revlogutils: for issue6528 fix, pre-cache nullrev as metadata-free

revlogutils: for issue6528 fix, cache results for null changes

revlogutils: fix _chunk() reference _chunk is only found in the inner revlog object and not directly exposed outside.

rev-branch-cache: reenable memory mapping of the revision data Now that we are no longer truncating it, we can mmap it again. This provide a sizeable speedup on repository with a very large amount of revision for example for a mozilla-try clone with 5 793 383 revisions, this provide a speedup of 5ms - 10ms. Since they happens within the "critical" locked path during push. These miliseconds are important. In addition, the v3 branchmap format is use the rev-branch-cache more than the v2 branchmap cache so this will be important. On smaller repository we consistently see an improvement of one or two percents, but the gain in absolute time is usually < 10 ms. #### benchmark.name = hg.command.unbundle # benchmark.variants.issue6528 = disabled # benchmark.variants.reuse-external-delta-parent = yes # benchmark.variants.revs = any-1-extra-rev # benchmark.variants.source = unbundle # benchmark.variants.verbosity = quiet ### data-env-vars.name = mozilla-try-2024-03-26-zstd-sparse-revlog ## bin-env-vars.hg.flavor = default e51161b12c7e: 3.527923 ebdcfe85b070: 3.468178 (-1.69%, -0.06) ## bin-env-vars.hg.flavor = rust e51161b12c7e: 3.580158 ebdcfe85b070: 3.480564 (-2.78%, -0.10) ### data-env-vars.name = mozilla-try-2024-03-26-ds2-pnm ## bin-env-vars.hg.flavor = rust e51161b12c7e: 3.527923 ebdcfe85b070: 3.468178 (-1.69%, -0.06)

rev-branch-cache: have debugupdatecache warm rbc too Since the "v2" format can be more performant than the "v1" format (thanks to mmap), it is useful to be able to make sure it is present

rev-branch-cache: schedule a write of the "v2" format if we read from "v1" The new file can be memorymapped, while the old one cannot. So there is value in having the v2 format around as soon a possible.

rev-branch-cache: fallback on "v1" data if no v2 is found This will help smooth the transition to the v2 format for existing large repository.

rev-branch-cache: increment the version to "v2" We want to ensure no older clients will truncate the file under us. So we need to change their name. We don't change the rest of the format (unfortunaly).

rev-branch-cache: stop truncating cache file Truncating the file prevent the safe use of mmap. So instead of overwrite the existing data. If more than 20% of the file is to be overwritten, we rewrite the whole file instead. Such whole rewrite is done by replacing the old one with a new one, so mmap of the old file would be affected. This prepare a more aggressive use of mmap in later patches.

rev-branch-cache: make sure we close the name file we open We were various opening without with or try. Adding a try would not hurt.