view tests/test-branch-change.t @ 40326:fed697fa1734

sqlitestore: file storage backend using SQLite

This commit provides an extension which uses SQLite to store file
data (as opposed to revlogs).

As the inline documentation describes, there are still several
aspects to the extension that are incomplete. But it's a start.
The extension does support basic clone, checkout, and commit
workflows, which makes it suitable for simple use cases.

One notable missing feature is support for "bundlerepos." This is
probably responsible for the most test failures when the extension
is activated as part of the test suite.

All revision data is stored in SQLite. Data is stored as zstd
compressed chunks (default if zstd is available), zlib compressed
chunks (default if zstd is not available), or raw chunks (if
configured or if a compressed delta is not smaller than the raw
delta). This makes things very similar to revlogs.

Unlike revlogs, the extension doesn't yet enforce a limit on delta
chain length. This is an obvious limitation and should be addressed.
This is somewhat mitigated by the use of zstd, which is much faster
than zlib to decompress.

There is a dedicated table for storing deltas. Deltas are stored
by the SHA-1 hash of their uncompressed content. The "fileindex" table
has columns that reference the delta for each revision and the base
delta that delta should be applied against. A recursive SQL query
is used to resolve the delta chain along with the delta data.

By storing deltas by hash, we are able to de-duplicate delta storage!
With revlogs, the same deltas in different revlogs would result in
duplicate storage of that delta. In this scheme, inserting the
duplicate delta is a no-op and delta chains simply reference the
existing delta.

When initially implementing this extension, I did not have
content-indexed deltas and deltas could be duplicated across files
(just like revlogs). When I implemented content-indexed deltas, the
size of the SQLite database for a full clone of mozilla-unified
dropped:

before: 2,554,261,504 bytes
after:  2,488,754,176 bytes

Surprisingly, this is still larger than the bytes size of revlog
files:

revlog files: 2,104,861,230 bytes
du -b:        2,254,381,614

I would have expected storage to be smaller since we're not limiting
delta chain length and since we're using zstd instead of zlib. I
suspect the SQLite indexes and per-column overhead account for the
bulk of the differences. (Keep in mind that revlog uses a 64-byte
packed struct for revision index data and deltas are stored without
padding. Aside from the 12 unused bytes in the 32 byte node field,
revlogs are pretty efficient.) Another source of overhead is file
name storage. With revlogs, file names are stored in the filesystem.
But with SQLite, we need to store file names in the database. This is
roughly equivalent to the size of the fncache file, which for the
mozilla-unified repository is ~34MB.

Since the SQLite database isn't append-only and since delta chains
can reference any delta, this opens some interesting possibilities.
For example, we could store deltas in reverse, such that fulltexts
are stored for newer revisions and deltas are applied to reconstruct
older revisions. This is likely a more optimal storage strategy for
version control, as new data tends to be more frequently accessed
than old data. We would obviously need wire protocol support for
transferring revision data from newest to oldest. And we would
probably need some kind of mechanism for "re-encoding" stores. But
it should be doable.

This extension is very much experimental quality. There are a handful
of features that don't work. It probably isn't suitable for day-to-day
use. But it could be used in limited cases (e.g. read-only checkouts
like in CI). And it is also a good proving ground for alternate
storage backends. As we continue to define interfaces for all things
storage, it will be useful to have a viable alternate storage backend
to see how things shake out in practice.

test-storage.py passes on Python 2 and introduces no new test failures on
Python 3. Having the storage-level unit tests has proved to be insanely
useful when developing this extension. Those tests caught numerous bugs
during development and I'm convinced this style of testing is the way
forward for ensuring alternate storage backends work as intended. Of
course, test coverage isn't close to what it needs to be. But it is
a start. And what coverage we have gives me confidence that basic store
functionality is implemented properly.

Differential Revision: https://phab.mercurial-scm.org/D4928

Testing changing branch on commits
==================================

Setup

  $ cat >> $HGRCPATH << EOF
  > [alias]
  > glog = log -G -T "{rev}:{node|short} {desc}\n{branch} ({bookmarks})"
  > [experimental]
  > evolution = createmarkers
  > [extensions]
  > rebase=
  > EOF

  $ hg init repo
  $ cd repo
  $ for ch in a b c d e; do echo foo >> $ch; hg ci -Aqm "Added "$ch; done
  $ hg glog
  @  4:aa98ab95a928 Added e
  |  default ()
  o  3:62615734edd5 Added d
  |  default ()
  o  2:28ad74487de9 Added c
  |  default ()
  o  1:29becc82797a Added b
  |  default ()
  o  0:18d04c59bb5d Added a
     default ()

  $ hg branches
  default                        4:aa98ab95a928

Try without passing a new branch name

  $ hg branch -r .
  abort: no branch name specified for the revisions
  [255]

Setting an invalid branch name

  $ hg branch -r . a:b
  abort: ':' cannot be used in a name
  [255]
  $ hg branch -r . tip
  abort: the name 'tip' is reserved
  [255]
  $ hg branch -r . 1234
  abort: cannot use an integer as a name
  [255]

Change on non-linear set of commits

  $ hg branch -r 2 -r 4 foo
  abort: cannot change branch of non-linear revisions
  [255]

Change in middle of the stack (linear commits)

  $ hg branch -r 1::3 foo
  abort: cannot change branch of changeset with children
  [255]

Change with dirty working directory

  $ echo bar > a
  $ hg branch -r . foo
  abort: uncommitted changes
  [255]

  $ hg revert --all
  reverting a

Change on empty revision set

  $ hg branch -r 'draft() - all()' foo
  abort: empty revision set
  [255]

Changing branch on linear set of commits from head

Without obsmarkers

  $ hg branch -r 3:4 foo --config experimental.evolution=!
  changed branch on 2 changesets
  saved backup bundle to $TESTTMP/repo/.hg/strip-backup/62615734edd5-e86bd13a-branch-change.hg
  $ hg glog
  @  4:3938acfb5c0f Added e
  |  foo ()
  o  3:9435da006bdc Added d
  |  foo ()
  o  2:28ad74487de9 Added c
  |  default ()
  o  1:29becc82797a Added b
  |  default ()
  o  0:18d04c59bb5d Added a
     default ()

  $ hg branches
  foo                            4:3938acfb5c0f
  default                        2:28ad74487de9 (inactive)

With obsmarkers

  $ hg branch -r 3::4 bar
  changed branch on 2 changesets
  $ hg glog
  @  6:7c1991464886 Added e
  |  bar ()
  o  5:1ea05e93925f Added d
  |  bar ()
  o  2:28ad74487de9 Added c
  |  default ()
  o  1:29becc82797a Added b
  |  default ()
  o  0:18d04c59bb5d Added a
     default ()

  $ hg branches
  bar                            6:7c1991464886
  default                        2:28ad74487de9 (inactive)

Change branch name to an existing branch

  $ hg branch -r . default
  abort: a branch of the same name already exists
  [255]

Changing on a branch head which is not topological head

  $ hg branch -r 2 stable
  abort: cannot change branch of changeset with children
  [255]

Enabling the allowunstable config and trying to change branch on a branch head
which is not a topological head

  $ echo "[experimental]" >> .hg/hgrc
  $ echo "evolution.allowunstable=yes" >> .hg/hgrc
  $ hg branch -r 2 foo
  changed branch on 1 changesets
  2 new orphan changesets

Changing branch of an obsoleted changeset

  $ hg branch -r 4 foobar
  abort: hidden revision '4' was rewritten as: 7c1991464886!
  (use --hidden to access hidden revisions)
  [255]

  $ hg branch -r 4 --hidden foobar
  abort: cannot change branch of a obsolete changeset
  [255]

Make sure bookmark movement is correct

  $ hg bookmark b1
  $ hg glog -r '.^::'
  @  6:7c1991464886 Added e
  |  bar (b1)
  *  5:1ea05e93925f Added d
  |  bar ()
  ~

  $ hg branch -r '(.^)::' wat --debug
  changing branch of '1ea05e93925f806d875a2163f9b76764be644636' from 'bar' to 'wat'
  committing files:
  d
  committing manifest
  committing changelog
  new node id is 343660ccab7400da637bd6a211d07f413536d718
  changing branch of '7c19914648869f5b02fc7fed31ddee9783fdd680' from 'bar' to 'wat'
  committing files:
  e
  committing manifest
  committing changelog
  new node id is de1404b45a69f8cc6437d7679033ee33e9efb4ba
  moving bookmarks ['b1'] from 7c19914648869f5b02fc7fed31ddee9783fdd680 to de1404b45a69f8cc6437d7679033ee33e9efb4ba
  resolving manifests
   branchmerge: False, force: False, partial: False
   ancestor: 7c1991464886, local: 7c1991464886+, remote: de1404b45a69
  starting 4 threads for background file closing (?)
  changed branch on 2 changesets
  updating the branch cache
  invalid branchheads cache (served): tip differs

  $ hg glog -r '(.^)::'
  @  9:de1404b45a69 Added e
  |  wat (b1)
  *  8:343660ccab74 Added d
  |  wat ()
  ~

Make sure phase handling is correct

  $ echo foo >> bar
  $ hg ci -Aqm "added bar" --secret
  1 new orphan changesets
  $ hg glog -r .
  @  10:8ad1294c1660 added bar
  |  wat (b1)
  ~
  $ hg branch -r . secret
  changed branch on 1 changesets
  $ hg phase -r .
  11: secret

  $ hg branches
  secret                        11:38a9b2d53f98
  foo                            7:8a4729a5e2b8
  wat                            9:de1404b45a69 (inactive)
  default                        2:28ad74487de9 (inactive)
  $ hg branch
  secret

Changing branch of another head, different from one on which we are

  $ hg glog
  @  11:38a9b2d53f98 added bar
  |  secret (b1)
  *  9:de1404b45a69 Added e
  |  wat ()
  *  8:343660ccab74 Added d
  |  wat ()
  | o  7:8a4729a5e2b8 Added c
  | |  foo ()
  x |  2:28ad74487de9 Added c
  |/   default ()
  o  1:29becc82797a Added b
  |  default ()
  o  0:18d04c59bb5d Added a
     default ()

  $ hg branch
  secret

  $ hg branch -r 7 foobar
  changed branch on 1 changesets

The current branch must be preserved
  $ hg branch
  secret

Changing branch on multiple heads at once

  $ hg rebase -s 8 -d 12 --keepbranches -q

  $ hg rebase -s 14 -d 1 --keepbranches -q

  $ hg branch -r 0: stable
  changed branch on 6 changesets
  $ hg glog
  @  23:6a5ddbcfb870 added bar
  |  stable (b1)
  o  22:baedc6e98a67 Added e
  |  stable ()
  | o  21:99ac7bf8aad1 Added d
  | |  stable ()
  | o  20:0ecb4d39c4bd Added c
  |/   stable ()
  o  19:fd45b986b109 Added b
  |  stable ()
  o  18:204d2769eca2 Added a
     stable ()

  $ hg branches
  stable                        23:6a5ddbcfb870

  $ hg branch
  stable

Changing to same branch is no-op

  $ hg branch -r 19::21 stable
  changed branch on 0 changesets

Changing branch name to existing branch name if the branch of parent of root of
revs is same as the new branch name

  $ hg branch -r 20::21 bugfix
  changed branch on 2 changesets
  $ hg glog
  o  25:714defe1cf34 Added d
  |  bugfix ()
  o  24:98394def28fc Added c
  |  bugfix ()
  | @  23:6a5ddbcfb870 added bar
  | |  stable (b1)
  | o  22:baedc6e98a67 Added e
  |/   stable ()
  o  19:fd45b986b109 Added b
  |  stable ()
  o  18:204d2769eca2 Added a
     stable ()

  $ hg branch -r 24:25 stable
  changed branch on 2 changesets
  $ hg glog
  o  27:4ec342341562 Added d
  |  stable ()
  o  26:83f48859c2de Added c
  |  stable ()
  | @  23:6a5ddbcfb870 added bar
  | |  stable (b1)
  | o  22:baedc6e98a67 Added e
  |/   stable ()
  o  19:fd45b986b109 Added b
  |  stable ()
  o  18:204d2769eca2 Added a
     stable ()

Testing on merge

  $ hg merge -r 26
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)

  $ hg branch -r . abcd
  abort: outstanding uncommitted merge
  [255]
  $ hg ci -m "Merge commit"
  $ hg branch -r '(.^)::' def
  abort: cannot change branch of a merge commit
  [255]

Changing branch on public changeset

  $ hg phase -r 27 -p
  $ hg branch -r 27 def
  abort: cannot change branch of public changesets
  (see 'hg help phases' for details)
  [255]