[Bug 5457] New: rebase could be smarter reading obs graph to avoid merge conflict

[mercurial-bugs at mercurial-scm.org]

https://bz.mercurial-scm.org/show_bug.cgi?id=5457

            Bug ID: 5457
           Summary: rebase could be smarter reading obs graph to avoid
                    merge conflict
           Product: Mercurial
           Version: default branch
          Hardware: All
                OS: All
            Status: UNCONFIRMED
          Severity: feature
          Priority: wish
         Component: Mercurial
          Assignee: bugzilla at mercurial-scm.org
          Reporter: arcppzju+hgbug at gmail.com
                CC: mercurial-devel at selenic.com

Given 3 changesets, where the second has a "copy" and "modify" behavior, and
the third changes the same file again:

  @  2:b17cae3eec88 b3
  |
  |  diff --git a/b b/b
  |  --- a/b
  |  +++ b/b
  |  @@ -1,2 +1,3 @@
  |   1
  |   2
  |  +3
  |
  o  1:bf41062b3958 b2
  |
  |  diff --git a/a b/b
  |  copy from a
  |  copy to b
  |  --- a/a
  |  +++ b/b
  |  @@ -1,1 +1,2 @@
  |   1
  |  +2
  |
  o  0:d847f600340d a1

     diff --git a/a b/a
     new file mode 100644
     --- /dev/null
     +++ b/a
     @@ -0,0 +1,1 @@
     +1


If the user decides to modify the second changeset to use a new name, using
"hg mv", followed by "hg amend", they will get the following graph:

  @  4:5835b9fd5e6c b2
  |
  |  diff --git a/a b/c
  |  copy from a
  |  copy to c
  |  --- a/a
  |  +++ b/c
  |  @@ -1,1 +1,2 @@
  |   1
  |  +2
  |
  | o  2:35963a930f76 b3
  | |
  | |  diff --git a/b b/b
  | |  --- a/b
  | |  +++ b/b
  | |  @@ -1,2 +1,3 @@
  | |   1
  | |   2
  | |  +3
  | |
  | x  1:7521d3ae89e6 b2
  |/
  |    diff --git a/a b/b
  |    copy from a
  |    copy to b
  |    --- a/a
  |    +++ b/b
  |    @@ -1,1 +1,2 @@
  |     1
  |    +2
  |
  o  0:6ec6f1968549 a1

     diff --git a/a b/a
     new file mode 100644
     --- /dev/null
     +++ b/a
     @@ -0,0 +1,1 @@
     +1

And evolve (or rebase) will report merge conflict:

  $ hg rebase -s 2 -d 4
  rebasing 2:35963a930f76 "b3"
  other [source] changed b which local [dest] deleted
  use (c)hanged version, leave (d)eleted, or leave (u)nresolved? 

If, instead of amending the second changeset, the user creates a new commit
with the rename, the rebase works without merge conflict:

  @  3:a7c7cb534733 new mv
  |
  |  diff --git a/b b/c
  |  rename from b
  |  rename to c
  |
  | o  2:37d2a0dca739 b3
  |/
  |    diff --git a/b b/b
  |    --- a/b
  |    +++ b/b
  |    @@ -1,2 +1,3 @@
  |     1
  |     2
  |    +3
  |
  o  1:8b0747b69e58 b2
  |
  |  diff --git a/a b/b
  |  copy from a
  |  copy to b
  |  --- a/a
  |  +++ b/b
  |  @@ -1,1 +1,2 @@
  |   1
  |  +2
  |
  o  0:c784163efb7c a1

     diff --git a/a b/a
     new file mode 100644
     --- /dev/null
     +++ b/a
     @@ -0,0 +1,1 @@
     +1

  $ hg rebase -s 2 -d 3
  rebasing 2:37d2a0dca739 "b3"
  merging c and b to c

Therefore, during rebase, we may want to travel the obsolete graph to get more
information to resolve merge conflicts. The above example shows how it can be
useful in the file renaming (copy-tracing) case. While it also applies to the
general content-change case without renaming.

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