What's the best way to back out multiple changesets in mercurial?

Question

Is the most reliable method to go one-by-one, using the backout command for each of many changesets, or is there a way to create one big reversal changeset to cover a whole bunc

Answer 1

There is --collapse option for rebase.

Helgi's answer can be upgraded into:

1 -- A -- 2 -- B -- 3 -- C -- 4 -- 5
                          \
                           C' -- B' -- A'

$ hg update --clean C
$ hg backout --rev C --message "Backed out changeset: C"
$ hg backout --rev B
$ hg commit --message "Backed out changeset: B"
$ hg backout --rev A
$ hg commit --message "Backed out changeset: A"
$ hg rebase --collapse --source C' --dest 5
$ hg commit --message "Backed out C, B, A"

which will result in the following

1 -- A -- 2 -- B -- 3 -- C -- 4 -- 5 -- C'B'A'

However, backing out in separate branch may result in [logical] conflict in the subsequent merge.

1 -- A -- 2 -- B -- 3 -- X  --  4 
                     \           \
                      B' -- A' -- M

if X depends on A or B, then M will have conflict (at least logical conflict).