Sheriffing/How To/Backouts

When a patch is landed that causes build or test failures, it must be backed out to restore the tree to a passing state again. This page documents the procedure for performing backouts both for Gecko changes and Gaia.

Gecko Backouts

Prerequisites

• Install the qbackout extension from https://hg.mozilla.org/hgcustom/version-control-tools/file/default/hgext/qbackout
• Instructions: https://hg.mozilla.org/hgcustom/version-control-tools/file/default/hgext/qbackout/README

Usage

Backout of a single changeset

hg qbackout -e -r 123456789abc

Where -r is the specific revision to be backed out. The -e option opens an editor to edit the commit message prior to committing. This is useful for adding DONTBUILD, CLOSED TREE, etc before pushing.

Backout of multiple consecutive changesets

hg qbackout -e -s -r 123456789abc:cba987654321

Where the -r is now specifying a range of commits (from 123456789abc to cba987654321). The -s option commits the backouts as a single commit. Otherwise, each changeset in the range will be backed out as separate commits.

Backout of multiple non-consecutive changesets

hg qbackout -e -s -r 123456789abc+cba987654321

Useful when there are commits in between the ones that need backing out.

Backout of a combination of consecutive and non-consecutive changesets

hg qbackout -e -s -r 123456789abc:cba987654321+xyz456789123

Useful when a follow-up patch needs backing out in addition to the main one (i.e. someone had a follow-up bustage fix push) or an additional commit conflicts with the backout and needs backing out as well.

Pushing

hg qfinish -a

Finalizes the patch for pushing to the remote repo.

What Ifs

• If the remote repo has additional changesets (i.e. a push race), hg pull --rebase will pull the new changesets and rebase the local changes on top of them.
  • This requires the rebase extension to be enabled in .hgrc
• If the commit message needs to be modifying after qfinish (i.e. forgotten CLOSED TREE), hg commit --amend will open the backout in an editor for modification.

Gaia Backouts

If the changeset is not part of a merge (i.e. pushed directly instead instead of a merged Pull Request):

git revert 12345ab

If the changeset is a merged Pull Request:

git revert -m 1 12345ab

For reverting multiple git commits at the same time, see this StackOverflow post.