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 However having out of date cache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
#require no-symlink
# The following script was used to create the bundle:
#
# hg init symlinks
# cd symlinks
# echo a > a
# mkdir d
# echo b > d/b
# ln -s a a.lnk
# ln -s d/b d/b.lnk
# hg ci -Am t
# hg bundle --base null ../test-no-symlinks.hg
Extract a symlink on a platform not supporting them
$ hg init t
$ cd t
$ hg pull -q "$TESTDIR/bundles/test-no-symlinks.hg"
$ hg update
4 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cat a.lnk && echo
a
$ cat d/b.lnk && echo
d/b
Copy a symlink and move another
$ hg copy a.lnk d/a2.lnk
$ hg mv d/b.lnk b2.lnk
$ hg ci -Am copy
$ cat d/a2.lnk && echo
a
$ cat b2.lnk && echo
d/b
Bundle and extract again
$ hg bundle --base null ../symlinks.hg
2 changesets found
$ cd ..
$ hg init t2
$ cd t2
$ hg pull ../symlinks.hg
pulling from ../symlinks.hg
requesting all changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 6 changes to 6 files
new changesets d326ae2d01ee:71d85cf3ba90 (2 drafts)
(run 'hg update' to get a working copy)
$ hg update
5 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cat a.lnk && echo
a
$ cat d/a2.lnk && echo
a
$ cat b2.lnk && echo
d/b