emitrevision: consider ancestors revision to emit as available base
This should make more delta base valid. This notably affects:
* case where we skipped some parent with empty delta to directly delta against
an ancestors
* case where an intermediate snapshots is stored.
This change means we could sent largish intermediate snapshots over the wire.
However this is actually a sub goal here. Sending snapshots over the wire means
the client have a high odd of simply storing the pre-computed delta instead of
doing a lengthy process that will… end up doing the same intermediate snapshot.
In addition the overall size of snapshot (or any level) is "only" some or the
overall delta size. (0.17% for my mercurial clone, 20% for my clone of Mozilla
try). So Sending them other the wire is unlikely to change large impact on the
bandwidth used.
If we decide that minimising the bandwidth is an explicit goal, we should
introduce new logic to filter-out snapshot as delta. The current code has no
notion explicite of snapshot so far, they just tended to fall into the wobbly
filtering options.
In some cases, this patch can yield large improvement to the bundling time:
### data-env-vars.name = mozilla-try-2019-02-18-zstd-sparse-revlog
# benchmark.name = perf-bundle
# benchmark.variants.revs = last-100000
before: 68.787066 seconds
after: 47.552677 seconds (-30.87%)
That translate to large improvement to the pull time :
### data-env-vars.name = mozilla-try-2019-02-18-zstd-sparse-revlog
# benchmark.name = pull
# benchmark.variants.
issue6528 = disabled
# benchmark.variants.revs = last-100000
before: 142.186625 seconds
after: 75.897745 seconds (-46.62%)
No significant negative impact have been observed.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install::
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.
Notes for packagers
===================
Mercurial ships a copy of the python-zstandard sources. This is used to
provide support for zstd compression and decompression functionality. The
module is not intended to be replaced by the plain python-zstandard nor
is it intended to use a system zstd library. Patches can result in hard
to diagnose errors and are explicitly discouraged as unsupported
configuration.