contrib/catapipe.py

rhg: Don’t compare ambiguous files one byte at a time

Even though the use of `BufReader` reduces the number of syscalls to read
the file from disk, `.bytes()` yields a separate `Result` for every byte.
Creating those results and dispatching on them is most likely costly.

Instead, this commit opts for simplicity by reading the entire file into memory
and comparing a single pair of byte strings. Note that memory already needs to
contain the entire previous contents of the file, as read from the filelog.
So with an extremely large file this doubles memory use but does not make it
grow by orders of magnitude.

At first I wrote code that still avoids reading the entire file into memory
and compares one buffer at a time with `BufReader`. Find this code below for
posterity. However its correctness is subtle. I ended up preferring the
simplicity of the obviously-correct single comparison.

```rust
let mut reader = BufReader::new(fobj);
let mut expected = &contents_in_p1[..];
loop {
    let buf = reader.fill_buf().when_reading_file(&fs_path)?;
    if buf.is_empty() {
        // Found EOF
        return Ok(expected.is_empty());
    } else if let Some(rest) = expected.drop_prefix(buf) {
        // What we read so far matches the expected content, continue reading
        let buf_len = buf.len();
        reader.consume(buf_len);
        expected = rest
    } else {
        // Found different content
        return Ok(false);
    }
}
```

Differential Revision: https://phab.mercurial-scm.org/D11412

#!/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.

Usage:
    Terminal 1: $ catapipe.py /tmp/mypipe /tmp/trace.json
    Terminal 2: $ HGCATAPULTSERVERPIPE=/tmp/mypipe hg root
    <ctrl-c catapipe.py in Terminal 1>
    $ catapult/tracing/bin/trace2html /tmp/trace.json  # produce /tmp/trace.html
    <open trace.html in your browser of choice; the WASD keys are very useful>
    (catapult is located at https://github.com/catapult-project/catapult)

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. To trace *only* run-tests, use HGTESTCATAPULTSERVERPIPE instead.
"""
from __future__ import absolute_import, print_function

import argparse
import json
import os
import timeit

_TYPEMAP = {
    'START': 'B',
    'END': 'E',
    'COUNTER': 'C',
}

_threadmap = {}

# Timeit already contains the whole logic about which timer to use based on
# Python version and OS
timer = timeit.default_timer


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 json file to create where the traces ' 'will be stored.',
    )
    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 = timer()
            while True:
                ev = f.readline().strip()
                if not ev:
                    continue
                now = timer()
                if args.debug:
                    print(ev)
                verb, session, label = ev.split(' ', 2)
                if session not in _threadmap:
                    _threadmap[session] = len(_threadmap)
                if verb == 'COUNTER':
                    amount, label = label.split(' ', 1)
                    payload_args = {'value': int(amount)}
                else:
                    payload_args = {}
                pid = _threadmap[session]
                ts_micros = (now - start) * 1000000
                out.write(
                    json.dumps(
                        {
                            "name": label,
                            "cat": "misc",
                            "ph": _TYPEMAP[verb],
                            "ts": ts_micros,
                            "pid": pid,
                            "tid": 1,
                            "args": payload_args,
                        }
                    )
                )
                out.write(',\n')
    finally:
        os.unlink(fn)


if __name__ == '__main__':
    main()