Add start_sha to commits API

When passing start_branch on committing from the WebIDE, it's possible
that the branch has changed since editing started, which results in the
change being applied on top of the latest commit in the branch and
overwriting the new changes.

By passing the start_sha instead we can make sure that the change is
applied on top of the commit which the user started editing from.

4 files changed, 21 insertions, 11 deletions

diff --git a/lib/api/commits.rb b/lib/api/commits.rb
index c414ad75d9d..0aeb9584641 100644
--- a/lib/api/commits.rb
+++ b/lib/api/commits.rb
@@ -76,7 +76,7 @@ module API
         detail 'This feature was introduced in GitLab 8.13'
       end
       params do
-        requires :branch, type: String, desc: 'Name of the branch to commit into. To create a new branch, also provide `start_branch`.', allow_blank: false
+        requires :branch, type: String, desc: 'Name of the branch to commit into. To create a new branch, also provide either `start_branch` or `start_sha`, and optionally `start_project`.', allow_blank: false
         requires :commit_message, type: String, desc: 'Commit message'
         requires :actions, type: Array, desc: 'Actions to perform in commit' do
           requires :action, type: String, desc: 'The action to perform, `create`, `delete`, `move`, `update`, `chmod`', values: %w[create update move delete chmod].freeze
@@ -98,12 +98,16 @@ module API
             requires :execute_filemode, type: Boolean, desc: 'When `true/false` enables/disables the execute flag on the file.'
           end
         end
-        optional :start_branch, type: String, desc: 'Name of the branch to start the new commit from'
-        optional :start_project, types: [Integer, String], desc: 'The ID or path of the project to start the commit from'
+
+        optional :start_branch, type: String, desc: 'Name of the branch to start the new branch from'
+        optional :start_sha, type: String, desc: 'SHA of the commit to start the new branch from'
+        mutually_exclusive :start_branch, :start_sha
+
+        optional :start_project, types: [Integer, String], desc: 'The ID or path of the project to start the new branch from'
         optional :author_email, type: String, desc: 'Author email for commit'
         optional :author_name, type: String, desc: 'Author name for commit'
         optional :stats, type: Boolean, default: true, desc: 'Include commit stats'
-        optional :force, type: Boolean, default: false, desc: 'When `true` overwrites the target branch with a new commit based on the `start_branch`'
+        optional :force, type: Boolean, default: false, desc: 'When `true` overwrites the target branch with a new commit based on the `start_branch` or `start_sha`'
       end
       post ':id/repository/commits' do
         if params[:start_project]
@@ -118,7 +122,7 @@ module API
 
         attrs = declared_params
         attrs[:branch_name] = attrs.delete(:branch)
-        attrs[:start_branch] ||= attrs[:branch_name]
+        attrs[:start_branch] ||= attrs[:branch_name] unless attrs[:start_sha]
         attrs[:start_project] = start_project if start_project
 
         result = ::Files::MultiService.new(user_project, current_user, attrs).execute
diff --git a/lib/gitlab/git.rb b/lib/gitlab/git.rb
index 44a62586a23..df9f33baec2 100644
--- a/lib/gitlab/git.rb
+++ b/lib/gitlab/git.rb
@@ -9,6 +9,7 @@ module Gitlab
     # https://github.com/git/git/blob/3ad8b5bf26362ac67c9020bf8c30eee54a84f56d/cache.h#L1011-L1012
     EMPTY_TREE_ID = '4b825dc642cb6eb9a060e54bf8d69288fbee4904'.freeze
     BLANK_SHA = ('0' * 40).freeze
+    COMMIT_ID = /\A[0-9a-f]{40}\z/.freeze
     TAG_REF_PREFIX = "refs/tags/".freeze
     BRANCH_REF_PREFIX = "refs/heads/".freeze
 
@@ -65,6 +66,10 @@ module Gitlab
         ref == BLANK_SHA
       end
 
+      def commit_id?(ref)
+        COMMIT_ID.match?(ref)
+      end
+
       def version
         Gitlab::Git::Version.git_version
       end
diff --git a/lib/gitlab/git/repository.rb b/lib/gitlab/git/repository.rb
index a7d9ba51277..6e8aa5d578e 100644
--- a/lib/gitlab/git/repository.rb
+++ b/lib/gitlab/git/repository.rb
@@ -873,13 +873,13 @@ module Gitlab
       def multi_action(
         user, branch_name:, message:, actions:,
         author_email: nil, author_name: nil,
-        start_branch_name: nil, start_repository: self,
+        start_branch_name: nil, start_sha: nil, start_repository: self,
         force: false)
 
         wrapped_gitaly_errors do
           gitaly_operation_client.user_commit_files(user, branch_name,
               message, actions, author_email, author_name,
-              start_branch_name, start_repository, force)
+              start_branch_name, start_repository, force, start_sha)
         end
       end
       # rubocop:enable Metrics/ParameterLists
diff --git a/lib/gitlab/gitaly_client/operation_service.rb b/lib/gitlab/gitaly_client/operation_service.rb
index 783c2ff0915..33ca428a942 100644
--- a/lib/gitlab/gitaly_client/operation_service.rb
+++ b/lib/gitlab/gitaly_client/operation_service.rb
@@ -325,11 +325,11 @@ module Gitlab
       # rubocop:disable Metrics/ParameterLists
       def user_commit_files(
         user, branch_name, commit_message, actions, author_email, author_name,
-        start_branch_name, start_repository, force = false)
+        start_branch_name, start_repository, force = false, start_sha = nil)
         req_enum = Enumerator.new do |y|
           header = user_commit_files_request_header(user, branch_name,
           commit_message, actions, author_email, author_name,
-          start_branch_name, start_repository, force)
+          start_branch_name, start_repository, force, start_sha)
 
           y.yield Gitaly::UserCommitFilesRequest.new(header: header)
 
@@ -445,7 +445,7 @@ module Gitlab
       # rubocop:disable Metrics/ParameterLists
       def user_commit_files_request_header(
         user, branch_name, commit_message, actions, author_email, author_name,
-        start_branch_name, start_repository, force)
+        start_branch_name, start_repository, force, start_sha)
 
         Gitaly::UserCommitFilesRequestHeader.new(
           repository: @gitaly_repo,
@@ -456,7 +456,8 @@ module Gitlab
           commit_author_email: encode_binary(author_email),
           start_branch_name: encode_binary(start_branch_name),
           start_repository: start_repository.gitaly_repository,
-          force: force
+          force: force,
+          start_sha: encode_binary(start_sha)
         )
       end
       # rubocop:enable Metrics/ParameterLists