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neels, 06/17/2016 03:55 PM

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h1. Contributing using Gerrit
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{{>toc}}
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At [[OpenBSC:OsmoDevCon2016]] we discussed problems with our past contribution / patch submission process using mails on the mailing list as well as patchwork.  The result is that we want to give Gerrit a try for some time and see if it helps us to have a better process
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Gerrit is a review tool that integrates nicely with git and ssh. You can find general information about Gerrit at https://www.gerritcodereview.com/
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The advantages of Gerrit are:
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* patch submission status is automatically tracked, also with several revisions for a patch set.
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* patches are build-tested (and possibly even further tested) by jenkins before they are applied
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* developers + maintainers can formally vote on a patch (developer: -1/0/+1, maintainer: -2/0/+2)
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* once a patch has +2 score, it can be (automatically) merged into master
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* patch sumissions not via git send-email but direcly from git
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h2. Osmocom Subprojects using Gerrit
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The following projects use Gerrit to contribute changes:
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* libosmocore.git
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* libosmo-abis.git
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* libosmo-netif.git
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* libosmo-sccp.git
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* libsmpp34.git
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* openbsc.git
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* osmo-bts.git
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* osmo-iuh.git
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* osmo-pcu.git
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* cellmgr-ng.git
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* osmo-sip-connector.git
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h2. Configuring Gerrit/Account
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You will need to sign-up at https://gerrit.osmocom.org/login/. If you have an Osmocom Redmine account you can use https://osmocom.org/openid as OpenID provider. If you have no Osmocom redmine account, you can simply create one online at the "Register" link in the upper right corner.
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Even without an existing or new redmine account, you should also be able to use any other OpenID provider to authenticate against gerrit (untested).
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After the initial sign-up you will need to:
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* Pick a username (can not be changed)
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* Add your public ssh key(s)
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* Add email addresses you intend to use as author/comitter
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h2. Setting up Gerrit for commits and pushing
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* Add the remote to be able to fetch and push to gerrit
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* Fetch the commit hook that adds Change-Id to each commit to uniquely identify a commit
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<pre>
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USERNAME=gerrit_user_name
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PROJECT=$(basename $PWD)
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git remote add gerrit ssh://$USERNAME@gerrit.osmocom.org:29418/$PROJECT.git
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scp -P 29418 $USERNAME@gerrit.osmocom.org:hooks/commit-msg .git/hooks/
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</pre>
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* In case your local username matches the gerrit username, the setup shortens to
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<pre>
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PROJECT=$(basename $PWD)
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git remote add gerrit ssh://gerrit.osmocom.org:29418/$PROJECT.git
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scp -P 29418 gerrit.osmocom.org:hooks/commit-msg .git/hooks/
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</pre>
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Then
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* Push for review
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<pre>
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git push gerrit HEAD:refs/for/master
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</pre>
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* Push a user branch
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<pre>
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git push gerrit HEAD:refs/heads/users/$USERNAME/topic
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</pre>
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* Directly push to master if you are allowed to
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<pre>
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git push gerrit HEAD:refs/heads/master
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</pre>
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* List changesets in gerrit
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<pre>
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git ls-remote gerrit changes/*
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</pre>
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h2. Tips and Tricks
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If you need to adjust and re-submit patches the easiest way is to create throw-away branch ("R D" in magit-gerrit in emacs for example), make you changes/amendments and than send patch(es) back to gerrit while removing temporary branch automatically with "git review -f".
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h2. Submitting Branches
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On a feature branch, one typically has numerous commits that depend on their preceding commits.
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Unfortunately, Gerrit has some issues with those, which boil down to these rules:
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1. Do not push your branch "privately" to the gerrit repository. You can, but Gerrit will force
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   you to remove all Change-Ids from commit logs once you want to submit it to master.
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2. To re-submit a branch, make sure to cosmetically tweak the branch's first commit log message
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   before each re-submission (keep the Change-Id, really just a cosmetic change).
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How to edit commit logs:
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<pre>
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cd openbsc
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git co my-branch
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git rebase -i master
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# replace all 'pick' with 'r' (or 'reword'), exit your editor
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# git presents each commit log message for editing
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</pre>
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h3. Why is this is necessary?
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There are different merge strategies that Gerrit performs to accept your patches.
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Each project can be configured to a specific merge strategy, but unfortunately you can't
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decide on a strategy per patch submission.
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It seems that the "Merge if Necessary" strategy is best supported, but it creates non-linear
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history with numerous merge commits that are usually not at all necessary.
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Instead, the "Cherry Pick" strategy puts each patch onto current master's HEAD to create
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linear history. However, this will cause merge failures as soon as one patch depends on
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another submitted patch, as typical for a feature branch submission.
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So we prefer the "Rebase if Necessary" strategy, which always tries to apply your patches to
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the current master HEAD, in sequence with the previous patches on the same branch.
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However, some problems still remain, including some bugs in "Rebase if Necessary".
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There are in general three problems with "Rebase if Necessary", with different solutions:
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h4. Fast-Forward
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If your branch sits at master's HEAD, Gerrit may refuse to accept the submission, because
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it thinks that no new changes are submitted. This is a bug in Gerrit, which holger has
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fixed manually in our Gerrit installation.
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https://bugs.chromium.org/p/gerrit/issues/detail?id=4158
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h4. Amending a Submitted Branch, First Commit Unchanged
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Say you have submitted a branch with 5 commits, and the third commit has gotten a -1 vote.
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You have fixed the problem in your local git and now would like to re-submit it. Of course
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you want to keep the Change-Ids identical so that Gerrit shows them as amends.
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Gerrit will refuse to accept your patch submission if the first commit on the branch is
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still identical.
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The quickest work-around: slightly reword the first branch commit's commit log message.
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Yes, that's enough! All commits that are identical on the branch are now ignored,
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it's only the first branch commit that Gerrit will complain about.
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The cause: Gerrit refuses to accept a commit with a Change-Id that it already knows and
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where the commit hash is identical.
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Some details: you could modify all the Change-Ids, but now your branch submission would
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open entirely new review entries and you would have to abandon your previous submission.
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Comments on the first submission are lost and you cannot diff between patch sets.
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Now, if you just cosmetically tweak the first commit's log message, the commit hash
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is changed. Since the following commits contain their predecessor's commit hash, now
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all of the branch's commit hashes are modified, and gerrit happily accepts them as a 
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new patch set. It will still pick up the Change-Ids (which you shouldn't edit) and 
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notice if commits have remained identical (keeping the votes). But with the minor
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commit log tweak, it will no longer thwart your re-submission with an error message.
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h4. Private Branches
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Say you have an extensive feature in development, and you want to keep it on the
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upstream git repository to a) keep it safe and b) collaborate with other devs on it.
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So, of course, you have regularly pushed to refs/heads/users/me/feature.
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Since you have the gerrit commit hook installed, your feature branch already has
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Change-Id tags in all commit log messages.
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Now your feature is complete and you would like to submit it to master.
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Alas, Gerrit refuses to accept your patch submission for master, because it
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knows the Change-Ids are also on a different branch.
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IMHO this is a very clumsy non-feature of Gerrit. It enforces that a Change-Id
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must be unique across all branches, so that each submission for review is
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separate for each branch. Instead, it should handle Change-Ids per-branch, so
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that you can have the same change submitted to different branches, as separate
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patch submissions, without having to cosmetically adjust the Change-Id.
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Solutions:
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Currently the best solution is to *not* push private branches to the gerrit
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repository. This makes collaborating publicly on branches currently impossible.
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Since our git.osmocom.org now are non-pushable, the only openly public place
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would be the gerrit repos; intead, you need a separate, "private" git repository
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to collaborate. IMHO we must still find a good solution to this problem.
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If you have already pushed your Change-Ids to refs/heads/* somewhere, the only
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way to submit to master is to, yes, edit every single commit log message on
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your branch to remove the Change-Id, to let the commit hook re-add a new one.
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Of course you can't push your modified Change-Ids back to the private branch on
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the gerrit repos, because then Gerrit will have the same problem all over.
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That means you can't easily edit branch commits and keep them public until they
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are through review, because between pushes to your private branch and pushes for
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review, you have to switch the Change-Id for the proper branch. Note: If you remove
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and re-create the Change-Ids over and over, you will create new review items instead
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of amending previous patch submissions with fixes.
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A third solution might be to not have Change-Ids on your private branch, but that
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means that once you want to submit for review, you also need to edit every single
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commit log message on your branch, and then you're again stuck with the same problem.
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My wish would be to teach Gerrit to ignore all of our private branches
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and simply not look for Change-Id tags in those, so we can push them for review
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while also keeping them on a private branch at the same time.
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An easier way might be to have a public collaboration repository, e.g. to allow
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pushing to git.osmocom.org repositories again and only handle for/master reviews
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via the Gerrit repos. This won't work if the gerrit repos fetches upstream branches,
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so we might need yet another place for repositories -- we already have git.osmocom.org
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and gerrit, so it would be confusing to have even a third set of public repositories.
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Let's hope to find nicer solutions...
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h2. Project Config Details
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There are more non-obvious configuration items with Gerrit.
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h3. Allow content merges
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By default, gerrit compares patches only by the files' paths. If two paths are the same,
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it immediately shows them as conflicts (path conflicts).
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In software development, a conflict usually means an actual content conflict, so if the
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edits are in two entirely separate places in the file, we don't consider this a conflict.
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By setting 'Allow content merges' to TRUE in the git project config, we tell Gerrit to
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perform text merges of the submitted patches and only complain about actual content
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conflicts, in the usual software engineering sense.
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h3. Create a new change for every commit not in the target branch
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If this is set to FALSE, Gerrit makes re-submitting a branch with fixes impossible.
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