rebase: rewrite core algorithm (issue5578) (issue5630) "defineparents" is the core algorithm of rebase. The old code has too many tech debts (like outdated comments, confusing assertions, etc) and is very error-prone to be improved. This patch rewrites "defineparents" to make the code easier to reason about, and solve a bunch of issues, including: - Assertion error: no base found (demonstrated by D212, issue5578) - Asymmetric result (demonstrated by D211, "F with one parent") - Wrong new parent (demonstrated by D262, "C':A'A'") - "revlog index out of range" (demonstrated by D262, issue5630) - "nothing to merge" (demonstrated by D262) As a side effect, merge base decision has been made more solid - rebase now prints out explicitly what could go wrong when it cannot find a unique suitable merge base. .. fix:: Issue 5578, Issue 5630 Core rebase algorithm has been rewritten to be more robust. Differential Revision: https://phab.mercurial-scm.org/D21

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  $ cat >> $HGRCPATH <<EOF

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  > [extensions]

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  > drawdag=$TESTDIR/drawdag.py

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  > bruterebase=$TESTDIR/bruterebase.py

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  > [experimental]

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  > evolution=createmarkers,allowunstable

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  > EOF

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  $ init() {

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  >   N=`expr ${N:-0} + 1`

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  >   cd $TESTTMP && hg init repo$N && cd repo$N

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  >   hg debugdrawdag

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  > }

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Source looks like "N"

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  $ init <<'EOS'

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  > C D

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  > |\|

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  > A B Z

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  > EOS

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  $ hg debugbruterebase 'all()-Z' Z

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     A: A':Z

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     B: B':Z

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    AB: A':Z B':Z

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     C: ABORT: cannot use revision 3 as base, result would have 3 parents

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    AC: A':Z C':A'B

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    BC: B':Z C':B'A

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   ABC: A':Z B':Z C':A'B'

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     D: D':Z

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    AD: A':Z D':Z

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    BD: B':Z D':B'

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   ABD: A':Z B':Z D':B'

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    CD: ABORT: cannot use revision 3 as base, result would have 3 parents

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   ACD: A':Z C':A'B D':Z

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   BCD: B':Z C':B'A D':B'

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  ABCD: A':Z B':Z C':A'B' D':B'

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Moving backwards

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  $ init <<'EOS'

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  > C

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  > |\

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  > A B

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  > |

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  > Z

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  > EOS

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  $ hg debugbruterebase 'all()-Z' Z

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    B: B':Z

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    A: 

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   BA: B':Z

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    C: ABORT: cannot use revision 3 as base, result would have 3 parents

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   BC: B':Z C':B'A

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   AC: 

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  BAC: B':Z C':B'A