view tests/test-mq-safety.t @ 26623:5a95fe44121d

clonebundles: support for seeding clones from pre-generated bundles

Cloning can be an expensive operation for servers because the server
generates a bundle from existing repository data at request time. For
a large repository like mozilla-central, this consumes 4+ minutes
of CPU time on the server. It also results in significant network
utilization. Multiplied by hundreds or even thousands of clients and
the ensuing load can result in difficulties scaling the Mercurial server.

Despite generation of bundles being deterministic until the next
changeset is added, the generation of bundles to service a clone request
is not cached. Each clone thus performs redundant work. This is
wasteful.

This patch introduces the "clonebundles" extension and related
client-side functionality to help alleviate this deficiency. The
client-side feature is behind an experimental flag and is not enabled by
default.

It works as follows:

1) Server operator generates a bundle and makes it available on a
   server (likely HTTP).
2) Server operator defines the URL of a bundle file in a
   .hg/clonebundles.manifest file.
3) Client `hg clone`ing sees the server is advertising bundle URLs.
4) Client fetches and applies the advertised bundle.
5) Client performs equivalent of `hg pull` to fetch changes made since
   the bundle was created.

Essentially, the server performs the expensive work of generating a
bundle once and all subsequent clones fetch a static file from
somewhere. Scaling static file serving is a much more manageable
problem than scaling a Python application like Mercurial. Assuming your
repository grows less than 1% per day, the end result is 99+% of CPU
and network load from clones is eliminated, allowing Mercurial servers
to scale more easily. Serving static files also means data can be
transferred to clients as fast as they can consume it, rather than as
fast as servers can generate it. This makes clones faster.

Mozilla has implemented similar functionality of this patch on
hg.mozilla.org using a custom extension. We are hosting bundle files in
Amazon S3 and CloudFront (a CDN) and have successfully offloaded
>1 TB/day in data transfer from hg.mozilla.org, freeing up significant
bandwidth and CPU resources. The positive impact has been stellar and
I believe it has proved its value to be included in Mercurial core. I
feel it is important for the client-side support to be enabled in core
by default because it means that clients will get faster, more reliable
clones and will enable server operators to reduce load without
requiring any client-side configuration changes (assuming clients are
up to date, of course).

The scope of this feature is narrowly and specifically tailored to
cloning, despite "serve pulls from pre-generated bundles" being a valid
and useful feature. I would eventually like for Mercurial servers to
support transferring *all* repository data via statically hosted files.
You could imagine a server that siphons all pushed data to bundle files
and instructs clients to apply a stream of bundles to reconstruct all
repository data. This feature, while useful and powerful, is
significantly more work to implement because it requires the server
component have awareness of discovery and a mapping of which changesets
are in which files. Full, clone bundles, by contrast, are much simpler.

The wire protocol command is named "clonebundles" instead of something
more generic like "staticbundles" to leave the door open for a new, more
powerful and more generic server-side component with minimal backwards
compatibility implications. The name "bundleclone" is used by Mozilla's
extension and would cause problems since there are subtle differences
in Mozilla's extension.

Mozilla's experience with this idea has taught us that some form of
"content negotiation" is required. Not all clients will support all
bundle formats or even URLs (advanced TLS requirements, etc). To ensure
the highest uptake possible, a server needs to advertise multiple
versions of bundles and clients need to be able to choose the most
appropriate from that list one. The "attributes" in each
server-advertised entry facilitate this filtering and sorting. Their
use will become apparent in subsequent patches.

Initial inspiration and credit for the idea of cloning from static files
belongs to Augie Fackler and his "lookaside clone" extension proof of
concept.

  $ echo '[extensions]' >> $HGRCPATH
  $ echo 'hgext.mq =' >> $HGRCPATH

  $ hg init repo
  $ cd repo

  $ echo foo > foo
  $ hg ci -qAm 'add a file'

  $ hg qinit

  $ hg qnew foo
  $ echo foo >> foo
  $ hg qrefresh -m 'append foo'

  $ hg qnew bar
  $ echo bar >> foo
  $ hg qrefresh -m 'append bar'

Try to operate on public mq changeset

  $ hg qpop
  popping bar
  now at: foo
  $ hg phase --public qbase
  $ echo babar >> foo
  $ hg qref
  abort: cannot refresh public revision
  (see "hg help phases" for details)
  [255]
  $ hg revert -a
  reverting foo
  $ hg qpop
  abort: popping would remove a public revision
  (see "hg help phases" for details)
  [255]
  $ hg qfold bar
  abort: cannot refresh public revision
  (see "hg help phases" for details)
  [255]
  $ hg revert -a
  reverting foo

restore state for remaining test

  $ hg qpush
  applying bar
  now at: bar

try to commit on top of a patch

  $ echo quux >> foo
  $ hg ci -m 'append quux'
  abort: cannot commit over an applied mq patch
  [255]


cheat a bit...

  $ mv .hg/patches .hg/patches2
  $ hg ci -m 'append quux'
  $ mv .hg/patches2 .hg/patches


qpop/qrefresh on the wrong revision

  $ hg qpop
  abort: popping would remove a revision not managed by this patch queue
  [255]
  $ hg qpop -n patches
  using patch queue: $TESTTMP/repo/.hg/patches (glob)
  abort: popping would remove a revision not managed by this patch queue
  [255]
  $ hg qrefresh
  abort: working directory revision is not qtip
  [255]

  $ hg up -C qtip
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg qpop
  abort: popping would remove a revision not managed by this patch queue
  [255]
  $ hg qrefresh
  abort: cannot refresh a revision with children
  [255]
  $ hg tip --template '{rev} {desc}\n'
  3 append quux


qpush warning branchheads

  $ cd ..
  $ hg init branchy
  $ cd branchy
  $ echo q > q
  $ hg add q
  $ hg qnew -f qp
  $ hg qpop
  popping qp
  patch queue now empty
  $ echo a > a
  $ hg ci -Ama
  adding a
  $ hg up null
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ hg branch b
  marked working directory as branch b
  (branches are permanent and global, did you want a bookmark?)
  $ echo c > c
  $ hg ci -Amc
  adding c
  $ hg merge default
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ hg ci -mmerge
  $ hg up default
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ hg log
  changeset:   2:65309210bf4e
  branch:      b
  tag:         tip
  parent:      1:707adb4c8ae1
  parent:      0:cb9a9f314b8b
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     merge
  
  changeset:   1:707adb4c8ae1
  branch:      b
  parent:      -1:000000000000
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     c
  
  changeset:   0:cb9a9f314b8b
  user:        test
  date:        Thu Jan 01 00:00:00 1970 +0000
  summary:     a
  
  $ hg qpush
  applying qp
  now at: qp

Testing applied patches, push and --force

  $ cd ..
  $ hg init forcepush
  $ cd forcepush
  $ echo a > a
  $ hg ci -Am adda
  adding a
  $ echo a >> a
  $ hg ci -m changea
  $ hg up 0
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg branch branch
  marked working directory as branch branch
  (branches are permanent and global, did you want a bookmark?)
  $ echo b > b
  $ hg ci -Am addb
  adding b
  $ hg up 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ hg --cwd .. clone -r 0 forcepush forcepush2
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files
  updating to branch default
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ echo a >> a
  $ hg qnew patch

Pushing applied patch with --rev without --force

  $ hg push -r . ../forcepush2
  pushing to ../forcepush2
  abort: source has mq patches applied
  [255]

Pushing applied patch with branchhash, without --force

  $ hg push ../forcepush2#default
  pushing to ../forcepush2
  abort: source has mq patches applied
  [255]

Pushing revs excluding applied patch

  $ hg push --new-branch -r 'branch(branch)' -r 2 ../forcepush2
  pushing to ../forcepush2
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files

Pushing applied patch with --force

  $ hg phase --force --secret 'mq()'
  $ hg push --force -r default ../forcepush2
  pushing to ../forcepush2
  searching for changes
  no changes found (ignored 1 secret changesets)
  [1]
  $ hg phase --draft 'mq()'
  $ hg push --force -r default ../forcepush2
  pushing to ../forcepush2
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 1 changes to 1 files (+1 heads)

  $ cd ..