view tests/test-show-work.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]
  > show =
  > EOF

  $ hg init repo0
  $ cd repo0

Command works on an empty repo

  $ hg show work

Single draft changeset shown

  $ echo 0 > foo
  $ hg -q commit -A -m 'commit 0'

  $ hg show work
  @  9f17 commit 0

Even when it isn't the wdir

  $ hg -q up null

  $ hg show work
  o  9f17 commit 0

Single changeset is still there when public because it is a head

  $ hg phase --public -r 0
  $ hg show work
  o  9f17 commit 0

A draft child will show both it and public parent

  $ hg -q up 0
  $ echo 1 > foo
  $ hg commit -m 'commit 1'

  $ hg show work
  @  181c commit 1
  o  9f17 commit 0

Multiple draft children will be shown

  $ echo 2 > foo
  $ hg commit -m 'commit 2'

  $ hg show work
  @  128c commit 2
  o  181c commit 1
  o  9f17 commit 0

Bumping first draft changeset to public will hide its parent

  $ hg phase --public -r 1
  $ hg show work
  @  128c commit 2
  o  181c commit 1
  ~

Multiple DAG heads will be shown

  $ hg -q up -r 1
  $ echo 3 > foo
  $ hg commit -m 'commit 3'
  created new head

  $ hg show work
  @  f0ab commit 3
  | o  128c commit 2
  |/
  o  181c commit 1
  ~

Even when wdir is something else

  $ hg -q up null

  $ hg show work
  o  f0ab commit 3
  | o  128c commit 2
  |/
  o  181c commit 1
  ~

Draft child shows public head (multiple heads)

  $ hg -q up 0
  $ echo 4 > foo
  $ hg commit -m 'commit 4'
  created new head

  $ hg show work
  @  668c commit 4
  | o  f0ab commit 3
  | | o  128c commit 2
  | |/
  | o  181c commit 1
  |/
  o  9f17 commit 0

  $ cd ..

Branch name appears in output

  $ hg init branches
  $ cd branches
  $ echo 0 > foo
  $ hg -q commit -A -m 'commit 0'
  $ echo 1 > foo
  $ hg commit -m 'commit 1'
  $ echo 2 > foo
  $ hg commit -m 'commit 2'
  $ hg phase --public -r .
  $ hg -q up -r 1
  $ hg branch mybranch
  marked working directory as branch mybranch
  (branches are permanent and global, did you want a bookmark?)
  $ echo 3 > foo
  $ hg commit -m 'commit 3'
  $ echo 4 > foo
  $ hg commit -m 'commit 4'

  $ hg show work
  @  f8dd (mybranch) commit 4
  o  90cf (mybranch) commit 3
  | o  128c commit 2
  |/
  o  181c commit 1
  ~

  $ cd ..

Bookmark name appears in output

  $ hg init bookmarks
  $ cd bookmarks
  $ echo 0 > foo
  $ hg -q commit -A -m 'commit 0'
  $ echo 1 > foo
  $ hg commit -m 'commit 1'
  $ echo 2 > foo
  $ hg commit -m 'commit 2'
  $ hg phase --public -r .
  $ hg bookmark @
  $ hg -q up -r 1
  $ echo 3 > foo
  $ hg commit -m 'commit 3'
  created new head
  $ echo 4 > foo
  $ hg commit -m 'commit 4'
  $ hg bookmark mybook

  $ hg show work
  @  cac8 (mybook) commit 4
  o  f0ab commit 3
  | o  128c (@) commit 2
  |/
  o  181c commit 1
  ~

  $ cd ..

Tags are rendered

  $ hg init tags
  $ cd tags
  $ echo 0 > foo
  $ hg -q commit -A -m 'commit 1'
  $ echo 1 > foo
  $ hg commit -m 'commit 2'
  $ hg tag 0.1
  $ hg phase --public -r .
  $ echo 2 > foo
  $ hg commit -m 'commit 3'
  $ hg tag 0.2

  $ hg show work
  @  3758 Added tag 0.2 for changeset 6379c25b76f1
  o  6379 (0.2) commit 3
  o  a2ad Added tag 0.1 for changeset 6a75536ea0b1
  ~

  $ cd ..

Multiple names on same changeset render properly

  $ hg init multiplenames
  $ cd multiplenames
  $ echo 0 > foo
  $ hg -q commit -A -m 'commit 1'
  $ hg phase --public -r .
  $ hg branch mybranch
  marked working directory as branch mybranch
  (branches are permanent and global, did you want a bookmark?)
  $ hg bookmark mybook
  $ echo 1 > foo
  $ hg commit -m 'commit 2'

  $ hg show work
  @  3483 (mybook) (mybranch) commit 2
  o  97fc commit 1

Multiple bookmarks on same changeset render properly

  $ hg book mybook2
  $ hg show work
  @  3483 (mybook mybook2) (mybranch) commit 2
  o  97fc commit 1

  $ cd ..

Extra namespaces are rendered

  $ hg init extranamespaces
  $ cd extranamespaces
  $ echo 0 > foo
  $ hg -q commit -A -m 'commit 1'
  $ hg phase --public -r .
  $ echo 1 > foo
  $ hg commit -m 'commit 2'
  $ echo 2 > foo
  $ hg commit -m 'commit 3'

  $ hg --config extensions.revnames=$TESTDIR/revnamesext.py show work
  @  32f3 (r2) commit 3
  o  6a75 (r1) commit 2
  o  97fc (r0) commit 1

Obsolescence information appears in labels.

  $ cat >> .hg/hgrc << EOF
  > [experimental]
  > evolution=createmarkers
  > EOF
  $ hg debugobsolete `hg log -r 'desc("commit 2")' -T "{node}"`
  obsoleted 1 changesets
  1 new orphan changesets
  $ hg show work --color=debug
  @  [log.changeset changeset.draft changeset.unstable instability.orphan|32f3] [log.description|commit 3]
  x  [log.changeset changeset.draft changeset.obsolete|6a75] [log.description|commit 2]
  ~

  $ cd ..

Prefix collision on hashes increases shortest node length

  $ hg init hashcollision
  $ cd hashcollision
  $ echo 0 > a
  $ hg -q commit -Am 0
  $ for i in 17 1057 2857 4025; do
  >   hg -q up 0
  >   echo $i > a
  >   hg -q commit -m $i
  >   echo 0 > a
  >   hg commit -m "$i commit 2"
  > done

  $ hg show work
  @  cfd04 4025 commit 2
  o  c562d 4025
  | o  08048 2857 commit 2
  | o  c5623 2857
  |/
  | o  6a6b6 1057 commit 2
  | o  c5625 1057
  |/
  | o  96b4e 17 commit 2
  | o  11424 17
  |/
  o  b4e73 0

  $ cd ..