nodemap: gate the feature behind a new requirement Now that the feature is working smoothly, a question was still open, should we gate the feature behind a new requirement or just treat it as a cache to be warmed by those who can and ignored by other. The advantage of using the cache approach is a transparent upgrade/downgrade story, making the feature easier to move to. However having out of date cache can come with a significant performance hit for process who expect an up to date cache but found none. In this case the file needs to be stored under `.hg/cache`. The "requirement" approach guarantee that the persistent nodemap is up to date. However, it comes with a less flexible activation story since an explicite upgrade is required. In this case the file can be stored in `.hg/store`. This wiki page is relevant to this questions: https://www.mercurial-scm.org/wiki/ComputedIndexPlan So which one should we take? Another element came into plan, the persistent nodemap use the `add` method of the transaction, it is used to keep track of a file content before a transaction in case we need to rollback it back. It turns out that the `transaction.add` API does not support file stored anywhere than `.hg/store`. Making it support file stored elsewhere is possible, require a change in on disk transaction format. Updating on disk file requires… introducing a new requirements. As a result, we pick the second option "gating the persistent nodemap behind a new requirements". Differential Revision: https://phab.mercurial-scm.org/D8417

#!/usr/bin/env python
#
# Dumps output generated by Mercurial's command server in a formatted style to a
# given file or stderr if '-' is specified. Output is also written in its raw
# format to stdout.
#
# $ ./hg serve --cmds pipe | ./contrib/debugcmdserver.py -
# o, 52   -> 'capabilities: getencoding runcommand\nencoding: UTF-8'

from __future__ import absolute_import, print_function
import struct
import sys

if len(sys.argv) != 2:
    print('usage: debugcmdserver.py FILE')
    sys.exit(1)

outputfmt = '>cI'
outputfmtsize = struct.calcsize(outputfmt)

if sys.argv[1] == '-':
    log = sys.stderr
else:
    log = open(sys.argv[1], 'a')


def read(size):
    data = sys.stdin.read(size)
    if not data:
        raise EOFError
    sys.stdout.write(data)
    sys.stdout.flush()
    return data


try:
    while True:
        header = read(outputfmtsize)
        channel, length = struct.unpack(outputfmt, header)
        log.write('%s, %-4d' % (channel, length))
        if channel in 'IL':
            log.write(' -> waiting for input\n')
        else:
            data = read(length)
            log.write(' -> %r\n' % data)
        log.flush()
except EOFError:
    pass
finally:
    if log != sys.stderr:
        log.close()