revlog: skeleton support for version 2 revlogs
There are a number of improvements we want to make to revlogs
that will require a new version - version 2. It is unclear what the
full set of improvements will be or when we'll be done with them.
What I do know is that the process will likely take longer than a
single release, will require input from various stakeholders to
evaluate changes, and will have many contentious debates and
bikeshedding.
It is unrealistic to develop revlog version 2 up front: there
are just too many uncertainties that we won't know until things
are implemented and experiments are run. Some changes will also
be invasive and prone to bit rot, so sitting on dozens of patches
is not practical.
This commit introduces skeleton support for version 2 revlogs in
a way that is flexible and not bound by backwards compatibility
concerns.
An experimental repo requirement for denoting revlog v2 has been
added. The requirement string has a sub-version component to it.
This will allow us to declare multiple requirements in the course
of developing revlog v2. Whenever we change the in-development
revlog v2 format, we can tweak the string, creating a new
requirement and locking out old clients. This will allow us to
make as many backwards incompatible changes and experiments to
revlog v2 as we want. In other words, we can land code and make
meaningful progress towards revlog v2 while still maintaining
extreme format flexibility up until the point we freeze the
format and remove the experimental labels.
To enable the new repo requirement, you must supply an experimental
and undocumented config option. But not just any boolean flag
will do: you need to explicitly use a value that no sane person
should ever type. This is an additional guard against enabling
revlog v2 on an installation it shouldn't be enabled on. The
specific scenario I'm trying to prevent is say a user with a
4.4 client with a frozen format enabling the option but then
downgrading to 4.3 and accidentally creating repos with an
outdated and unsupported repo format. Requiring a "challenge"
string should prevent this.
Because the format is not yet finalized and I don't want to take
any chances, revlog v2's version is currently 0xDEAD. I figure
squatting on a value we're likely never to use as an actual revlog
version to mean "internal testing only" is acceptable. And
"dead" is easily recognized as something meaningful.
There is a bunch of cleanup that is needed before work on revlog
v2 begins in earnest. I plan on doing that work once this patch
is accepted and we're comfortable with the idea of starting down
this path.
$ cat >> $HGRCPATH << EOF
> [extensions]
> show =
> EOF
No arguments shows available views
$ hg init empty
$ cd empty
$ hg show
available views:
bookmarks -- bookmarks and their associated changeset
work -- changesets that aren't finished
abort: no view requested
(use "hg show VIEW" to choose a view)
[255]
`hg help show` prints available views
$ hg help show
hg show VIEW
show various repository information
A requested view of repository data is displayed.
If no view is requested, the list of available views is shown and the
command aborts.
Note:
There are no backwards compatibility guarantees for the output of this
command. Output may change in any future Mercurial release.
Consumers wanting stable command output should specify a template via
"-T/--template".
List of available views:
bookmarks bookmarks and their associated changeset
work changesets that aren't finished
(use 'hg help -e show' to show help for the show extension)
options:
-T --template TEMPLATE display with template
(some details hidden, use --verbose to show complete help)
Unknown view prints error
$ hg show badview
abort: unknown view: badview
(run "hg show" to see available views)
[255]
HGPLAIN results in abort
$ HGPLAIN=1 hg show bookmarks
abort: must specify a template in plain mode
(invoke with -T/--template to control output format)
[255]
But not if a template is specified
$ HGPLAIN=1 hg show bookmarks -T '{bookmark}\n'
(no bookmarks set)
$ cd ..
bookmarks view with no bookmarks prints empty message
$ hg init books
$ cd books
$ touch f0
$ hg -q commit -A -m initial
$ hg show bookmarks
(no bookmarks set)
bookmarks view shows bookmarks in an aligned table
$ echo book1 > f0
$ hg commit -m 'commit for book1'
$ echo book2 > f0
$ hg commit -m 'commit for book2'
$ hg bookmark -r 1 book1
$ hg bookmark a-longer-bookmark
$ hg show bookmarks
* a-longer-bookmark 7b570
book1 b757f
A custom bookmarks template works
$ hg show bookmarks -T '{node} {bookmark} {active}\n'
7b5709ab64cbc34da9b4367b64afff47f2c4ee83 a-longer-bookmark True
b757f780b8ffd71267c6ccb32e0882d9d32a8cc0 book1 False
bookmarks JSON works
$ hg show bookmarks -T json
[
{
"active": true,
"bookmark": "a-longer-bookmark",
"longestbookmarklen": 17,
"node": "7b5709ab64cbc34da9b4367b64afff47f2c4ee83"
},
{
"active": false,
"bookmark": "book1",
"longestbookmarklen": 17,
"node": "b757f780b8ffd71267c6ccb32e0882d9d32a8cc0"
}
]
JSON works with no bookmarks
$ hg book -d a-longer-bookmark
$ hg book -d book1
$ hg show bookmarks -T json
[
]
$ cd ..