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 However having out of date cache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
$ hg init
$ touch a
$ hg add a
$ hg commit -m "Added a"
$ touch main
$ hg add main
$ hg commit -m "Added main"
$ hg checkout 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
'main' should be gone:
$ ls -A
.hg
a
$ touch side1
$ hg add side1
$ hg commit -m "Added side1"
created new head
$ touch side2
$ hg add side2
$ hg commit -m "Added side2"
$ hg log
changeset: 3:91ebc10ed028
tag: tip
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: Added side2
changeset: 2:b932d7dbb1e1
parent: 0:c2eda428b523
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: Added side1
changeset: 1:71a760306caf
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: Added main
changeset: 0:c2eda428b523
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: Added a
$ hg heads
changeset: 3:91ebc10ed028
tag: tip
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: Added side2
changeset: 1:71a760306caf
user: test
date: Thu Jan 01 00:00:00 1970 +0000
summary: Added main
$ ls -A
.hg
a
side1
side2
$ hg update --debug -C 1
resolving manifests
branchmerge: False, force: True, partial: False
ancestor: 91ebc10ed028+, local: 91ebc10ed028+, remote: 71a760306caf
side1: other deleted -> r
removing side1
side2: other deleted -> r
removing side2
main: remote created -> g
getting main
1 files updated, 0 files merged, 2 files removed, 0 files unresolved
$ ls -A
.hg
a
main