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
# destutil.py - Mercurial utility function for command destination
#
# Copyright Matt Mackall <mpm@selenic.com> and other
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
from __future__ import absolute_import
from .i18n import _
from . import (
bookmarks,
error,
obsutil,
scmutil,
stack
)
def orphanpossibledestination(repo, rev):
"""Return all changesets that may be a new parent for orphan `rev`.
This function works fine on non-orphan revisions, it's just silly
because there's no destination implied by obsolete markers, so
it'll return nothing.
"""
tonode = repo.changelog.node
parents = repo.changelog.parentrevs
torev = repo.changelog.rev
dest = set()
tovisit = list(parents(rev))
while tovisit:
r = tovisit.pop()
succsets = obsutil.successorssets(repo, tonode(r))
if not succsets:
# if there are no successors for r, r was probably pruned
# and we should walk up to r's parents to try and find
# some successors.
tovisit.extend(parents(r))
else:
# We should probably pick only one destination from split
# (case where '1 < len(ss)'), This could be the currently
# tipmost, but the correct result is less clear when
# results of the split have been moved such that they
# reside on multiple branches.
for ss in succsets:
for n in ss:
dr = torev(n)
if dr != -1:
dest.add(dr)
return dest
def _destupdateobs(repo, clean):
"""decide of an update destination from obsolescence markers"""
node = None
wc = repo[None]
p1 = wc.p1()
movemark = None
if p1.obsolete() and not p1.children():
# allow updating to successors
successors = obsutil.successorssets(repo, p1.node())
# behavior of certain cases is as follows,
#
# divergent changesets: update to highest rev, similar to what
# is currently done when there are more than one head
# (i.e. 'tip')
#
# replaced changesets: same as divergent except we know there
# is no conflict
#
# pruned changeset: no update is done; though, we could
# consider updating to the first non-obsolete parent,
# similar to what is current done for 'hg prune'
if successors:
# flatten the list here handles both divergent (len > 1)
# and the usual case (len = 1)
successors = [n for sub in successors for n in sub]
# get the max revision for the given successors set,
# i.e. the 'tip' of a set
node = repo.revs('max(%ln)', successors).first()
if bookmarks.isactivewdirparent(repo):
movemark = repo['.'].node()
return node, movemark, None
def _destupdatebook(repo, clean):
"""decide on an update destination from active bookmark"""
# we also move the active bookmark, if any
node = None
activemark, movemark = bookmarks.calculateupdate(repo.ui, repo)
if activemark is not None:
node = repo._bookmarks[activemark]
return node, movemark, activemark
def _destupdatebranch(repo, clean):
"""decide on an update destination from current branch
This ignores closed branch heads.
"""
wc = repo[None]
movemark = node = None
currentbranch = wc.branch()
if clean:
currentbranch = repo['.'].branch()
if currentbranch in repo.branchmap():
heads = repo.branchheads(currentbranch)
if heads:
node = repo.revs('max(.::(%ln))', heads).first()
if bookmarks.isactivewdirparent(repo):
movemark = repo['.'].node()
elif currentbranch == 'default' and not wc.p1():
# "null" parent belongs to "default" branch, but it doesn't exist, so
# update to the tipmost non-closed branch head
node = repo.revs('max(head() and not closed())').first()
else:
node = repo['.'].node()
return node, movemark, None
def _destupdatebranchfallback(repo, clean):
"""decide on an update destination from closed heads in current branch"""
wc = repo[None]
currentbranch = wc.branch()
movemark = None
if currentbranch in repo.branchmap():
# here, all descendant branch heads are closed
heads = repo.branchheads(currentbranch, closed=True)
assert heads, "any branch has at least one head"
node = repo.revs('max(.::(%ln))', heads).first()
assert node is not None, ("any revision has at least "
"one descendant branch head")
if bookmarks.isactivewdirparent(repo):
movemark = repo['.'].node()
else:
# here, no "default" branch, and all branches are closed
node = repo.lookup('tip')
assert node is not None, "'tip' exists even in empty repository"
return node, movemark, None
# order in which each step should be evaluated
# steps are run until one finds a destination
destupdatesteps = ['evolution', 'bookmark', 'branch', 'branchfallback']
# mapping to ease extension overriding steps.
destupdatestepmap = {'evolution': _destupdateobs,
'bookmark': _destupdatebook,
'branch': _destupdatebranch,
'branchfallback': _destupdatebranchfallback,
}
def destupdate(repo, clean=False):
"""destination for bare update operation
return (rev, movemark, activemark)
- rev: the revision to update to,
- movemark: node to move the active bookmark from
(cf bookmark.calculate update),
- activemark: a bookmark to activate at the end of the update.
"""
node = movemark = activemark = None
for step in destupdatesteps:
node, movemark, activemark = destupdatestepmap[step](repo, clean)
if node is not None:
break
rev = repo[node].rev()
return rev, movemark, activemark
msgdestmerge = {
# too many matching divergent bookmark
'toomanybookmarks':
{'merge':
(_("multiple matching bookmarks to merge -"
" please merge with an explicit rev or bookmark"),
_("run 'hg heads' to see all heads")),
'rebase':
(_("multiple matching bookmarks to rebase -"
" please rebase to an explicit rev or bookmark"),
_("run 'hg heads' to see all heads")),
},
# no other matching divergent bookmark
'nootherbookmarks':
{'merge':
(_("no matching bookmark to merge - "
"please merge with an explicit rev or bookmark"),
_("run 'hg heads' to see all heads")),
'rebase':
(_("no matching bookmark to rebase - "
"please rebase to an explicit rev or bookmark"),
_("run 'hg heads' to see all heads")),
},
# branch have too many unbookmarked heads, no obvious destination
'toomanyheads':
{'merge':
(_("branch '%s' has %d heads - please merge with an explicit rev"),
_("run 'hg heads .' to see heads")),
'rebase':
(_("branch '%s' has %d heads - please rebase to an explicit rev"),
_("run 'hg heads .' to see heads")),
},
# branch have no other unbookmarked heads
'bookmarkedheads':
{'merge':
(_("heads are bookmarked - please merge with an explicit rev"),
_("run 'hg heads' to see all heads")),
'rebase':
(_("heads are bookmarked - please rebase to an explicit rev"),
_("run 'hg heads' to see all heads")),
},
# branch have just a single heads, but there is other branches
'nootherbranchheads':
{'merge':
(_("branch '%s' has one head - please merge with an explicit rev"),
_("run 'hg heads' to see all heads")),
'rebase':
(_("branch '%s' has one head - please rebase to an explicit rev"),
_("run 'hg heads' to see all heads")),
},
# repository have a single head
'nootherheads':
{'merge':
(_('nothing to merge'),
None),
'rebase':
(_('nothing to rebase'),
None),
},
# repository have a single head and we are not on it
'nootherheadsbehind':
{'merge':
(_('nothing to merge'),
_("use 'hg update' instead")),
'rebase':
(_('nothing to rebase'),
_("use 'hg update' instead")),
},
# We are not on a head
'notatheads':
{'merge':
(_('working directory not at a head revision'),
_("use 'hg update' or merge with an explicit revision")),
'rebase':
(_('working directory not at a head revision'),
_("use 'hg update' or rebase to an explicit revision"))
},
'emptysourceset':
{'merge':
(_('source set is empty'),
None),
'rebase':
(_('source set is empty'),
None),
},
'multiplebranchessourceset':
{'merge':
(_('source set is rooted in multiple branches'),
None),
'rebase':
(_('rebaseset is rooted in multiple named branches'),
_('specify an explicit destination with --dest')),
},
}
def _destmergebook(repo, action='merge', sourceset=None, destspace=None):
"""find merge destination in the active bookmark case"""
node = None
bmheads = bookmarks.headsforactive(repo)
curhead = repo._bookmarks[repo._activebookmark]
if len(bmheads) == 2:
if curhead == bmheads[0]:
node = bmheads[1]
else:
node = bmheads[0]
elif len(bmheads) > 2:
msg, hint = msgdestmerge['toomanybookmarks'][action]
raise error.ManyMergeDestAbort(msg, hint=hint)
elif len(bmheads) <= 1:
msg, hint = msgdestmerge['nootherbookmarks'][action]
raise error.NoMergeDestAbort(msg, hint=hint)
assert node is not None
return node
def _destmergebranch(repo, action='merge', sourceset=None, onheadcheck=True,
destspace=None):
"""find merge destination based on branch heads"""
node = None
if sourceset is None:
sourceset = [repo[repo.dirstate.p1()].rev()]
branch = repo.dirstate.branch()
elif not sourceset:
msg, hint = msgdestmerge['emptysourceset'][action]
raise error.NoMergeDestAbort(msg, hint=hint)
else:
branch = None
for ctx in repo.set('roots(%ld::%ld)', sourceset, sourceset):
if branch is not None and ctx.branch() != branch:
msg, hint = msgdestmerge['multiplebranchessourceset'][action]
raise error.ManyMergeDestAbort(msg, hint=hint)
branch = ctx.branch()
bheads = repo.branchheads(branch)
onhead = repo.revs('%ld and %ln', sourceset, bheads)
if onheadcheck and not onhead:
# Case A: working copy if not on a head. (merge only)
#
# This is probably a user mistake We bailout pointing at 'hg update'
if len(repo.heads()) <= 1:
msg, hint = msgdestmerge['nootherheadsbehind'][action]
else:
msg, hint = msgdestmerge['notatheads'][action]
raise error.Abort(msg, hint=hint)
# remove heads descendants of source from the set
bheads = list(repo.revs('%ln - (%ld::)', bheads, sourceset))
# filters out bookmarked heads
nbhs = list(repo.revs('%ld - bookmark()', bheads))
if destspace is not None:
# restrict search space
# used in the 'hg pull --rebase' case, see issue 5214.
nbhs = list(repo.revs('%ld and %ld', destspace, nbhs))
if len(nbhs) > 1:
# Case B: There is more than 1 other anonymous heads
#
# This means that there will be more than 1 candidate. This is
# ambiguous. We abort asking the user to pick as explicit destination
# instead.
msg, hint = msgdestmerge['toomanyheads'][action]
msg %= (branch, len(bheads) + 1)
raise error.ManyMergeDestAbort(msg, hint=hint)
elif not nbhs:
# Case B: There is no other anonymous heads
#
# This means that there is no natural candidate to merge with.
# We abort, with various messages for various cases.
if bheads:
msg, hint = msgdestmerge['bookmarkedheads'][action]
elif len(repo.heads()) > 1:
msg, hint = msgdestmerge['nootherbranchheads'][action]
msg %= branch
elif not onhead:
# if 'onheadcheck == False' (rebase case),
# this was not caught in Case A.
msg, hint = msgdestmerge['nootherheadsbehind'][action]
else:
msg, hint = msgdestmerge['nootherheads'][action]
raise error.NoMergeDestAbort(msg, hint=hint)
else:
node = nbhs[0]
assert node is not None
return node
def destmerge(repo, action='merge', sourceset=None, onheadcheck=True,
destspace=None):
"""return the default destination for a merge
(or raise exception about why it can't pick one)
:action: the action being performed, controls emitted error message
"""
# destspace is here to work around issues with `hg pull --rebase` see
# issue5214 for details
if repo._activebookmark:
node = _destmergebook(repo, action=action, sourceset=sourceset,
destspace=destspace)
else:
node = _destmergebranch(repo, action=action, sourceset=sourceset,
onheadcheck=onheadcheck, destspace=destspace)
return repo[node].rev()
def desthistedit(ui, repo):
"""Default base revision to edit for `hg histedit`."""
default = ui.config('histedit', 'defaultrev')
if default is None:
revs = stack.getstack(repo)
elif default:
revs = scmutil.revrange(repo, [default])
if revs:
# Take the first revision of the revset as the root
return revs.min()
return None
def stackbase(ui, repo):
revs = stack.getstack(repo)
return revs.first() if revs else None
def _statusotherbook(ui, repo):
bmheads = bookmarks.headsforactive(repo)
curhead = repo._bookmarks[repo._activebookmark]
if repo.revs('%n and parents()', curhead):
# we are on the active bookmark
bmheads = [b for b in bmheads if curhead != b]
if bmheads:
msg = _('%i other divergent bookmarks for "%s"\n')
ui.status(msg % (len(bmheads), repo._activebookmark))
def _statusotherbranchheads(ui, repo):
currentbranch = repo.dirstate.branch()
allheads = repo.branchheads(currentbranch, closed=True)
heads = repo.branchheads(currentbranch)
if repo.revs('%ln and parents()', allheads):
# we are on a head, even though it might be closed
#
# on closed otherheads
# ========= ==========
# o 0 all heads for current branch are closed
# N only descendant branch heads are closed
# x 0 there is only one non-closed branch head
# N there are some non-closed branch heads
# ========= ==========
otherheads = repo.revs('%ln - parents()', heads)
if repo['.'].closesbranch():
ui.warn(_('no open descendant heads on branch "%s", '
'updating to a closed head\n') %
(currentbranch))
if otherheads:
ui.warn(_("(committing will reopen the head, "
"use 'hg heads .' to see %i other heads)\n") %
(len(otherheads)))
else:
ui.warn(_('(committing will reopen branch "%s")\n') %
(currentbranch))
elif otherheads:
curhead = repo['.']
ui.status(_('updated to "%s: %s"\n') % (curhead,
curhead.description().split('\n')[0]))
ui.status(_('%i other heads for branch "%s"\n') %
(len(otherheads), currentbranch))
def statusotherdests(ui, repo):
"""Print message about other head"""
# XXX we should probably include a hint:
# - about what to do
# - how to see such heads
if repo._activebookmark:
_statusotherbook(ui, repo)
else:
_statusotherbranchheads(ui, repo)