Mercurial - backout an old merge

Question

I have a branch that looks like this:

A->B->C->D->...->Z
     ^
1->2-^

where C is a merge from 2 an

Answer 1

Merge is public and there are public commits on this merge

Assume we have this published history of commits (top is the newest):

                  revZ
                   |
                  ...  
                   |
                  revD
                   |
                  revC     <- unwated merge commit (rev2 to revB)
                   |   \    
wanted branch ->  revB  rev2    <- unwanted branch
                   |     |

Instead, we would like to have top state as if we had this history:

                  revZ'
                   |
                  ...  
                   |
                  revD'
                   |       
wanted branch ->  revB  rev2    <- unwanted branch
                   |     |

How to achive this (step by step)

1. Callapse the history after merge commit (revD - revZ) into one commit

   $ hg update -r revC                # Update to merge commit
   $ hg revert --all -r revZ          # revert to the newest commit
   $ hg commit -m "collapsed commits" # Create new commit (revTmp1)
   
                      revZ
                       |  
                      ...
                       |  
              revTmp1 revD
                    \ /
                    revC
                     |  \    
                   revB  rev2
                     |    |
   

2. Copy changes after merge (revTmp1) to wanted branch (revB)

   $ hg update -r revB    # Update to the last "wanted" commit before merge
   $ hg graft -r revTmp1  # Copy changes from revTmp1 (create revTmp2 commit)
   
                      revZ
                       |  
                      ...
                       |  
              revTmp1 revD
                    \ /
           revTmp2  revC
                 \  / \    
                revB  rev2
                  |     |
   

3. Create a "backout" commit

   $ hg update -r revZ                   # Update to the top commit
   $ hg revert --all -r revTmp2          # Copy state revTmp2
   $ hg commit -m "reverted revC merge"  # Create revZ' commit
   
                      revZ'
                       |
                      revZ
                       |  
                      ...
                       |  
              revTmp1 revD
                    \ /
           revTmp2  revC
                 \  / \    
                 revB  rev2
                  |     |
   

4. Clean the temporary commits

   $ hg strip revTmp1 revTmp2
   
                     revZ'          <- reverted revC merge
                      |  
                     revZ
                      |  
                     revD
                      |
                     revC     <- unwated merge commit (rev2 to revB)
                      |   \    
   wanted branch ->  revB  rev2    <- unwanted branch
                      |     |