bisect: avoid copying ancestor list for non-merge commits During a bisection, hg needs to compute a list of all ancestors for every candidate commit. This is accomplished via a bottom-up traversal of the set of candidates, during which each revision's ancestor list is populated using the ancestor list of its parent(s). Previously, this involved copying the entire list, which could be very long in if the bisection range was large. To help improve this, we can observe that each candidate commit is visited exactly once, at which point its ancestor list is copied into its children's lists and then dropped. In the case of non-merge commits, a commit's ancestor list consists exactly of its parent's list plus itself. This means that we can trivially reuse the parent's existing list for one of its non-merge children, which avoids copying entirely if that commit is the parent's only child. This makes bisections over linear ranges of commits much faster. During some informal testing in the large publicly-available `mozilla-central` repository, this noticeably sped up bisections over large ranges of history: Setup: $ cd mozilla-central $ hg bisect --reset $ hg bisect --good 0 $ hg log -r tip -T '{rev}\n' 628417 Test: $ time hg bisect --bad tip --noupdate Before: real 3m35.927s user 3m35.553s sys 0m0.319s After: real 1m41.142s user 1m40.810s sys 0m0.285s

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  $ hg init

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  $ echo foo > a

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  $ hg add a

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  $ hg commit -m "1"

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  $ echo bar > b

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  $ hg add b

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  $ hg remove a

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Should show a removed and b added:

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  $ hg status

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  A b

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  R a

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  $ hg revert --all

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  forgetting b

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  undeleting a

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Should show b unknown and a back to normal:

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  $ hg status

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  ? b

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  $ rm b

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  $ hg co -C 0

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  0 files updated, 0 files merged, 0 files removed, 0 files unresolved

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  $ echo foo-a > a

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  $ hg commit -m "2a"

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  $ hg co -C 0

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  1 files updated, 0 files merged, 0 files removed, 0 files unresolved

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  $ echo foo-b > a

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  $ hg commit -m "2b"

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  created new head

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  $ HGMERGE=true hg merge 1

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  merging a

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  0 files updated, 1 files merged, 0 files removed, 0 files unresolved

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  (branch merge, don't forget to commit)

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Should show foo-b:

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  $ cat a

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  foo-b

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  $ echo bar > b

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  $ hg add b

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  $ rm a

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  $ hg remove a

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Should show a removed and b added:

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  $ hg status

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  A b

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  R a

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Revert should fail:

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  $ hg revert

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  abort: uncommitted merge with no revision specified

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  (use 'hg update' or see 'hg help revert')

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  [10]

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Revert should be ok now:

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  $ hg revert -r2 --all

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  forgetting b

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  undeleting a

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Should show b unknown and a marked modified (merged):

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  $ hg status

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  M a

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  ? b

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Should show foo-b:

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  $ cat a

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  foo-b

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