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 a

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  $ cd a

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  $ touch empty1

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  $ hg add empty1

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  $ hg commit -m 'add empty1'

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  $ touch empty2

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  $ hg add empty2

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  $ hg commit -m 'add empty2'

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  $ hg up -C 0

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

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  $ touch empty3

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  $ hg add empty3

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  $ hg commit -m 'add empty3'

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  created new head

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  $ hg heads

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  changeset:   2:a1cb177e0d44

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  tag:         tip

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  parent:      0:1e1d9c4e5b64

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  user:        test

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  date:        Thu Jan 01 00:00:00 1970 +0000

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  summary:     add empty3

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  changeset:   1:097d2b0e17f6

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  user:        test

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  date:        Thu Jan 01 00:00:00 1970 +0000

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  summary:     add empty2

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  $ hg merge 1

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

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  (branch merge, don't forget to commit)

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Before changeset 05257fd28591, we didn't notice the

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empty file that came from rev 1:

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  $ hg status

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  M empty2

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  $ hg commit -m merge

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  $ hg manifest --debug tip

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  b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty1

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  b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty2

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  b80de5d138758541c5f05265ad144ab9fa86d1db 644   empty3

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  $ cd ..