Support dry-run cherry-picks and reverts

This will perform a cherry-pick or revert with the specified arguments,
but not actually create a resulting commit. This allows a user to check
for a `CreateTreeError` on an operation but not modify the repository.

One example use case is in the Delivery team. We want to know if a
security merge request targeting `master` will cherry-pick cleanly to
the previous stable branches. If it does, we can simply cherry-pick it
prior to release; if it doesn't, we need to inform the developer that
they need to create a backport.

3 files changed, 52 insertions, 24 deletions

diff --git a/changelogs/unreleased/rs-cherry-pick-revert-dry-run.yml b/changelogs/unreleased/rs-cherry-pick-revert-dry-run.yml
new file mode 100644
index 000000000..18b3632be
--- /dev/null
+++ b/changelogs/unreleased/rs-cherry-pick-revert-dry-run.yml
@@ -0,0 +1,5 @@
+---
+title: Support dry-run cherry-picks and reverts
+merge_request: 2382
+author:
+type: added
diff --git a/ruby/lib/gitaly_server/operations_service.rb b/ruby/lib/gitaly_server/operations_service.rb
index a6cd3f13d..f85ba5c9c 100644
--- a/ruby/lib/gitaly_server/operations_service.rb
+++ b/ruby/lib/gitaly_server/operations_service.rb
@@ -168,7 +168,8 @@ module GitalyServer
         branch_name: request.branch_name,
         message: request.message.dup,
         start_branch_name: request.start_branch_name.presence,
-        start_repository: start_repository
+        start_repository: start_repository,
+        dry_run: request.dry_run
       )
 
       branch_update = branch_update_result(result)
@@ -196,7 +197,8 @@ module GitalyServer
         branch_name: request.branch_name,
         message: request.message.dup,
         start_branch_name: request.start_branch_name.presence,
-        start_repository: start_repository
+        start_repository: start_repository,
+        dry_run: request.dry_run
       )
 
       branch_update = branch_update_result(result)
diff --git a/ruby/lib/gitlab/git/repository.rb b/ruby/lib/gitlab/git/repository.rb
index 540e61a18..43c0b2a85 100644
--- a/ruby/lib/gitlab/git/repository.rb
+++ b/ruby/lib/gitlab/git/repository.rb
@@ -321,7 +321,7 @@ module Gitlab
         raise ArgumentError, 'Invalid merge source'
       end
 
-      def revert(user:, commit:, branch_name:, message:, start_branch_name:, start_repository:)
+      def revert(user:, commit:, branch_name:, message:, start_branch_name:, start_repository:, dry_run: false)
         OperationService.new(user, self).with_branch(
           branch_name,
           start_branch_name: start_branch_name,
@@ -330,24 +330,35 @@ module Gitlab
 
           revert_tree_id = check_revert_content(commit, start_commit.sha)
 
-          committer = user_to_committer(user)
-
-          create_commit(message: message,
-                        author: committer,
-                        committer: committer,
-                        tree: revert_tree_id,
-                        parents: [start_commit.sha])
+          if dry_run
+            # At this point the tree has been written to the object database but
+            # not committed, so we'll leave it to be cleaned up by `gc`.
+            #
+            # The response expects a SHA, so just return the starting one.
+            start_commit.sha
+          else
+            committer = user_to_committer(user)
+
+            create_commit(
+              message: message,
+              author: committer,
+              committer: committer,
+              tree: revert_tree_id,
+              parents: [start_commit.sha]
+            )
+          end
         end
       end
 
-      def cherry_pick(user:, commit:, branch_name:, message:, start_branch_name:, start_repository:)
+      def cherry_pick(user:, commit:, branch_name:, message:, start_branch_name:, start_repository:, dry_run: false)
         args = {
           user: user,
           commit: commit,
           branch_name: branch_name,
           message: message,
           start_branch_name: start_branch_name,
-          start_repository: start_repository
+          start_repository: start_repository,
+          dry_run: dry_run
         }
 
         rugged_cherry_pick(args)
@@ -822,7 +833,7 @@ module Gitlab
         raise GitError, "Could not delete refs #{ref_names}: #{message}" unless status.zero?
       end
 
-      def rugged_cherry_pick(user:, commit:, branch_name:, message:, start_branch_name:, start_repository:)
+      def rugged_cherry_pick(user:, commit:, branch_name:, message:, start_branch_name:, start_repository:, dry_run: false)
         OperationService.new(user, self).with_branch(
           branch_name,
           start_branch_name: start_branch_name,
@@ -831,17 +842,27 @@ module Gitlab
 
           cherry_pick_tree_id = check_cherry_pick_content(commit, start_commit.sha)
 
-          committer = user_to_committer(user)
-
-          create_commit(message: message,
-                        author: {
-                          email: commit.author_email,
-                          name: commit.author_name,
-                          time: commit.authored_date
-                        },
-                        committer: committer,
-                        tree: cherry_pick_tree_id,
-                        parents: [start_commit.sha])
+          if dry_run
+            # At this point the tree has been written to the object database but
+            # not committed, so we'll leave it to be cleaned up by `gc`.
+            #
+            # The response expects a SHA, so just return the starting one.
+            start_commit.sha
+          else
+            committer = user_to_committer(user)
+
+            create_commit(
+              message: message,
+              author: {
+                email: commit.author_email,
+                name: commit.author_name,
+                time: commit.authored_date
+              },
+              committer: committer,
+              tree: cherry_pick_tree_id,
+              parents: [start_commit.sha]
+            )
+          end
         end
       end