view tests/test-bundle-phases.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

  $ cat >> $HGRCPATH <<EOF
  > [experimental]
  > bundle-phases=yes
  > [extensions]
  > strip=
  > drawdag=$TESTDIR/drawdag.py
  > EOF

Set up repo with linear history
  $ hg init linear
  $ cd linear
  $ hg debugdrawdag <<'EOF'
  > E
  > |
  > D
  > |
  > C
  > |
  > B
  > |
  > A
  > EOF
  $ hg phase --public A
  $ hg phase --force --secret D
  $ hg log -G -T '{desc} {phase}\n'
  o  E secret
  |
  o  D secret
  |
  o  C draft
  |
  o  B draft
  |
  o  A public
  
Phases are restored when unbundling
  $ hg bundle --base B -r E bundle
  3 changesets found
  $ hg debugbundle bundle
  Stream params: {Compression: BZ}
  changegroup -- {nbchanges: 3, targetphase: 2, version: 02} (mandatory: True)
      26805aba1e600a82e93661149f2313866a221a7b
      f585351a92f85104bff7c284233c338b10eb1df7
      9bc730a19041f9ec7cb33c626e811aa233efb18c
  cache:rev-branch-cache -- {} (mandatory: False)
  phase-heads -- {} (mandatory: True)
      26805aba1e600a82e93661149f2313866a221a7b draft
  $ hg strip --no-backup C
  $ hg unbundle -q bundle
  $ rm bundle
  $ hg log -G -T '{desc} {phase}\n'
  o  E secret
  |
  o  D secret
  |
  o  C draft
  |
  o  B draft
  |
  o  A public
  
Root revision's phase is preserved
  $ hg bundle -a bundle
  5 changesets found
  $ hg strip --no-backup A
  $ hg unbundle -q bundle
  $ rm bundle
  $ hg log -G -T '{desc} {phase}\n'
  o  E secret
  |
  o  D secret
  |
  o  C draft
  |
  o  B draft
  |
  o  A public
  
Completely public history can be restored
  $ hg phase --public E
  $ hg bundle -a bundle
  5 changesets found
  $ hg strip --no-backup A
  $ hg unbundle -q bundle
  $ rm bundle
  $ hg log -G -T '{desc} {phase}\n'
  o  E public
  |
  o  D public
  |
  o  C public
  |
  o  B public
  |
  o  A public
  
Direct transition from public to secret can be restored
  $ hg phase --secret --force D
  $ hg bundle -a bundle
  5 changesets found
  $ hg strip --no-backup A
  $ hg unbundle -q bundle
  $ rm bundle
  $ hg log -G -T '{desc} {phase}\n'
  o  E secret
  |
  o  D secret
  |
  o  C public
  |
  o  B public
  |
  o  A public
  
Revisions within bundle preserve their phase even if parent changes its phase
  $ hg phase --draft --force B
  $ hg bundle --base B -r E bundle
  3 changesets found
  $ hg strip --no-backup C
  $ hg phase --public B
  $ hg unbundle -q bundle
  $ rm bundle
  $ hg log -G -T '{desc} {phase}\n'
  o  E secret
  |
  o  D secret
  |
  o  C draft
  |
  o  B public
  |
  o  A public
  
Phase of ancestors of stripped node get advanced to accommodate child
  $ hg bundle --base B -r E bundle
  3 changesets found
  $ hg strip --no-backup C
  $ hg phase --force --secret B
  $ hg unbundle -q bundle
  $ rm bundle
  $ hg log -G -T '{desc} {phase}\n'
  o  E secret
  |
  o  D secret
  |
  o  C draft
  |
  o  B draft
  |
  o  A public
  
Unbundling advances phases of changesets even if they were already in the repo.
To test that, create a bundle of everything in draft phase and then unbundle
to see that secret becomes draft, but public remains public.
  $ hg phase --draft --force A
  $ hg phase --draft E
  $ hg bundle -a bundle
  5 changesets found
  $ hg phase --public A
  $ hg phase --secret --force E
  $ hg unbundle -q bundle
  $ rm bundle
  $ hg log -G -T '{desc} {phase}\n'
  o  E draft
  |
  o  D draft
  |
  o  C draft
  |
  o  B draft
  |
  o  A public
  
Unbundling change in the middle of a stack does not affect later changes
  $ hg strip --no-backup E
  $ hg phase --secret --force D
  $ hg log -G -T '{desc} {phase}\n'
  o  D secret
  |
  o  C draft
  |
  o  B draft
  |
  o  A public
  
  $ hg bundle --base A -r B bundle
  1 changesets found
  $ hg unbundle -q bundle
  $ rm bundle
  $ hg log -G -T '{desc} {phase}\n'
  o  D secret
  |
  o  C draft
  |
  o  B draft
  |
  o  A public
  

  $ cd ..

Set up repo with non-linear history
  $ hg init non-linear
  $ cd non-linear
  $ hg debugdrawdag <<'EOF'
  > D E
  > |\|
  > B C
  > |/
  > A
  > EOF
  $ hg phase --public C
  $ hg phase --force --secret B
  $ hg log -G -T '{node|short} {desc} {phase}\n'
  o  03ca77807e91 E draft
  |
  | o  4e4f9194f9f1 D secret
  |/|
  o |  dc0947a82db8 C public
  | |
  | o  112478962961 B secret
  |/
  o  426bada5c675 A public
  

Restore bundle of entire repo
  $ hg bundle -a bundle
  5 changesets found
  $ hg debugbundle bundle
  Stream params: {Compression: BZ}
  changegroup -- {nbchanges: 5, targetphase: 2, version: 02} (mandatory: True)
      426bada5c67598ca65036d57d9e4b64b0c1ce7a0
      112478962961147124edd43549aedd1a335e44bf
      dc0947a82db884575bb76ea10ac97b08536bfa03
      4e4f9194f9f181c57f62e823e8bdfa46ab9e4ff4
      03ca77807e919db8807c3749086dc36fb478cac0
  cache:rev-branch-cache -- {} (mandatory: False)
  phase-heads -- {} (mandatory: True)
      dc0947a82db884575bb76ea10ac97b08536bfa03 public
      03ca77807e919db8807c3749086dc36fb478cac0 draft
  $ hg strip --no-backup A
  $ hg unbundle -q bundle
  $ rm bundle
  $ hg log -G -T '{node|short} {desc} {phase}\n'
  o  03ca77807e91 E draft
  |
  | o  4e4f9194f9f1 D secret
  |/|
  o |  dc0947a82db8 C public
  | |
  | o  112478962961 B secret
  |/
  o  426bada5c675 A public
  

  $ hg bundle --base 'A + C' -r D bundle
  2 changesets found
  $ hg debugbundle bundle
  Stream params: {Compression: BZ}
  changegroup -- {nbchanges: 2, targetphase: 2, version: 02} (mandatory: True)
      112478962961147124edd43549aedd1a335e44bf
      4e4f9194f9f181c57f62e823e8bdfa46ab9e4ff4
  cache:rev-branch-cache -- {} (mandatory: False)
  phase-heads -- {} (mandatory: True)
  $ rm bundle

  $ hg bundle --base A -r D bundle
  3 changesets found
  $ hg debugbundle bundle
  Stream params: {Compression: BZ}
  changegroup -- {nbchanges: 3, targetphase: 2, version: 02} (mandatory: True)
      112478962961147124edd43549aedd1a335e44bf
      dc0947a82db884575bb76ea10ac97b08536bfa03
      4e4f9194f9f181c57f62e823e8bdfa46ab9e4ff4
  cache:rev-branch-cache -- {} (mandatory: False)
  phase-heads -- {} (mandatory: True)
      dc0947a82db884575bb76ea10ac97b08536bfa03 public
  $ rm bundle

  $ hg bundle --base 'B + C' -r 'D + E' bundle
  2 changesets found
  $ hg debugbundle bundle
  Stream params: {Compression: BZ}
  changegroup -- {nbchanges: 2, targetphase: 2, version: 02} (mandatory: True)
      4e4f9194f9f181c57f62e823e8bdfa46ab9e4ff4
      03ca77807e919db8807c3749086dc36fb478cac0
  cache:rev-branch-cache -- {} (mandatory: False)
  phase-heads -- {} (mandatory: True)
      03ca77807e919db8807c3749086dc36fb478cac0 draft
  $ rm bundle