view tests/test-histedit-outgoing.t @ 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

  $ cat >> $HGRCPATH <<EOF
  > [extensions]
  > histedit=
  > EOF

  $ initrepos ()
  > {
  >     hg init r
  >     cd r
  >     for x in a b c ; do
  >         echo $x > $x
  >         hg add $x
  >         hg ci -m $x
  >     done
  >     cd ..
  >     hg clone r r2 | grep -v updating
  >     cd r2
  >     for x in d e f ; do
  >         echo $x > $x
  >         hg add $x
  >         hg ci -m $x
  >     done
  >     cd ..
  >     hg init r3
  >     cd r3
  >     for x in g h i ; do
  >         echo $x > $x
  >         hg add $x
  >         hg ci -m $x
  >     done
  >     cd ..
  > }

  $ initrepos
  3 files updated, 0 files merged, 0 files removed, 0 files unresolved

show the edit commands offered by outgoing
  $ cd r2
  $ HGEDITOR=cat hg histedit --outgoing ../r | grep -v comparing | grep -v searching
  pick 055a42cdd887 3 d
  pick e860deea161a 4 e
  pick 652413bf663e 5 f
  
  # Edit history between 055a42cdd887 and 652413bf663e
  #
  # Commits are listed from least to most recent
  #
  # You can reorder changesets by reordering the lines
  #
  # Commands:
  #
  #  e, edit = use commit, but stop for amending
  #  m, mess = edit commit message without changing commit content
  #  p, pick = use commit
  #  b, base = checkout changeset and apply further changesets from there
  #  d, drop = remove commit from history
  #  f, fold = use commit, but combine it with the one above
  #  r, roll = like fold, but discard this commit's description and date
  #
  $ cd ..

show the error from unrelated repos
  $ cd r3
  $ HGEDITOR=cat hg histedit --outgoing ../r | grep -v comparing | grep -v searching
  abort: repository is unrelated
  [1]
  $ cd ..

show the error from unrelated repos
  $ cd r3
  $ HGEDITOR=cat hg histedit --force --outgoing ../r
  comparing with ../r
  searching for changes
  warning: repository is unrelated
  pick 2a4042b45417 0 g
  pick 68c46b4927ce 1 h
  pick 51281e65ba79 2 i
  
  # Edit history between 2a4042b45417 and 51281e65ba79
  #
  # Commits are listed from least to most recent
  #
  # You can reorder changesets by reordering the lines
  #
  # Commands:
  #
  #  e, edit = use commit, but stop for amending
  #  m, mess = edit commit message without changing commit content
  #  p, pick = use commit
  #  b, base = checkout changeset and apply further changesets from there
  #  d, drop = remove commit from history
  #  f, fold = use commit, but combine it with the one above
  #  r, roll = like fold, but discard this commit's description and date
  #
  $ cd ..

test sensitivity to branch in URL:

  $ cd r2
  $ hg -q update 2
  $ hg -q branch foo
  $ hg commit -m 'create foo branch'
  $ HGEDITOR=cat hg histedit --outgoing '../r#foo' | grep -v comparing | grep -v searching
  pick f26599ee3441 6 create foo branch
  
  # Edit history between f26599ee3441 and f26599ee3441
  #
  # Commits are listed from least to most recent
  #
  # You can reorder changesets by reordering the lines
  #
  # Commands:
  #
  #  e, edit = use commit, but stop for amending
  #  m, mess = edit commit message without changing commit content
  #  p, pick = use commit
  #  b, base = checkout changeset and apply further changesets from there
  #  d, drop = remove commit from history
  #  f, fold = use commit, but combine it with the one above
  #  r, roll = like fold, but discard this commit's description and date
  #

test to check number of roots in outgoing revisions

  $ hg -q outgoing -G --template '{node|short}({branch})' '../r'
  @  f26599ee3441(foo)
  
  o  652413bf663e(default)
  |
  o  e860deea161a(default)
  |
  o  055a42cdd887(default)
  
  $ HGEDITOR=cat hg -q histedit --outgoing '../r'
  abort: there are ambiguous outgoing revisions
  (see 'hg help histedit' for more detail)
  [255]

  $ hg -q update -C 2
  $ echo aa >> a
  $ hg -q commit -m 'another head on default'
  $ hg -q outgoing -G --template '{node|short}({branch})' '../r#default'
  @  3879dc049647(default)
  
  o  652413bf663e(default)
  |
  o  e860deea161a(default)
  |
  o  055a42cdd887(default)
  
  $ HGEDITOR=cat hg -q histedit --outgoing '../r#default'
  abort: there are ambiguous outgoing revisions
  (see 'hg help histedit' for more detail)
  [255]

  $ cd ..