Add experimental batch support

README.md

@@ -105,6 +105,8 @@ optional arguments:
                            [env var: MARGE_BRANCH_REGEXP] (default: .*)
   --debug               Debug logging (includes all HTTP requests etc).
                            [env var: MARGE_DEBUG] (default: False)
+  --batch               Enable processing MRs in batches.
+                           [env var: MARGE_BATCH] (default: False)
 ```
 Here is a config file example
 ```yaml
@@ -117,6 +119,7 @@ auth-token-file: token.FILE
 branch-regexp: .*
 ci-timeout: 15min
 embargo: Friday 1pm - Monday 9am
+batch: false
 git-timeout: 120s
 gitlab-url: "https://gitlab.example.com"
 impersonate-approvers: true
@@ -178,7 +181,7 @@ docker run \
   --gitlab-url='http://your.gitlab.instance.com'
 ```
 
-For completeness sake, here's how we run marge-bot at smarkets ourselves:
+For completeness sake, here's how we run marge-bot at Smarkets ourselves:
 ```bash
 docker run \
   -e MARGE_AUTH_TOKEN="$(cat marge-bot.token)" \
@@ -206,7 +209,7 @@ Once running, the bot will continuously monitor all projects that have its user
 will pick up any changes in membership at runtime.
 
 
-## Suggested worfklow
+## Suggested workflow
 1. Alice creates a new merge request and assigns Bob and Charlie as reviewers
 
 2. Both review the code and after all issues they raise are resolved by Alice,
@@ -229,7 +232,7 @@ two [standardized git commit trailers](https://www.kernel.org/doc/html/v4.11/pro
 the name of the `marge-bot` user in GitLab).
 
 If you pass `--add-reviewers` and the list of approvers is non-empty and you
-have enough approvers to meet the required approver count, Marge will add a the
+have enough approvers to meet the required approver count, Marge will add the
 following header to each commit message and each reviewer as it rebases the
 target branch into your PR branch:
 
@@ -281,6 +284,66 @@ More than one embargo period can be specified, separated by commas. Any merge
 request assigned to her during an embargo period, will be merged in only once all
 embargoes are over.
 
+
+## Batching Merge Requests
+
+The flag --batch enables testing and merging merge requests in batches. This can
+significantly speed up the rate at which marge-bot processes jobs - not just
+because merge requests can be tested together, but because marge-bot will ensure
+the whole set of merge requests is mergeable first. This includes, for example,
+checking if a merge request is marked as WIP, or does not have enough approvals.
+Essentially, users get faster feedback if there is an issue.
+
+This is currently an experimental feature.
+
+### How it works
+
+If marge-bot finds multiple merge requests to deal with, she attempts to create
+a batch job. She filters the merge requests such that they have all have a
+common target branch, and eliminates those that have not yet passed CI (a
+heuristic to help guarantee the batch will pass CI later).
+
+Once the merge requests have been gathered, a batch branch is created using the
+commits from each merge request in sequence. Any merge request that cannot be
+merged to this branch (e.g. due to a rebase conflict) is filtered out. A new
+merge request is then created for this branch, and tested in CI.
+
+If CI passes, the original merge requests will be merged one by one.
+
+If the batch job fails for any reason, we fall back to merging the first merge
+request, before attempting a new batch job.
+
+### Limitations
+
+* Batch mode is incompatible with the tested-by trailer, as trailers are only
+  added to the original merge requests. This means the tested-by trailer would
+  be added to each merge request's last commit, as opposed to the last commit of
+  the last merge request in the batch, which is the only that should be
+  considered tested.
+* As trailers are added to the original merge requests only, their branches
+  would need to be pushed to in order to reflect this change. This would trigger
+  CI in each of the branches again that would have to be passed before merging,
+  which effectively defeats the point of batching. To workaround this, the
+  current implementation merges to the target branch through git, instead of the
+  GitLab API. GitLab will detect the merge request as having been merged, and
+  update the merge request status accordingly, regardless of whether it has
+  passed CI. This does still mean the triggered CI jobs will be running even
+  though the merge requests are merged. marge-bot will attempt to cancel these
+  pipelines, although this doesn't work too effectively if external CI is used.
+* There is what can be considered to be a flaw in this implementation that could
+  potentially result in a non-green master; consider the following situation:
+
+  1. A batch merge request is created, and passes CI.
+  2. Several merge requests are then merged to master, but one could fail
+     (perhaps due to someone pushing directly to master in between).
+  3. At this point, marge-bot will abort the batch job, resulting in a subset of
+     the batch merge requests having been merged.
+
+  We've guaranteed that individually, each of these merge requests pass CI, and
+  together with some extra merge requests they also pass CI, but this does not
+  guarantee that the subset will. However, this would only happen in a rather
+  convoluted situation that can be considered to be very rare.
+
 ## Restricting the list of projects marge-bot considers
 
 By default marge-bot will work on all projects that she is a member of.
@@ -298,7 +361,7 @@ look-ahead and will match any string not starting with `project/with_reviewers`.
 ## Restricting the list of branches marge-bot considers
 
 It is also possible to restrict the branches marge-bot watches for incoming
-merge requests. By default, marge-bot will process MRs targetted for any branch.
+merge requests. By default, marge-bot will process MRs targeted for any branch.
 You may specify a regexp that target branches must match with `--branch-regexp`.
 
 This could be useful, if for instance, you wanted to set a regular freeze
@@ -324,7 +387,7 @@ git config --global alias.revert-mr '!f() { REVS=$(git mr-revs "$1"); URL="$(git
 ```
 
 E.g. `git revert-mr 123`. This will create a single commit reverting all commits
-that are part of MR 123 with a a commit message that looks like this:
+that are part of MR 123 with a commit message that looks like this:
 
 ```
 Revert <http://gitlab.example.com/mygropup/myproject/merge_requests/123>

marge/app.py

@@ -99,20 +99,25 @@ def regexp(str_regex):
         metavar='INTERVAL[,..]',
         help='Time(s) during which no merging is to take place, e.g. "Friday 1pm - Monday 9am".\n',
     )
-    merge_group = parser.add_mutually_exclusive_group(required=False)
-    merge_group.add_argument(
+    experimental_group = parser.add_mutually_exclusive_group(required=False)
+    experimental_group.add_argument(
         '--use-merge-strategy',
         action='store_true',
         help=(
             'Use git merge instead of git rebase (EXPERIMENTAL)\n'
             'Enable if you use a workflow based on merge-commits and not linear history.\n'
         ),
     )
-    merge_group.add_argument(
+    experimental_group.add_argument(
         '--add-tested',
         action='store_true',
         help='Add "Tested: marge-bot <$MR_URL>" for the final commit on branch after it passed CI.\n',
     )
+    experimental_group.add_argument(
+        '--batch',
+        action='store_true',
+        help='Enable processing MRs in batches (EXPERIMENTAL)\n',
+    )
     parser.add_argument(
         '--add-part-of',
         action='store_true',
@@ -223,6 +228,9 @@ def main(args=None):
             )
             options.ci_timeout = timedelta(minutes=options.max_ci_time_in_minutes)
 
+        if options.batch:
+            logging.warning('Experimental batch mode enabled')
+
         config = bot.BotConfig(
             user=user,
             ssh_key_file=ssh_key_file,
@@ -238,7 +246,8 @@ def main(args=None):
                 embargo=options.embargo,
                 ci_timeout=options.ci_timeout,
                 use_merge_strategy=options.use_merge_strategy,
-            )
+            ),
+            batch=options.batch,
         )
 
         marge_bot = bot.Bot(api=api, config=config)