relnotes/5.6

stream-clone: avoid opening a revlog in case we do not need it

Opening an revlog has a cost, especially if it is inline as we have to scan the
file and construct an index.

To prevent the associated slowdown, we just do a minimal scan to check that an
inline file is still inline, and simply stream the file without creating a
revlog when we can.

This provides a big boost compared to the previous changeset, even if the full
generation is still penalized by the initial gathering of information.

All benchmarks are run on linux with Python 3.10.7.
# benchmark.name             = hg.exchange.stream.generate
# benchmark.variants.version = v2

### Compared to the previous changesets

We get a large win all across the board!

# mercurial-2018-08-01-zstd-sparse-revlog
before:  0.250694 seconds
after:   0.105986 seconds  (-57.72%)
# pypy-2018-08-01-zstd-sparse-revlog
before:  3.885657 seconds
after:   1.709748 seconds  (-56.00%)
# netbeans-2018-08-01-zstd-sparse-revlog
before: 16.679371 seconds
after:   7.687469 seconds  (-53.91%)
# mozilla-central-2018-08-01-zstd-sparse-revlog
before: 38.575482 seconds
after:  17.520316 seconds  (-54.58%)
# mozilla-try-2019-02-18-zstd-sparse-revlog
before: 81.160994 seconds
after:  37.073753 seconds  (-54.32%)



### Compared to 6.4.3

We are still significantly slower than 6.4.3, the extra time is usually twice
slower than the extra time we observe on the locked section, which is a quite
interesting information.

Except for mercurial-central that is much faster. That discrepancy is not really
explained yet.

# mercurial-2018-08-01-zstd-sparse-revlog
6.4.3:  0.072560 seconds
after:  0.105986 seconds  (+46.07%) (- 0.03 seconds)
# pypy-2018-08-01-zstd-sparse-revlog
6.4.3:  1.211193 seconds
after:  1.709748 seconds  (+41.16%) (-0.45 seconds)
# netbeans-2018-08-01-zstd-sparse-revlog
6.4.3:  4.932843 seconds
after:  7.687469 seconds  (+55.84%) (-2.75 seconds)
# mozilla-central-2018-08-01-zstd-sparse-revlog
6.4.3: 34.012226 seconds
after: 17.520316 seconds  (-48.49%) (-16.49 seconds)
# mozilla-try-2019-02-18-zstd-sparse-revlog
6.4.3: 23.850555 seconds
after: 37.073753 seconds  (+55.44%) (+13.22 seconds)

== New Features ==

 * `hg mv -A` can now be used with `--at-rev`. It behaves just like
   `hg cp -A --at-rev`, i.e. it marks the destination as a copy of the
   source whether or not the source still exists (but the source must
   exist in the parent revision).

 * New revset predicate `diffcontains(pattern)` for filtering revisions
   in the same way as `hg grep --diff pattern`.

 * The memory footprint per changeset and per file during pull/unbundle
   operations has been significantly reduced.


== New Experimental Features ==



== Bug Fixes ==



== Backwards Compatibility Changes ==



== Internal API Changes ==

 * `merge.update()` is now private (renamed to `_update()`). Hopefully
   the higher-level functions available in the same module cover your
   use cases.

 * `phases.registernew` now takes a set of revisions instead of a list
   of nodes. `phases.advanceboundary` takes an optional set of revisions
   in addition to the list of nodes. The corresponeding members of the
   `phasecache` class follow this change.

 * The `addgroup` member of `revlog` classes no longer keeps a list of
   all found nodes. It now returns True iff a node was found in the group.
   An optional callback for duplicated nodes can be used by callers to keep
   track of all nodes themselve.

 * The `_chaininfocache` of `revlog` classes has been changed from a dict
   to a LRU cache.