Sheriffing/How To/Merge Conflicts

During merges, sheriffs might run into merge conflicts if code in the same line or near each other has changed in the same file both on the repository to which the merge is done and from which the changesets got pulled.

Resolving merge conflicts without the domain knowledge of the original patch author is prone to error. Sheriffs should not to resolve merge conflicts except for trivial cases or they possess the required domain knowledge. In almost all cases, it is safer to simply backout the changeset causing the merge conflict.

Example

1. mozilla-central was on revision c2fe4b3b1b93
2. Then autoland got "merged" up to revision 597025d8888f to central. Technically it got updated because there were no changes on central since the last merge, so autoland already contained all the changes from central.
3. The attempted merge of the revisions up to a84fff04d938 from inbound to central failed:
   

   ~/mozilla/mozilla-unified$ hg merge -r a84fff04d938 
   merging dom/base/nsRange.cpp
   merging mobile/android/base/java/org/mozilla/gecko/customtabs/CustomTabsActivity.java
   merging mobile/android/base/java/org/mozilla/gecko/webapps/WebAppActivity.java
   merging mobile/android/base/moz.build
   merging taskcluster/scripts/misc/build-sccache.sh
   merging toolkit/library/gtest/rust/Cargo.lock
   merging toolkit/library/rust/Cargo.lock
   merging widget/android/GeneratedJNINatives.h
   merging widget/android/GeneratedJNIWrappers.cpp
   merging widget/android/GeneratedJNIWrappers.h
   warning: conflicts while merging mobile/android/base/moz.build! (edit, then use 'hg resolve --mark')
   warning: conflicts while merging widget/android/GeneratedJNINatives.h! (edit, then use 'hg resolve --mark')
   warning: conflicts while merging widget/android/GeneratedJNIWrappers.cpp! (edit, then use 'hg resolve --mark')
   warning: conflicts while merging widget/android/GeneratedJNIWrappers.h! (edit, then use 'hg resolve --mark')
   282 files updated, 6 files merged, 0 files removed, 4 files unresolved
   use 'hg resolve' to retry unresolved file merges or 'hg update -C .' to abandon

   These files with the conflicts got changed on both inbound and central (which got the change from the previous merge from autoland.
4. Let's abandon the merge and back out the conflicting changes to these files from the integration repository (inbound!). Update the working repository to the inbound revision which shall be merged: hg update -C a84fff04d938
5. Now the bug with the changesets which cause the merge conflicts have to be identified. On hg.mozilla.org, open the repository to which shall be merged (in this case: mozilla-central).
6. Click on 'Files' in the header and navigate to the folder which contains the file with the merge conflict. This is mobile/android/base/ in this case.
7. Click on the revisions name next to the file name, in this case moz.build.
8. Now you can see the recent changes to the file. The latest changes got pushed 2017-11-07 10:28 +0000 and are from bug 1413698.