Clean up how we handle cherrypy's calls of os._exit(70)

With this solution, we can catch it cleanly in the standalone
nilmdb-server, and test the error paths in our normal test suite.

nilmdb/scripts/nilmdb_server.py

@@ -5,6 +5,7 @@ import argparse
 import os
 import socket
 import cherrypy
+import sys
 
 def main():
     """Main entry point for the 'nilmdb-server' command line script"""
@@ -61,27 +62,30 @@ def main():
         print("----")
 
     # Run it
-    if args.yappi:
-        print("Running in yappi")
-        try:
-            import yappi
-            yappi.start()
+    try:
+        if args.yappi:
+            print("Running in yappi")
+            try:
+                import yappi
+                yappi.start()
+                server.start(blocking = True)
+            finally:
+                yappi.stop()
+                stats = yappi.get_func_stats()
+                stats.sort("ttot")
+                stats.print_all()
+                from IPython import embed
+                embed(header = "Use the `yappi` or `stats` object to explore "
+                      "further, quit to exit")
+        else:
             server.start(blocking = True)
-        finally:
-            yappi.stop()
-            stats = yappi.get_func_stats()
-            stats.sort("ttot")
-            stats.print_all()
-            from IPython import embed
-            embed(header = "Use the `yappi` or `stats` object to explore "
-                  "further, quit to exit")
-    else:
-        server.start(blocking = True)
-
-    # Clean up
-    if not args.quiet:
-        print("Closing database")
-        db.close()
+    except nilmdb.server.serverutil.CherryPyExit as e:
+        print("Exiting due to CherryPy error", file=sys.stderr)
+        raise
+    finally:
+        if not args.quiet:
+            print("Closing database")
+            db.close()
 
 if __name__ == "__main__":
     main()

nilmdb/server/serverutil.py

@@ -143,28 +143,38 @@ def json_error_page(status, message, traceback, version,
         errordata[k] = v
     return json.dumps(errordata, separators=(',',':'))
 
-# Start/stop CherryPy standalone server
-def cherrypy_start(blocking = False, event = False):
-    """Start the CherryPy server, handling errors and signals
-    somewhat gracefully."""
+class CherryPyExit(SystemExit):
+    pass
 
+def cherrypy_patch_exit():
     # Cherrypy stupidly calls os._exit(70) when it can't bind the port
-    # and exits.  Replace os._exit with a version that at least prints
-    # a message, since otherwise it's totally unclear what happened
+    # and exits.  Instead of that, raise a CherryPyExit (derived from
+    # SystemExit).  This exception may not make it back up to the caller
+    # due to internal thread use in the CherryPy engine, but there should
+    # be at least some indication that it happened.
+    bus = cherrypy.process.wspbus.bus
+    if "_patched_exit" in bus.__dict__:
+        return
+    bus._patched_exit = True
+
     def patched_exit(orig):
         real_exit = os._exit
-        def fake_exit(code): # pragma: no cover (hard to invoke from tests;
-                             # standalone server isn't used in production)
-            print("Cherrypy called os._exit(%d)" % code, file=sys.stderr)
-            real_exit(code)
+        def fake_exit(code):
+            raise CherryPyExit(code)
         os._exit = fake_exit
         try:
             orig()
         finally:
             os._exit = real_exit
-    bus = cherrypy.process.wspbus.bus
     bus.exit = functools.partial(patched_exit, bus.exit)
 
+# Start/stop CherryPy standalone server
+def cherrypy_start(blocking = False, event = False):
+    """Start the CherryPy server, handling errors and signals
+    somewhat gracefully."""
+
+    cherrypy_patch_exit()
+
     # Start the server
     cherrypy.engine.start()
 

tests/test_misc.py

@@ -6,6 +6,8 @@ import io
 import os
 import sys
 import time
+import socket
+import cherrypy
 
 import nilmdb.server
 from nilmdb.utils import timer, lock
@@ -95,7 +97,7 @@ class TestMisc(object):
             nilmdb.utils.diskusage.du("/dev/null/bogus")
         nilmdb.utils.diskusage.du("super-bogus-does-not-exist")
 
-    def tests_cors_allow(self):
+    def test_cors_allow(self):
         # Just to get some test coverage; these code paths aren't actually
         # used in current code
         cpy = nilmdb.server.serverutil.cherrypy
@@ -110,3 +112,28 @@ class TestMisc(object):
         with assert_raises(cpy.HTTPError):
             nilmdb.server.serverutil.CORS_allow(methods=[])
         (cpy.request, cpy.response) = (req, resp)
+
+    def test_cherrypy_failure(self):
+        # Test failure of cherrypy to start up because the port is
+        # already in use.  This also tests the functionality of
+        # serverutil:cherrypy_patch_exit()
+        sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
+        sock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
+        try:
+            sock.bind(("127.0.0.1", 32180))
+            sock.listen(1)
+        except OSError:
+            raise AssertionError("port 32180 must be free for tests")
+
+        nilmdb.server.serverutil.cherrypy_patch_exit()
+        cherrypy.config.update({
+            'environment': 'embedded',
+            'server.socket_host': '127.0.0.1',
+            'server.socket_port': 32180,
+            'engine.autoreload.on': False,
+            })
+        with assert_raises(Exception) as e:
+            cherrypy.engine.start()
+        in_("Address already in use", str(e.exception))
+
+        sock.close()