Mercurial > hg
view mercurial/pvec.py @ 35569:964212780daf
rust: implementation of `hg`
This commit provides a mostly-working implementation of the
`hg` script in Rust along with scaffolding to support Rust in
the repository.
If you are familiar with Rust, the contents of the added rust/
directory should be pretty straightforward. We create an "hgcli"
package that implements a binary application to run Mercurial.
The output of this package is an "hg" binary.
Our Rust `hg` (henceforth "rhg") essentially is a port of the existing
`hg` Python script. The main difference is the creation of the embedded
CPython interpreter is handled by the binary itself instead of relying
on the shebang. In that sense, rhg is more similar to the "exe wrapper"
we currently use on Windows. However, unlike the exe wrapper, rhg does
not call the `hg` Python script. Instead, it uses the CPython APIs to
import mercurial modules and call appropriate functions. The amount of
code here is surprisingly small.
It is my intent to replace the existing C-based exe wrapper with rhg.
Preferably in the next Mercurial release. This should be achievable -
at least for some Mercurial distributions. The future/timeline for
rhg on other platforms is less clear. We already ship a hg.exe on
Windows. So if we get the quirks with Rust worked out, shipping a
Rust-based hg.exe should hopefully not be too contentious.
Now onto the implementation.
We're using python27-sys and the cpython crates for talking to the
CPython API. We currently don't use too much functionality of the
cpython crate and could have probably cut it out. However, it does
provide a reasonable abstraction over unsafe {} CPython function
calls. While we still have our fair share of those, at least we're
not dealing with too much refcounting, error checking, etc. So I
think the use of the cpython crate is justified. Plus, there is
not-yet-implemented functionality that could benefit from cpython. I
see our use of this crate only increasing.
The cpython and python27-sys crates are not without their issues.
The cpython crate didn't seem to account for the embedding use case
in its design. Instead, it seems to assume that you are building
a Python extension. It is making some questionable decisions around
certain CPython APIs. For example, it insists that
PyEval_ThreadsInitialized() is called and that the Python code
likely isn't the main thread in the underlying application. It
is also missing some functionality that is important for embedded
use cases (such as exporting the path to the Python interpreter
from its build script). After spending several hours trying to
wrangle python27-sys and cpython, I gave up and forked the project
on GitHub. Our Cargo.toml tracks this fork. I'm optimistic that
the upstream project will accept our contributions and we can
eventually unfork.
There is a non-trivial amount of code in our custom Cargo build
script. Our build.rs (which is called as part of building the hgcli
crate):
* Validates that the Python interpreter that was detected by the
python27-sys crate provides a shared library (we only support
shared library linking at this time - although this restriction
could be loosened).
* Validates that the Python is built with UCS-4 support. This ensures
maximum Unicode compatibility.
* Exports variables to the crate build allowing the built crate to e.g.
find the path to the Python interpreter.
The produced rhg should be considered alpha quality. There are several
known deficiencies. Many of these are documented with inline TODOs.
Probably the biggest limitation of rhg is that it assumes it is
running from the ./rust/target/<target> directory of a source
distribution. So, rhg is currently not very practical for real-world
use. But, if you can `cargo build` it, running the binary *should*
yield a working Mercurial CLI.
In order to support using rhg with the test harness, we needed to hack
up run-tests.py so the path to Mercurial's Python files is set properly.
The change is extremely hacky and is only intended to be a stop-gap
until the test harness gains first-class support for installing rhg.
This will likely occur after we support running rhg outside the
source directory.
Despite its officially alpha quality, rhg copes extremely well with
the test harness (at least on Linux). Using
`run-tests.py --with-hg ../rust/target/debug/hg`, I only encounter
the following failures:
* test-run-tests.t -- Warnings emitted about using an unexpected
Mercurial library. This is due to the hacky nature of setting the
Python directory when run-tests.py detected rhg.
* test-devel-warnings.t -- Expected stack trace missing frame for `hg`
(This is expected since we no longer have an `hg` script!)
* test-convert.t -- Test running `$PYTHON "$BINDIR"/hg`, which obviously
assumes `hg` is a Python script.
* test-merge-tools.t -- Same assumption about `hg` being executable with
Python.
* test-http-bad-server.t -- Seeing exit code 255 instead of 1 around
line 358.
* test-blackbox.t -- Exit code 255 instead of 1.
* test-basic.t -- Exit code 255 instead of 1.
It certainly looks like we have a bug around exit code handling. I
don't think it is severe enough to hold up review and landing of this
initial implementation. Perfect is the enemy of good.
Differential Revision: https://phab.mercurial-scm.org/D1581
author | Gregory Szorc <gregory.szorc@gmail.com> |
---|---|
date | Wed, 10 Jan 2018 08:53:22 -0800 |
parents | 4462a981e8df |
children | e7aa113b14f7 |
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# pvec.py - probabilistic vector clocks for Mercurial # # Copyright 2012 Matt Mackall <mpm@selenic.com> # # This software may be used and distributed according to the terms of the # GNU General Public License version 2 or any later version. ''' A "pvec" is a changeset property based on the theory of vector clocks that can be compared to discover relatedness without consulting a graph. This can be useful for tasks like determining how a disconnected patch relates to a repository. Currently a pvec consist of 448 bits, of which 24 are 'depth' and the remainder are a bit vector. It is represented as a 70-character base85 string. Construction: - a root changeset has a depth of 0 and a bit vector based on its hash - a normal commit has a changeset where depth is increased by one and one bit vector bit is flipped based on its hash - a merge changeset pvec is constructed by copying changes from one pvec into the other to balance its depth Properties: - for linear changes, difference in depth is always <= hamming distance - otherwise, changes are probably divergent - when hamming distance is < 200, we can reliably detect when pvecs are near Issues: - hamming distance ceases to work over distances of ~ 200 - detecting divergence is less accurate when the common ancestor is very close to either revision or total distance is high - this could probably be improved by modeling the relation between delta and hdist Uses: - a patch pvec can be used to locate the nearest available common ancestor for resolving conflicts - ordering of patches can be established without a DAG - two head pvecs can be compared to determine whether push/pull/merge is needed and approximately how many changesets are involved - can be used to find a heuristic divergence measure between changesets on different branches ''' from __future__ import absolute_import from .node import nullrev from . import ( util, ) _size = 448 # 70 chars b85-encoded _bytes = _size / 8 _depthbits = 24 _depthbytes = _depthbits / 8 _vecbytes = _bytes - _depthbytes _vecbits = _vecbytes * 8 _radius = (_vecbits - 30) / 2 # high probability vectors are related def _bin(bs): '''convert a bytestring to a long''' v = 0 for b in bs: v = v * 256 + ord(b) return v def _str(v, l): bs = "" for p in xrange(l): bs = chr(v & 255) + bs v >>= 8 return bs def _split(b): '''depth and bitvec''' return _bin(b[:_depthbytes]), _bin(b[_depthbytes:]) def _join(depth, bitvec): return _str(depth, _depthbytes) + _str(bitvec, _vecbytes) def _hweight(x): c = 0 while x: if x & 1: c += 1 x >>= 1 return c _htab = [_hweight(x) for x in xrange(256)] def _hamming(a, b): '''find the hamming distance between two longs''' d = a ^ b c = 0 while d: c += _htab[d & 0xff] d >>= 8 return c def _mergevec(x, y, c): # Ideally, this function would be x ^ y ^ ancestor, but finding # ancestors is a nuisance. So instead we find the minimal number # of changes to balance the depth and hamming distance d1, v1 = x d2, v2 = y if d1 < d2: d1, d2, v1, v2 = d2, d1, v2, v1 hdist = _hamming(v1, v2) ddist = d1 - d2 v = v1 m = v1 ^ v2 # mask of different bits i = 1 if hdist > ddist: # if delta = 10 and hdist = 100, then we need to go up 55 steps # to the ancestor and down 45 changes = (hdist - ddist + 1) / 2 else: # must make at least one change changes = 1 depth = d1 + changes # copy changes from v2 if m: while changes: if m & i: v ^= i changes -= 1 i <<= 1 else: v = _flipbit(v, c) return depth, v def _flipbit(v, node): # converting bit strings to longs is slow bit = (hash(node) & 0xffffffff) % _vecbits return v ^ (1<<bit) def ctxpvec(ctx): '''construct a pvec for ctx while filling in the cache''' r = ctx.repo() if not util.safehasattr(r, "_pveccache"): r._pveccache = {} pvc = r._pveccache if ctx.rev() not in pvc: cl = r.changelog for n in xrange(ctx.rev() + 1): if n not in pvc: node = cl.node(n) p1, p2 = cl.parentrevs(n) if p1 == nullrev: # start with a 'random' vector at root pvc[n] = (0, _bin((node * 3)[:_vecbytes])) elif p2 == nullrev: d, v = pvc[p1] pvc[n] = (d + 1, _flipbit(v, node)) else: pvc[n] = _mergevec(pvc[p1], pvc[p2], node) bs = _join(*pvc[ctx.rev()]) return pvec(util.b85encode(bs)) class pvec(object): def __init__(self, hashorctx): if isinstance(hashorctx, str): self._bs = hashorctx self._depth, self._vec = _split(util.b85decode(hashorctx)) else: self._vec = ctxpvec(hashorctx) def __str__(self): return self._bs def __eq__(self, b): return self._vec == b._vec and self._depth == b._depth def __lt__(self, b): delta = b._depth - self._depth if delta < 0: return False # always correct if _hamming(self._vec, b._vec) > delta: return False return True def __gt__(self, b): return b < self def __or__(self, b): delta = abs(b._depth - self._depth) if _hamming(self._vec, b._vec) <= delta: return False return True def __sub__(self, b): if self | b: raise ValueError("concurrent pvecs") return self._depth - b._depth def distance(self, b): d = abs(b._depth - self._depth) h = _hamming(self._vec, b._vec) return max(d, h) def near(self, b): dist = abs(b.depth - self._depth) if dist > _radius or _hamming(self._vec, b._vec) > _radius: return False