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 However having out of date cache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
# Helper script used for generating history and working copy files and content.
# The file's name corresponds to its history. The number of changesets can
# be specified on the command line. With 2 changesets, files with names like
# content1_content2_content1-untracked are generated. The first two filename
# segments describe the contents in the two changesets. The third segment
# ("content1-untracked") describes the state in the working copy, i.e.
# the file has content "content1" and is untracked (since it was previously
# tracked, it has been forgotten).
#
# This script generates the filenames and their content, but it's up to the
# caller to tell hg about the state.
#
# There are two subcommands:
# filelist <numchangesets>
# state <numchangesets> (<changeset>|wc)
#
# Typical usage:
#
# $ python $TESTDIR/generate-working-copy-states.py state 2 1
# $ hg addremove --similarity 0
# $ hg commit -m 'first'
#
# $ python $TESTDIR/generate-working-copy-states.py state 2 1
# $ hg addremove --similarity 0
# $ hg commit -m 'second'
#
# $ python $TESTDIR/generate-working-copy-states.py state 2 wc
# $ hg addremove --similarity 0
# $ hg forget *_*_*-untracked
# $ rm *_*_missing-*
from __future__ import absolute_import, print_function
import os
import sys
# Generates pairs of (filename, contents), where 'contents' is a list
# describing the file's content at each revision (or in the working copy).
# At each revision, it is either None or the file's actual content. When not
# None, it may be either new content or the same content as an earlier
# revisions, so all of (modified,clean,added,removed) can be tested.
def generatestates(maxchangesets, parentcontents):
depth = len(parentcontents)
if depth == maxchangesets + 1:
for tracked in (b'untracked', b'tracked'):
filename = (
b"_".join(
[
(content is None and b'missing' or content)
for content in parentcontents
]
)
+ b"-"
+ tracked
)
yield (filename, parentcontents)
else:
for content in {None, b'content' + (b"%d" % (depth + 1))} | set(
parentcontents
):
for combination in generatestates(
maxchangesets, parentcontents + [content]
):
yield combination
# retrieve the command line arguments
target = sys.argv[1]
maxchangesets = int(sys.argv[2])
if target == 'state':
depth = sys.argv[3]
# sort to make sure we have stable output
combinations = sorted(generatestates(maxchangesets, []))
# compute file content
content = []
for filename, states in combinations:
if target == 'filelist':
print(filename.decode('ascii'))
elif target == 'state':
if depth == 'wc':
# Make sure there is content so the file gets written and can be
# tracked. It will be deleted outside of this script.
content.append((filename, states[maxchangesets] or b'TOBEDELETED'))
else:
content.append((filename, states[int(depth) - 1]))
else:
print("unknown target:", target, file=sys.stderr)
sys.exit(1)
# write actual content
for filename, data in content:
if data is not None:
f = open(filename, 'wb')
f.write(data + b'\n')
f.close()
elif os.path.exists(filename):
os.remove(filename)