tests/test-exchange-obsmarkers-case-B6.t

bundle2: extract logic for seeking bundle2 part into own class

Currently, unbundlepart classes support bi-directional seeking.
Most consumers of unbundlepart only ever seek forward - typically
as part of moving to the end of the bundle part so they can move
on to the next one. But regardless of the actual usage of the
part, instances maintain an index mapping offsets within the
underlying raw payload to offsets within the decoded payload.

Maintaining the mapping of offset data can be expensive in terms of
memory use. Furthermore, many bundle2 consumers don't have access
to an underlying seekable stream. This includes all compressed
bundles. So maintaining offset data when the underlying stream
can't be seeked anyway is wasteful. And since many bundle2 streams
can't be seeked, it seems like a bad idea to expose a seek API
in bundle2 parts by default. If you provide them, people will
attempt to use them.

Seekable bundle2 parts should be the exception, not the rule. This
commit starts the process dividing unbundlepart into 2 classes: a
base class that supports linear, one-time reads and a child class
that supports bi-directional seeking. In this first commit, we
split various methods and attributes out into a new
"seekableunbundlepart" class. Previous instantiators of "unbundlepart"
now instantiate "seekableunbundlepart." This preserves backwards
compatibility. The coupling between the classes is still tight:
"unbundlepart" cannot be used on its own. This will be addressed
in subsequent commits.

Differential Revision: https://phab.mercurial-scm.org/D1386

============================================
Testing obsolescence markers push: Cases B.6
============================================

Mercurial pushes obsolescences markers relevant to the "pushed-set", the set of
all changesets that requested to be "in sync" after the push (even if they are
already on both side).

This test belongs to a series of tests checking such set is properly computed
and applied. This does not tests "obsmarkers" discovery capabilities.

Category B: pruning case
TestCase 6: Pruned changeset with precursors not in pushed set

B.6 Pruned changeset with precursors not in pushed set
======================================================

.. {{{
..   B ø⇠⊗ B'
..     | |
..   A ○ |
..     |/
..     ● O
.. }}}
..
.. Marker exist from:
..
..  * `B ø⇠⊗ B'`
..  * B' prune
..
.. Command run:
..
..  * hg push -r O
..
.. Expected exchange:
..
..  * `B ø⇠⊗ B'`
..  * B' prune

Setup
-----

  $ . $TESTDIR/testlib/exchange-obsmarker-util.sh

Initial

  $ setuprepos B.6
  creating test repo for test case B.6
  - pulldest
  - main
  - pushdest
  cd into `main` and proceed with env setup
  $ cd main
  $ mkcommit A
  $ mkcommit B0
  $ hg up --quiet 0
  $ mkcommit B1
  created new head
  $ hg debugobsolete `getid 'desc(B0)'` `getid 'desc(B1)'`
  obsoleted 1 changesets
  $ hg prune -qd '0 0' .
  $ hg log -G --hidden
  x  f6298a8ac3a4 (draft): B1
  |
  | x  962ecf6b1afc (draft): B0
  | |
  | o  f5bc6836db60 (draft): A
  |/
  @  a9bdc8b26820 (public): O
  
  $ inspect_obsmarkers
  obsstore content
  ================
  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  $ cd ..
  $ cd ..

Actual Test
-------------------------------------

  $ dotest B.6 O
  ## Running testcase B.6
  # testing echange of "O" (a9bdc8b26820)
  ## initial state
  # obstore: main
  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  # obstore: pushdest
  # obstore: pulldest
  ## pushing "O" from main to pushdest
  pushing to pushdest
  searching for changes
  no changes found
  remote: 2 new obsolescence markers
  ## post push state
  # obstore: main
  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  # obstore: pushdest
  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  # obstore: pulldest
  ## pulling "a9bdc8b26820" from main into pulldest
  pulling from main
  no changes found
  2 new obsolescence markers
  ## post pull state
  # obstore: main
  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  # obstore: pushdest
  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  # obstore: pulldest
  962ecf6b1afc94e15c7e48fdfb76ef8abd11372b f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}
  f6298a8ac3a4b78bbeae5f1d3dc5bc3c3812f0f3 0 {a9bdc8b26820b1b87d585b82eb0ceb4a2ecdbc04} (Thu Jan 01 00:00:00 1970 +0000) {'user': 'test'}