.py
@@ -189,7 +189,7 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None):
 
                 if alive_r in rfds:
                     # EOF because no more client processes left
-                    assert os.read(alive_r, 1) == b''
+                    assert os.read(alive_r, 1) == b'', "Not at EOF?"
                     raise SystemExit
 
                 if sig_r in rfds:
@@ -208,7 +208,10 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None):
                             if os.WIFSIGNALED(sts):
                                 returncode = -os.WTERMSIG(sts)
                             else:
-                                assert os.WIFEXITED(sts)
+                                if not os.WIFEXITED(sts):
+                                    raise AssertionError(
+                                        "Child {0:n} status is {1:n}".format(
+                                            pid,sts))
                                 returncode = os.WEXITSTATUS(sts)
                             # Send exit code to client process
                             try:
@@ -227,7 +230,10 @@ def main(listener_fd, alive_r, preload, main_path=None, sys_path=None):
                     with listener.accept()[0] as s:
                         # Receive fds from client
                         fds = reduction.recvfds(s, MAXFDS_TO_SEND + 1)
-                        assert len(fds) <= MAXFDS_TO_SEND
+                        if len(fds) > MAXFDS_TO_SEND:
+                            raise RuntimeError(
+                                "Too many ({0:n}) fds to send".format(
+                                    len(fds)))
                         child_r, child_w, *fds = fds
                         s.close()
                         pid = os.fork()