contrib: new script to read events from a named pipe and emit catapult traces

I'm starting to get more serious about getting some insight into where
we're spending our time, both in hg itself but also in the test
suite. As a first pass, I'm going to try and produce catapult
traces[0] that can be viewed with Chrome's `about:tracing` tool.

0: https://docs.google.com/document/d/1CvAClvFfyA5R-PhYUmn5OOQtYMH4h6I0nSsKchNAySU/edit#heading=h.nso4gcezn7n1

Differential Revision: https://phab.mercurial-scm.org/D4342

contrib/catapipe.py

@@ -0,0 +1,85 b''
+#!/usr/bin/env python3
+#
+# Copyright 2018 Google LLC.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+"""Tool read primitive events from a pipe to produce a catapult trace.
+
+For now the event stream supports
+
+  START $SESSIONID ...
+
+and
+
+  END $SESSIONID ...
+
+events. Everything after the SESSIONID (which must not contain spaces)
+is used as a label for the event. Events are timestamped as of when
+they arrive in this process and are then used to produce catapult
+traces that can be loaded in Chrome's about:tracing utility. It's
+important that the event stream *into* this process stay simple,
+because we have to emit it from the shell scripts produced by
+run-tests.py.
+
+Typically you'll want to place the path to the named pipe in the
+HGCATAPULTSERVERPIPE environment variable, which both run-tests and hg
+understand.
+"""
+from __future__ import absolute_import, print_function
+
+import argparse
+import datetime
+import json
+import os
+
+_TYPEMAP = {
+    'START': 'B',
+    'END': 'E',
+}
+
+_threadmap = {}
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument('pipe', type=str, nargs=1,
+                        help='Path of named pipe to create and listen on.')
+    parser.add_argument('output', default='trace.json', type=str, nargs='?',
+                        help='Path of named pipe to create and listen on.')
+    parser.add_argument('--debug', default=False, action='store_true',
+                        help='Print useful debug messages')
+    args = parser.parse_args()
+    fn = args.pipe[0]
+    os.mkfifo(fn)
+    try:
+        with open(fn) as f, open(args.output, 'w') as out:
+            out.write('[\n')
+            start = datetime.datetime.now()
+            while True:
+                ev = f.readline().strip()
+                if not ev:
+                    continue
+                now = datetime.datetime.now()
+                if args.debug:
+                    print(ev)
+                verb, session, label = ev.split(' ', 2)
+                if session not in _threadmap:
+                    _threadmap[session] = len(_threadmap)
+                pid = _threadmap[session]
+                ts_micros = (now - start).total_seconds() * 1000000
+                out.write(json.dumps(
+                    {
+                        "name": label,
+                        "cat": "misc",
+                        "ph": _TYPEMAP[verb],
+                        "ts": ts_micros,
+                        "pid": pid,
+                        "tid": 1,
+                        "args": {}
+                    }))
+                out.write(',\n')
+    finally:
+        os.unlink(fn)
+
+if __name__ == '__main__':
+    main()