annotate hgext/infinitepush/fileindexapi.py @ 40021:c537144fdbef

wireprotov2: support response caching

One of the things I've learned from managing VCS servers over the
years is that they are hard to scale. It is well known that some
companies have very beefy (read: very expensive) servers to power
their VCS needs. It is also known that specialized servers for
various VCS exist in order to facilitate scaling servers. (Mercurial
is in this boat.)

One of the aspects that make a VCS server hard to scale is the
high CPU load incurred by constant client clone/pull operations.

To alleviate the scaling pain associated with data retrieval
operations, I want to integrate caching into the Mercurial wire
protocol server as robustly as possible such that servers can
aggressively cache responses and defer as much server load as
possible.

This commit represents the initial implementation of a general
caching layer in wire protocol version 2.

We define a new interface and behavior for a wire protocol cacher
in repository.py. (This is probably where a reviewer should look
first to understand what is going on.)

The bulk of the added code is in wireprotov2server.py, where we
define how a command can opt in to being cached and integrate
caching into command dispatching.

From a very high-level:

* A command can declare itself as cacheable by providing a callable
  that can be used to derive a cache key.
* At dispatch time, if a command is cacheable, we attempt to
  construct a cacher and use it for serving the request and/or
  caching the request.
* The dispatch layer handles the bulk of the business logic for
  caching, making cachers mostly "dumb content stores."
* The mechanism for invalidating cached entries (one of the harder
  parts about caching in general) is by varying the cache key when
  state changes. As such, cachers don't need to be concerned with
  cache invalidation.

Initially, we've hooked up support for caching "manifestdata" and
"filedata" commands. These are the simplest to cache, as they should
be immutable over time. Caching of commands related to changeset
data is a bit harder (because cache validation is impacted by
changes to bookmarks, phases, etc). This will be implemented later.
(Strictly speaking, censoring a file should invalidate caches. I've
added an inline TODO to track this edge case.)

To prove it works, this commit implements a test-only extension
providing in-memory caching backed by an lrucachedict. A new test
showing this extension behaving properly is added. FWIW, the
cacher is ~50 lines of code, demonstrating the relative ease with
which a cache can be added to a server.

While the test cacher is not suitable for production workloads, just
for kicks I performed a clone of just the changeset and manifest data
for the mozilla-unified repository. With a fully warmed cache (of just
the manifest data since changeset data is not cached), server-side
CPU usage dropped from ~73s to ~28s. That's pretty significant and
demonstrates the potential that response caching has on server
scalability!

Differential Revision: https://phab.mercurial-scm.org/D4773

# Infinite push
#
# Copyright 2016 Facebook, Inc.
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
"""
    [infinitepush]
    # Server-side option. Used only if indextype=disk.
    # Filesystem path to the index store
    indexpath = PATH
"""

from __future__ import absolute_import

import os

from mercurial import util

from mercurial.utils import stringutil

from . import indexapi

class fileindexapi(indexapi.indexapi):
    def __init__(self, repo):
        super(fileindexapi, self).__init__()
        self._repo = repo
        root = repo.ui.config('infinitepush', 'indexpath')
        if not root:
            root = os.path.join('scratchbranches', 'index')

        self._nodemap = os.path.join(root, 'nodemap')
        self._bookmarkmap = os.path.join(root, 'bookmarkmap')
        self._metadatamap = os.path.join(root, 'nodemetadatamap')
        self._lock = None

    def __enter__(self):
        self._lock = self._repo.wlock()
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        if self._lock:
            self._lock.__exit__(exc_type, exc_val, exc_tb)

    def addbundle(self, bundleid, nodesctx):
        for node in nodesctx:
            nodepath = os.path.join(self._nodemap, node.hex())
            self._write(nodepath, bundleid)

    def addbookmark(self, bookmark, node):
        bookmarkpath = os.path.join(self._bookmarkmap, bookmark)
        self._write(bookmarkpath, node)

    def addmanybookmarks(self, bookmarks):
        for bookmark, node in bookmarks.items():
            self.addbookmark(bookmark, node)

    def deletebookmarks(self, patterns):
        for pattern in patterns:
            for bookmark, _ in self._listbookmarks(pattern):
                bookmarkpath = os.path.join(self._bookmarkmap, bookmark)
                self._delete(bookmarkpath)

    def getbundle(self, node):
        nodepath = os.path.join(self._nodemap, node)
        return self._read(nodepath)

    def getnode(self, bookmark):
        bookmarkpath = os.path.join(self._bookmarkmap, bookmark)
        return self._read(bookmarkpath)

    def getbookmarks(self, query):
        return dict(self._listbookmarks(query))

    def saveoptionaljsonmetadata(self, node, jsonmetadata):
        vfs = self._repo.vfs
        vfs.write(os.path.join(self._metadatamap, node), jsonmetadata)

    def _listbookmarks(self, pattern):
        if pattern.endswith('*'):
            pattern = 're:^' + pattern[:-1] + '.*'
        kind, pat, matcher = stringutil.stringmatcher(pattern)
        prefixlen = len(self._bookmarkmap) + 1
        for dirpath, _, books in self._repo.vfs.walk(self._bookmarkmap):
            for book in books:
                bookmark = os.path.join(dirpath, book)[prefixlen:]
                bookmark = util.pconvert(bookmark)
                if not matcher(bookmark):
                    continue
                yield bookmark, self._read(os.path.join(dirpath, book))

    def _write(self, path, value):
        vfs = self._repo.vfs
        dirname = vfs.dirname(path)
        if not vfs.exists(dirname):
            vfs.makedirs(dirname)

        vfs.write(path, value)

    def _read(self, path):
        vfs = self._repo.vfs
        if not vfs.exists(path):
            return None
        return vfs.read(path)

    def _delete(self, path):
        vfs = self._repo.vfs
        if not vfs.exists(path):
            return
        return vfs.unlink(path)