streamclone: define first iteration of version 2 of stream format
(This patch is based on a first draft from Gregory Szorc, with deeper rework)
Version 1 of the stream clone format was invented many years ago and suffers
from a few deficiencies:
1) Filenames are stored in store-encoded (on filesystem) form rather than in
their internal form. This makes future compatibility with new store
filename encodings more difficult.
2) File entry "headers" consist of a newline of the file name followed by the
string file size. Converting strings to integers is avoidable overhead. We
can't store filenames with newlines (manifests have this limitation as
well, so it isn't a major concern). But the big concern here is the
necessity for readline(). Scanning for newlines means reading ahead and
that means extra buffer allocations and slicing (in Python) and this makes
performance suffer.
3) Filenames aren't compressed optimally. Filenames should be compressed well
since there is a lot of repeated data. However, since they are scattered
all over the stream (with revlog data in between), they typically fall
outside the window size of the compressor and don't compress.
4) It can only exchange stored based content, being able to exchange caches
too would be nice.
5) It is limited to a stream-based protocol and isn't suitable for an on-disk
format for general repository reading because the offset of individual file
entries requires scanning the entire file to find file records.
As part of enabling streaming clones to work in bundle2, #2 proved to have a
significant negative impact on performance. Since bundle2 provides the
opportunity to start fresh, Gregory Szorc figured he would take the
opportunity to invent a new streaming clone data format.
The new format devised in this series addresses #1, #2, and #4. It punts on #3
because it was complex without yielding a significant gain and on #5 because
devising a new store format that "packs" multiple revlogs into a single
"packed revlog" is massive scope bloat. However, this v2 format might be
suitable for streaming into a "packed revlog" with minimal processing. If it
works, great. If not, we can always invent stream format when it is needed.
This patch only introduces the bases of the format. We'll get it usable through
bundle2 first, then we'll extend the format in future patches to bring it to its
full potential (especially #4).
Mercurial allows you to customize output of commands through
templates. You can either pass in a template or select an existing
template-style from the command line, via the --template option.
You can customize output for any "log-like" command: log,
outgoing, incoming, tip, parents, and heads.
Some built-in styles are packaged with Mercurial. These can be listed
with :hg:`log --template list`. Example usage::
$ hg log -r1.0::1.1 --template changelog
A template is a piece of text, with markup to invoke variable
expansion::
$ hg log -r1 --template "{node}\n"
b56ce7b07c52de7d5fd79fb89701ea538af65746
Keywords
========
Strings in curly braces are called keywords. The availability of
keywords depends on the exact context of the templater. These
keywords are usually available for templating a log-like command:
.. keywordsmarker
The "date" keyword does not produce human-readable output. If you
want to use a date in your output, you can use a filter to process
it. Filters are functions which return a string based on the input
variable. Be sure to use the stringify filter first when you're
applying a string-input filter to a list-like input variable.
You can also use a chain of filters to get the desired output::
$ hg tip --template "{date|isodate}\n"
2008-08-21 18:22 +0000
Filters
=======
List of filters:
.. filtersmarker
Note that a filter is nothing more than a function call, i.e.
``expr|filter`` is equivalent to ``filter(expr)``.
Functions
=========
In addition to filters, there are some basic built-in functions:
.. functionsmarker
Operators
=========
We provide a limited set of infix arithmetic operations on integers::
+ for addition
- for subtraction
* for multiplication
/ for floor division (division rounded to integer nearest -infinity)
Division fulfills the law x = x / y + mod(x, y).
Also, for any expression that returns a list, there is a list operator::
expr % "{template}"
As seen in the above example, ``{template}`` is interpreted as a template.
To prevent it from being interpreted, you can use an escape character ``\{``
or a raw string prefix, ``r'...'``.
The dot operator can be used as a shorthand for accessing a sub item:
- ``expr.member`` is roughly equivalent to ``expr % '{member}'`` if ``expr``
returns a non-list/dict. The returned value is not stringified.
- ``dict.key`` is identical to ``get(dict, 'key')``.
Aliases
=======
New keywords and functions can be defined in the ``templatealias`` section of
a Mercurial configuration file::
<alias> = <definition>
Arguments of the form `a1`, `a2`, etc. are substituted from the alias into
the definition.
For example,
::
[templatealias]
r = rev
rn = "{r}:{node|short}"
leftpad(s, w) = pad(s, w, ' ', True)
defines two symbol aliases, ``r`` and ``rn``, and a function alias
``leftpad()``.
It's also possible to specify complete template strings, using the
``templates`` section. The syntax used is the general template string syntax.
For example,
::
[templates]
nodedate = "{node|short}: {date(date, "%Y-%m-%d")}\n"
defines a template, ``nodedate``, which can be called like::
$ hg log -r . -Tnodedate
A template defined in ``templates`` section can also be referenced from
another template::
$ hg log -r . -T "{rev} {nodedate}"
but be aware that the keywords cannot be overridden by templates. For example,
a template defined as ``templates.rev`` cannot be referenced as ``{rev}``.
A template defined in ``templates`` section may have sub templates which
are inserted before/after/between items::
[templates]
myjson = ' {dict(rev, node|short)|json}'
myjson:docheader = '\{\n'
myjson:docfooter = '\n}\n'
myjson:separator = ',\n'
Examples
========
Some sample command line templates:
- Format lists, e.g. files::
$ hg log -r 0 --template "files:\n{files % ' {file}\n'}"
- Join the list of files with a ", "::
$ hg log -r 0 --template "files: {join(files, ', ')}\n"
- Join the list of files ending with ".py" with a ", "::
$ hg log -r 0 --template "pythonfiles: {join(files('**.py'), ', ')}\n"
- Separate non-empty arguments by a " "::
$ hg log -r 0 --template "{separate(' ', node, bookmarks, tags}\n"
- Modify each line of a commit description::
$ hg log --template "{splitlines(desc) % '**** {line}\n'}"
- Format date::
$ hg log -r 0 --template "{date(date, '%Y')}\n"
- Display date in UTC::
$ hg log -r 0 --template "{localdate(date, 'UTC')|date}\n"
- Output the description set to a fill-width of 30::
$ hg log -r 0 --template "{fill(desc, 30)}"
- Use a conditional to test for the default branch::
$ hg log -r 0 --template "{ifeq(branch, 'default', 'on the main branch',
'on branch {branch}')}\n"
- Append a newline if not empty::
$ hg tip --template "{if(author, '{author}\n')}"
- Label the output for use with the color extension::
$ hg log -r 0 --template "{label('changeset.{phase}', node|short)}\n"
- Invert the firstline filter, i.e. everything but the first line::
$ hg log -r 0 --template "{sub(r'^.*\n?\n?', '', desc)}\n"
- Display the contents of the 'extra' field, one per line::
$ hg log -r 0 --template "{join(extras, '\n')}\n"
- Mark the active bookmark with '*'::
$ hg log --template "{bookmarks % '{bookmark}{ifeq(bookmark, active, '*')} '}\n"
- Find the previous release candidate tag, the distance and changes since the tag::
$ hg log -r . --template "{latesttag('re:^.*-rc$') % '{tag}, {changes}, {distance}'}\n"
- Mark the working copy parent with '@'::
$ hg log --template "{ifcontains(rev, revset('.'), '@')}\n"
- Show details of parent revisions::
$ hg log --template "{revset('parents(%d)', rev) % '{desc|firstline}\n'}"
- Show only commit descriptions that start with "template"::
$ hg log --template "{startswith('template', firstline(desc))}\n"
- Print the first word of each line of a commit message::
$ hg log --template "{word(0, desc)}\n"