Merge branch '52568-external-mr-diffs' into 'master'

Allow merge request diffs to be placed into an object store

Closes #52568

See merge request gitlab-org/gitlab-ce!24276

16 files changed, 581 insertions, 75 deletions

diff --git a/app/models/merge_request_diff.rb b/app/models/merge_request_diff.rb
index a3029a54604..712347e76ed 100644
--- a/app/models/merge_request_diff.rb
+++ b/app/models/merge_request_diff.rb
@@ -7,6 +7,7 @@ class MergeRequestDiff < ActiveRecord::Base
   include IgnorableColumn
   include EachBatch
   include Gitlab::Utils::StrongMemoize
+  include ObjectStorage::BackgroundMove
 
   # Don't display more than 100 commits at once
   COMMITS_SAFE_SIZE = 100
@@ -15,9 +16,13 @@ class MergeRequestDiff < ActiveRecord::Base
                 :st_diffs
 
   belongs_to :merge_request
+
   manual_inverse_association :merge_request, :merge_request_diff
 
-  has_many :merge_request_diff_files, -> { order(:merge_request_diff_id, :relative_order) }
+  has_many :merge_request_diff_files,
+    -> { order(:merge_request_diff_id, :relative_order) },
+    inverse_of: :merge_request_diff
+
   has_many :merge_request_diff_commits, -> { order(:merge_request_diff_id, :relative_order) }
 
   state_machine :state, initial: :empty do
@@ -45,10 +50,14 @@ class MergeRequestDiff < ActiveRecord::Base
 
   scope :recent, -> { order(id: :desc).limit(100) }
 
+  mount_uploader :external_diff, ExternalDiffUploader
+
   # All diff information is collected from repository after object is created.
   # It allows you to override variables like head_commit_sha before getting diff.
   after_create :save_git_content, unless: :importing?
 
+  after_save :update_external_diff_store, if: :external_diff_changed?
+
   def self.find_by_diff_refs(diff_refs)
     find_by(start_commit_sha: diff_refs.start_sha, head_commit_sha: diff_refs.head_sha, base_commit_sha: diff_refs.base_sha)
   end
@@ -241,10 +250,97 @@ class MergeRequestDiff < ActiveRecord::Base
     end
   end
 
+  # Carrierwave defines `write_uploader` dynamically on this class, so `super`
+  # does not work. Alias the carrierwave method so we can call it when needed
+  alias_method :carrierwave_write_uploader, :write_uploader
+
+  # The `external_diff`, `external_diff_store`, and `stored_externally`
+  # columns were introduced in GitLab 11.8, but some background migration specs
+  # use factories that rely on current code with an old schema. Without these
+  # `has_attribute?` guards, they fail with a `MissingAttributeError`.
+  #
+  # For more details, see: https://gitlab.com/gitlab-org/gitlab-ce/issues/44990
+
+  def write_uploader(column, identifier)
+    carrierwave_write_uploader(column, identifier) if has_attribute?(column)
+  end
+
+  def update_external_diff_store
+    update_column(:external_diff_store, external_diff.object_store) if
+      has_attribute?(:external_diff_store)
+  end
+
+  def external_diff_changed?
+    super if has_attribute?(:external_diff)
+  end
+
+  def stored_externally
+    super if has_attribute?(:stored_externally)
+  end
+  alias_method :stored_externally?, :stored_externally
+
+  # If enabled, yields the external file containing the diff. Otherwise, yields
+  # nil. This method is not thread-safe, but it *is* re-entrant, which allows
+  # multiple merge_request_diff_files to load their data efficiently
+  def opening_external_diff
+    return yield(nil) unless stored_externally?
+    return yield(@external_diff_file) if @external_diff_file
+
+    external_diff.open do |file|
+      begin
+        @external_diff_file = file
+
+        yield(@external_diff_file)
+      ensure
+        @external_diff_file = nil
+      end
+    end
+  end
+
   private
 
   def create_merge_request_diff_files(diffs)
-    rows = diffs.map.with_index do |diff, index|
+    rows =
+      if has_attribute?(:external_diff) && Gitlab.config.external_diffs.enabled
+        build_external_merge_request_diff_files(diffs)
+      else
+        build_merge_request_diff_files(diffs)
+      end
+
+    # Faster inserts
+    Gitlab::Database.bulk_insert('merge_request_diff_files', rows)
+  end
+
+  def build_external_merge_request_diff_files(diffs)
+    rows = build_merge_request_diff_files(diffs)
+    tempfile = build_external_diff_tempfile(rows)
+
+    self.external_diff = tempfile
+    self.stored_externally = true
+
+    rows
+  ensure
+    tempfile&.unlink
+  end
+
+  def build_external_diff_tempfile(rows)
+    Tempfile.open(external_diff.filename) do |file|
+      rows.inject(0) do |offset, row|
+        data = row.delete(:diff)
+        row[:external_diff_offset] = offset
+        row[:external_diff_size] = data.size
+
+        file.write(data)
+
+        offset + data.size
+      end
+
+      file
+    end
+  end
+
+  def build_merge_request_diff_files(diffs)
+    diffs.map.with_index do |diff, index|
       diff_hash = diff.to_hash.merge(
         binary: false,
         merge_request_diff_id: self.id,
@@ -261,18 +357,20 @@ class MergeRequestDiff < ActiveRecord::Base
         end
       end
     end
-
-    Gitlab::Database.bulk_insert('merge_request_diff_files', rows)
   end
 
   def load_diffs(options)
-    collection = merge_request_diff_files
+    # Ensure all diff files operate on the same external diff file instance if
+    # present. This reduces file open/close overhead.
+    opening_external_diff do
+      collection = merge_request_diff_files
 
-    if paths = options[:paths]
-      collection = collection.where('old_path IN (?) OR new_path IN (?)', paths, paths)
-    end
+      if paths = options[:paths]
+        collection = collection.where('old_path IN (?) OR new_path IN (?)', paths, paths)
+      end
 
-    Gitlab::Git::DiffCollection.new(collection.map(&:to_hash), options)
+      Gitlab::Git::DiffCollection.new(collection.map(&:to_hash), options)
+    end
   end
 
   def load_commits
diff --git a/app/models/merge_request_diff_file.rb b/app/models/merge_request_diff_file.rb
index a9f110bec5c..e8d936e265c 100644
--- a/app/models/merge_request_diff_file.rb
+++ b/app/models/merge_request_diff_file.rb
@@ -4,7 +4,7 @@ class MergeRequestDiffFile < ActiveRecord::Base
   include Gitlab::EncodingHelper
   include DiffFile
 
-  belongs_to :merge_request_diff
+  belongs_to :merge_request_diff, inverse_of: :merge_request_diff_files
 
   def utf8_diff
     return '' if diff.blank?
@@ -13,6 +13,16 @@ class MergeRequestDiffFile < ActiveRecord::Base
   end
 
   def diff
-    binary? ? super.unpack('m0').first : super
+    content =
+      if merge_request_diff&.stored_externally?
+        merge_request_diff.opening_external_diff do |file|
+          file.seek(external_diff_offset)
+          file.read(external_diff_size)
+        end
+      else
+        super
+      end
+
+    binary? ? content.unpack('m0').first : content
   end
 end
diff --git a/app/uploaders/external_diff_uploader.rb b/app/uploaders/external_diff_uploader.rb
new file mode 100644
index 00000000000..d2707cd0777
--- /dev/null
+++ b/app/uploaders/external_diff_uploader.rb
@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+class ExternalDiffUploader < GitlabUploader
+  include ObjectStorage::Concern
+
+  storage_options Gitlab.config.external_diffs
+
+  alias_method :upload, :model
+
+  def filename
+    "diff-#{model.id}"
+  end
+
+  def store_dir
+    dynamic_segment
+  end
+
+  private
+
+  def dynamic_segment
+    File.join(model.model_name.plural, "mr-#{model.merge_request_id}")
+  end
+end
diff --git a/changelogs/unreleased/52568-external-mr-diffs.yml b/changelogs/unreleased/52568-external-mr-diffs.yml
new file mode 100644
index 00000000000..b1c9d5cb809
--- /dev/null
+++ b/changelogs/unreleased/52568-external-mr-diffs.yml
@@ -0,0 +1,5 @@
+---
+title: Allow merge request diffs to be placed into an object store
+merge_request: 24276
+author:
+type: added
diff --git a/config/gitlab.yml.example b/config/gitlab.yml.example
index 6fc33e8971e..be23166cb7b 100644
--- a/config/gitlab.yml.example
+++ b/config/gitlab.yml.example
@@ -166,6 +166,23 @@ production: &base
     #     aws_signature_version: 4 # For creation of signed URLs. Set to 2 if provider does not support v4.
     #     endpoint: 'https://s3.amazonaws.com' # default: nil - Useful for S3 compliant services such as DigitalOcean Spaces
 
+  ## Merge request external diff storage
+  external_diffs:
+    # If disabled (the default), the diffs are in-database. Otherwise, they can
+    # be stored on disk, or in object storage
+    enabled: false
+    # The location where external diffs are stored (default: shared/lfs-external-diffs).
+    # storage_path: shared/external-diffs
+    # object_store:
+    #   enabled: false
+    #   remote_directory: external-diffs
+    #   background_upload: false
+    #   proxy_download: false
+    #   connection:
+    #     provider: AWS
+    #     aws_access_key_id: AWS_ACCESS_KEY_ID
+    #     aws_secret_access_key: AWS_SECRET_ACCESS_KEY
+    #     region: us-east-1
 
   ## Git LFS
   lfs:
@@ -733,6 +750,18 @@ test:
   <<: *base
   gravatar:
     enabled: true
+  external_diffs:
+    enabled: false
+    # The location where external diffs are stored (default: shared/external-diffs).
+    # storage_path: shared/external-diffs
+    object_store:
+      enabled: false
+      remote_directory: external-diffs # The bucket name
+      connection:
+        provider: AWS # Only AWS supported at the moment
+        aws_access_key_id: AWS_ACCESS_KEY_ID
+        aws_secret_access_key: AWS_SECRET_ACCESS_KEY
+        region: us-east-1
   lfs:
     enabled: false
     # The location where LFS objects are stored (default: shared/lfs-objects).
diff --git a/config/initializers/1_settings.rb b/config/initializers/1_settings.rb
index 1aed41e02ab..dfcf1e648b4 100644
--- a/config/initializers/1_settings.rb
+++ b/config/initializers/1_settings.rb
@@ -216,6 +216,14 @@ Settings.pages['admin'] ||= Settingslogic.new({})
 Settings.pages.admin['certificate'] ||= ''
 
 #
+# External merge request diffs
+#
+Settings['external_diffs'] ||= Settingslogic.new({})
+Settings.external_diffs['enabled']      = false if Settings.external_diffs['enabled'].nil?
+Settings.external_diffs['storage_path'] = Settings.absolute(Settings.external_diffs['storage_path'] || File.join(Settings.shared['path'], 'external-diffs'))
+Settings.external_diffs['object_store'] = ObjectStoreSettings.parse(Settings.external_diffs['object_store'])
+
+#
 # Git LFS
 #
 Settings['lfs'] ||= Settingslogic.new({})
diff --git a/db/migrate/20190109153125_add_merge_request_external_diffs.rb b/db/migrate/20190109153125_add_merge_request_external_diffs.rb
new file mode 100644
index 00000000000..c67903c7f67
--- /dev/null
+++ b/db/migrate/20190109153125_add_merge_request_external_diffs.rb
@@ -0,0 +1,25 @@
+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddMergeRequestExternalDiffs < ActiveRecord::Migration[5.0]
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  def change
+    # Allow the merge request diff to store details about an external file
+    add_column :merge_request_diffs, :external_diff, :string
+    add_column :merge_request_diffs, :external_diff_store, :integer
+    add_column :merge_request_diffs, :stored_externally, :boolean
+
+    # The diff for each file is mapped to a range in the external file
+    add_column :merge_request_diff_files, :external_diff_offset, :integer
+    add_column :merge_request_diff_files, :external_diff_size, :integer
+
+    # If the diff is in object storage, it will be null in the database
+    change_column_null :merge_request_diff_files, :diff, true
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 4b6e4992056..20c8dab4c3e 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -1203,8 +1203,10 @@ ActiveRecord::Schema.define(version: 20190131122559) do
     t.string "b_mode", null: false
     t.text "new_path", null: false
     t.text "old_path", null: false
-    t.text "diff", null: false
+    t.text "diff"
     t.boolean "binary"
+    t.integer "external_diff_offset"
+    t.integer "external_diff_size"
     t.index ["merge_request_diff_id", "relative_order"], name: "index_merge_request_diff_files_on_mr_diff_id_and_order", unique: true, using: :btree
   end
 
@@ -1218,6 +1220,9 @@ ActiveRecord::Schema.define(version: 20190131122559) do
     t.string "head_commit_sha"
     t.string "start_commit_sha"
     t.integer "commits_count"
+    t.string "external_diff"
+    t.integer "external_diff_store"
+    t.boolean "stored_externally"
     t.index ["merge_request_id", "id"], name: "index_merge_request_diffs_on_merge_request_id_and_id", using: :btree
   end
 
diff --git a/doc/administration/index.md b/doc/administration/index.md
index 0b673d61139..184754cd467 100644
--- a/doc/administration/index.md
+++ b/doc/administration/index.md
@@ -48,6 +48,7 @@ Learn how to install, configure, update, and maintain your GitLab instance.
 - [Third party offers](../user/admin_area/settings/third_party_offers.md)
 - [Compliance](compliance.md): A collection of features from across the application that you may configure to help ensure that your GitLab instance and DevOps workflow meet compliance standards.
 - [Diff limits](../user/admin_area/diff_limits.md): Configure the diff rendering size limits of branch comparison pages.
+- [Merge request diffs](merge_request_diffs.md): Configure the diffs shown on merge requests
 - [Broadcast Messages](../user/admin_area/broadcast_messages.md): Send messages to GitLab users through the UI.
 
 #### Customizing GitLab's appearance
diff --git a/doc/administration/merge_request_diffs.md b/doc/administration/merge_request_diffs.md
new file mode 100644
index 00000000000..94620c3d3a0
--- /dev/null
+++ b/doc/administration/merge_request_diffs.md
@@ -0,0 +1,154 @@
+# Merge request diffs administration
+
+> **Notes:**
+> - External merge request diffs introduced in GitLab 11.8
+
+Merge request diffs are size-limited copies of diffs associated with merge
+requests. When viewing a merge request, diffs are sourced from these copies
+wherever possible as a performance optimization.
+
+By default, merge request diffs are stored in the database, in a table named
+`merge_request_diff_files`. Larger installations may find this table grows too
+large, in which case, switching to external storage is recommended.
+
+### Using external storage
+
+Merge request diffs can be stored on disk, or in object storage. In general, it
+is better to store the diffs in the database than on disk.
+
+To enable external storage of merge request diffs:
+
+---
+
+**In Omnibus installations:**
+
+1. Edit `/etc/gitlab/gitlab.rb` and add the following line:
+
+    ```ruby
+    gitlab_rails['external_diffs_enabled'] = true
+    ```
+
+1. _The external diffs will be stored in in
+   `/var/opt/gitlab/gitlab-rails/shared/external-diffs`._  To change the path,
+   for example to `/mnt/storage/external-diffs`, edit `/etc/gitlab/gitlab.rb`
+   and add the following line:
+
+    ```ruby
+    gitlab_rails['external_diffs_storage_path'] = "/mnt/storage/external-diffs"
+    ```
+
+1. Save the file and [reconfigure GitLab][] for the changes to take effect.
+
+---
+
+**In installations from source:**
+
+1. Edit `/home/git/gitlab/config/gitlab.yml` and add or amend the following
+   lines:
+
+    ```yaml
+    external_diffs:
+      enabled: true
+    ```
+
+1. _The external diffs will be stored in 
+   `/home/git/gitlab/shared/external-diffs`._ To change the path,  for example
+   to `/mnt/storage/external-diffs`, edit `/home/git/gitlab/config/gitlab.yml`
+   and add or amend the following lines:
+
+    ```yaml
+    external_diffs:
+      enabled: true
+      storage_path: /mnt/storage/external-diffs
+    ```
+
+1. Save the file and [restart GitLab][] for the changes to take effect.
+
+### Using object storage
+
+Instead of storing the external diffs on disk, we recommended you use an object
+store like AWS S3 instead. This configuration relies on valid AWS credentials to
+be configured already.
+
+### Object Storage Settings
+
+For source installations, these settings are nested under `external_diffs:` and
+then `object_store:`. On omnibus installs, they are prefixed by
+`external_diffs_object_store_`.
+
+| Setting | Description | Default |
+|---------|-------------|---------|
+| `enabled` | Enable/disable object storage | `false` |
+| `remote_directory` | The bucket name where external diffs will be stored| |
+| `direct_upload` | Set to true to enable direct upload of external diffs without the need of local shared storage. Option may be removed once we decide to support only single storage for all files. | `false` |
+| `background_upload` | Set to false to disable automatic upload. Option may be removed once upload is direct to S3 | `true` |
+| `proxy_download` | Set to true to enable proxying all files served. Option allows to reduce egress traffic as this allows clients to download directly from remote storage instead of proxying all data | `false` |
+| `connection` | Various connection options described below | |
+
+#### S3 compatible connection settings
+
+The connection settings match those provided by [Fog](https://github.com/fog), and are as follows:
+
+| Setting | Description | Default |
+|---------|-------------|---------|
+| `provider` | Always `AWS` for compatible hosts | AWS |
+| `aws_access_key_id` | AWS credentials, or compatible | |
+| `aws_secret_access_key` | AWS credentials, or compatible | |
+| `aws_signature_version` | AWS signature version to use. 2 or 4 are valid options. Digital Ocean Spaces and other providers may need 2. | 4 |
+| `region` | AWS region | us-east-1 |
+| `host` | S3 compatible host for when not using AWS, e.g. `localhost` or `storage.example.com` | s3.amazonaws.com |
+| `endpoint` | Can be used when configuring an S3 compatible service such as [Minio](https://www.minio.io), by entering a URL such as `http://127.0.0.1:9000` | (optional) |
+| `path_style` | Set to true to use `host/bucket_name/object` style paths instead of `bucket_name.host/object`. Leave as false for AWS S3 | false |
+| `use_iam_profile` | Set to true to use IAM profile instead of access keys | false
+
+**In Omnibus installations:**
+
+1. Edit `/etc/gitlab/gitlab.rb` and add the following lines by replacing with
+   the values you want:
+
+    ```ruby
+    gitlab_rails['external_diffs_enabled'] = true
+    gitlab_rails['external_diffs_object_store_enabled'] = true
+    gitlab_rails['external_diffs_object_store_remote_directory'] = "external-diffs"
+    gitlab_rails['external_diffs_object_store_connection'] = {
+      'provider' => 'AWS',
+      'region' => 'eu-central-1',
+      'aws_access_key_id' => 'AWS_ACCESS_KEY_ID',
+      'aws_secret_access_key' => 'AWS_SECRET_ACCESS_KEY'
+    }
+    ```
+
+    NOTE: if you are using AWS IAM profiles, be sure to omit the
+    AWS access key and secret access key/value pairs. For example:
+
+    ```ruby
+    gitlab_rails['external_diffs_object_store_connection'] = {
+      'provider' => 'AWS',
+      'region' => 'eu-central-1',
+      'use_iam_profile' => true
+    }
+    ```
+
+1. Save the file and [reconfigure GitLab][] for the changes to take effect.
+
+---
+
+**In installations from source:**
+
+1. Edit `/home/git/gitlab/config/gitlab.yml` and add or amend the following
+   lines:
+
+    ```yaml
+    external_diffs:
+      enabled: true
+      object_store:
+        enabled: true
+        remote_directory: "external-diffs" # The bucket name
+        connection:
+          provider: AWS # Only AWS supported at the moment
+          aws_access_key_id: AWS_ACCESS_KEY_ID
+          aws_secret_access_key: AWS_SECRET_ACCESS_KEY
+          region: eu-central-1
+    ```
+
+1. Save the file and [restart GitLab][] for the changes to take effect.
diff --git a/doc/development/file_storage.md b/doc/development/file_storage.md
index b90dc90e424..597812c8c49 100644
--- a/doc/development/file_storage.md
+++ b/doc/development/file_storage.md
@@ -18,6 +18,7 @@ There are many places where file uploading is used, according to contexts:
   - Issues/MR/Notes Legacy Markdown attachments
   - CI Artifacts (archive, metadata, trace)
   - LFS Objects
+  - Merge request diffs
 
 
 ## Disk storage
@@ -37,6 +38,7 @@ they are still not 100% standardized. You can see them below:
 | Issues/MR/Notes Legacy Markdown attachments | no     | uploads/-/system/note/attachment/:id/:filename              | `AttachmentUploader`   | Note       |
 | CI Artifacts (CE)                     | yes    | shared/artifacts/:disk_hash[0..1]/:disk_hash[2..3]/:disk_hash/:year_:month_:date/:job_id/:job_artifact_id (:disk_hash is SHA256 digest of project_id) | `JobArtifactUploader`  | Ci::JobArtifact  |
 | LFS Objects  (CE)                     | yes    | shared/lfs-objects/:hex/:hex/:object_hash                   | `LfsObjectUploader`    | LfsObject  |
+| External merge request diffs          | yes    | shared/external-diffs/merge_request_diffs/mr-:parent_id/diff-:id | `ExternalDiffUploader` | MergeRequestDiff |
 
 CI Artifacts and LFS Objects behave differently in CE and EE. In CE they inherit the `GitlabUploader`
 while in EE they inherit the `ObjectStorage` and store files in and S3 API compatible object store.
diff --git a/lib/gitlab/import_export/import_export.yml b/lib/gitlab/import_export/import_export.yml
index add7ee58da6..099677a791c 100644
--- a/lib/gitlab/import_export/import_export.yml
+++ b/lib/gitlab/import_export/import_export.yml
@@ -130,9 +130,14 @@ excluded_attributes:
   snippets:
     - :expired_at
   merge_request_diff:
+    - :external_diff
+    - :stored_externally
+    - :external_diff_store
     - :st_diffs
   merge_request_diff_files:
     - :diff
+    - :external_diff_offset
+    - :external_diff_size
   issues:
     - :milestone_id
   merge_requests:
diff --git a/spec/models/merge_request_diff_spec.rb b/spec/models/merge_request_diff_spec.rb
index 33e984dc399..1849d3bac12 100644
--- a/spec/models/merge_request_diff_spec.rb
+++ b/spec/models/merge_request_diff_spec.rb
@@ -46,7 +46,7 @@ describe MergeRequestDiff do
     it { expect(first_diff.reload).not_to be_latest }
   end
 
-  describe '#diffs' do
+  shared_examples_for 'merge request diffs' do
     let(:merge_request) { create(:merge_request, :with_diffs) }
     let!(:diff) { merge_request.merge_request_diff.reload }
 
@@ -91,98 +91,110 @@ describe MergeRequestDiff do
         diff.diffs.diff_files
       end
     end
-  end
 
-  describe '#raw_diffs' do
-    context 'when the :ignore_whitespace_change option is set' do
-      it 'creates a new compare object instead of loading from the DB' do
-        expect(diff_with_commits).not_to receive(:load_diffs)
-        expect(diff_with_commits.compare).to receive(:diffs).and_call_original
+    describe '#raw_diffs' do
+      context 'when the :ignore_whitespace_change option is set' do
+        it 'creates a new compare object instead of using preprocessed data' do
+          expect(diff_with_commits).not_to receive(:load_diffs)
+          expect(diff_with_commits.compare).to receive(:diffs).and_call_original
 
-        diff_with_commits.raw_diffs(ignore_whitespace_change: true)
+          diff_with_commits.raw_diffs(ignore_whitespace_change: true)
+        end
       end
-    end
 
-    context 'when the raw diffs are empty' do
-      before do
-        MergeRequestDiffFile.where(merge_request_diff_id: diff_with_commits.id).delete_all
-      end
+      context 'when the raw diffs are empty' do
+        before do
+          MergeRequestDiffFile.where(merge_request_diff_id: diff_with_commits.id).delete_all
+        end
 
-      it 'returns an empty DiffCollection' do
-        expect(diff_with_commits.raw_diffs).to be_a(Gitlab::Git::DiffCollection)
-        expect(diff_with_commits.raw_diffs).to be_empty
+        it 'returns an empty DiffCollection' do
+          expect(diff_with_commits.raw_diffs).to be_a(Gitlab::Git::DiffCollection)
+          expect(diff_with_commits.raw_diffs).to be_empty
+        end
       end
-    end
 
-    context 'when the raw diffs exist' do
-      it 'returns the diffs' do
-        expect(diff_with_commits.raw_diffs).to be_a(Gitlab::Git::DiffCollection)
-        expect(diff_with_commits.raw_diffs).not_to be_empty
-      end
+      context 'when the raw diffs exist' do
+        it 'returns the diffs' do
+          expect(diff_with_commits.raw_diffs).to be_a(Gitlab::Git::DiffCollection)
+          expect(diff_with_commits.raw_diffs).not_to be_empty
+        end
 
-      context 'when the :paths option is set' do
-        let(:diffs) { diff_with_commits.raw_diffs(paths: ['files/ruby/popen.rb', 'files/ruby/popen.rb']) }
+        context 'when the :paths option is set' do
+          let(:diffs) { diff_with_commits.raw_diffs(paths: ['files/ruby/popen.rb', 'files/ruby/popen.rb']) }
 
-        it 'only returns diffs that match the (old path, new path) given' do
-          expect(diffs.map(&:new_path)).to contain_exactly('files/ruby/popen.rb')
-        end
+          it 'only returns diffs that match the (old path, new path) given' do
+            expect(diffs.map(&:new_path)).to contain_exactly('files/ruby/popen.rb')
+          end
 
-        it 'only serializes diff files found by query' do
-          expect(diff_with_commits.merge_request_diff_files.count).to be > 10
-          expect_any_instance_of(MergeRequestDiffFile).to receive(:to_hash).once
+          it 'only serializes diff files found by query' do
+            expect(diff_with_commits.merge_request_diff_files.count).to be > 10
+            expect_any_instance_of(MergeRequestDiffFile).to receive(:to_hash).once
 
-          diffs
-        end
+            diffs
+          end
 
-        it 'uses the diffs from the DB' do
-          expect(diff_with_commits).to receive(:load_diffs)
+          it 'uses the preprocessed diffs' do
+            expect(diff_with_commits).to receive(:load_diffs)
 
-          diffs
+            diffs
+          end
         end
       end
     end
-  end
 
-  describe '#save_diffs' do
-    it 'saves collected state' do
-      mr_diff = create(:merge_request).merge_request_diff
+    describe '#save_diffs' do
+      it 'saves collected state' do
+        mr_diff = create(:merge_request).merge_request_diff
 
-      expect(mr_diff.collected?).to be_truthy
-    end
+        expect(mr_diff.collected?).to be_truthy
+      end
 
-    it 'saves overflow state' do
-      allow(Commit).to receive(:max_diff_options)
-        .and_return(max_lines: 0, max_files: 0)
+      it 'saves overflow state' do
+        allow(Commit).to receive(:max_diff_options)
+          .and_return(max_lines: 0, max_files: 0)
 
-      mr_diff = create(:merge_request).merge_request_diff
+        mr_diff = create(:merge_request).merge_request_diff
 
-      expect(mr_diff.overflow?).to be_truthy
-    end
+        expect(mr_diff.overflow?).to be_truthy
+      end
 
-    it 'saves empty state' do
-      allow_any_instance_of(described_class).to receive_message_chain(:compare, :commits)
-        .and_return([])
+      it 'saves empty state' do
+        allow_any_instance_of(described_class).to receive_message_chain(:compare, :commits)
+          .and_return([])
 
-      mr_diff = create(:merge_request).merge_request_diff
+        mr_diff = create(:merge_request).merge_request_diff
 
-      expect(mr_diff.empty?).to be_truthy
-    end
+        expect(mr_diff.empty?).to be_truthy
+      end
 
-    it 'expands collapsed diffs before saving' do
-      mr_diff = create(:merge_request, source_branch: 'expand-collapse-lines', target_branch: 'master').merge_request_diff
-      diff_file = mr_diff.merge_request_diff_files.find_by(new_path: 'expand-collapse/file-5.txt')
+      it 'expands collapsed diffs before saving' do
+        mr_diff = create(:merge_request, source_branch: 'expand-collapse-lines', target_branch: 'master').merge_request_diff
+        diff_file = mr_diff.merge_request_diff_files.find_by(new_path: 'expand-collapse/file-5.txt')
 
-      expect(diff_file.diff).not_to be_empty
+        expect(diff_file.diff).not_to be_empty
+      end
+
+      it 'saves binary diffs correctly' do
+        path = 'files/images/icn-time-tracking.pdf'
+        mr_diff = create(:merge_request, source_branch: 'add-pdf-text-binary', target_branch: 'master').merge_request_diff
+        diff_file = mr_diff.merge_request_diff_files.find_by(new_path: path)
+
+        expect(diff_file).to be_binary
+        expect(diff_file.diff).to eq(mr_diff.compare.diffs(paths: [path]).to_a.first.diff)
+      end
     end
+  end
 
-    it 'saves binary diffs correctly' do
-      path = 'files/images/icn-time-tracking.pdf'
-      mr_diff = create(:merge_request, source_branch: 'add-pdf-text-binary', target_branch: 'master').merge_request_diff
-      diff_file = mr_diff.merge_request_diff_files.find_by(new_path: path)
+  describe 'internal diffs configured' do
+    include_examples 'merge request diffs'
+  end
 
-      expect(diff_file).to be_binary
-      expect(diff_file.diff).to eq(mr_diff.compare.diffs(paths: [path]).to_a.first.diff)
+  describe 'external diffs configured' do
+    before do
+      stub_external_diffs_setting(enabled: true)
     end
+
+    include_examples 'merge request diffs'
   end
 
   describe '#commit_shas' do
@@ -245,4 +257,55 @@ describe MergeRequestDiff do
       expect(subject.modified_paths).to eq(%w{foo bar baz})
     end
   end
+
+  describe '#opening_external_diff' do
+    subject(:diff) { diff_with_commits }
+
+    context 'external diffs disabled' do
+      it { expect(diff.external_diff).not_to be_exists }
+
+      it 'yields nil' do
+        expect { |b| diff.opening_external_diff(&b) }.to yield_with_args(nil)
+      end
+    end
+
+    context 'external diffs enabled' do
+      let(:test_dir) { 'tmp/tests/external-diffs' }
+
+      around do |example|
+        FileUtils.mkdir_p(test_dir)
+
+        begin
+          example.run
+        ensure
+          FileUtils.rm_rf(test_dir)
+        end
+      end
+
+      before do
+        stub_external_diffs_setting(enabled: true, storage_path: test_dir)
+      end
+
+      it { expect(diff.external_diff).to be_exists }
+
+      it 'yields an open file' do
+        expect { |b| diff.opening_external_diff(&b) }.to yield_with_args(File)
+      end
+
+      it 'is re-entrant' do
+        outer_file_a =
+          diff.opening_external_diff do |outer_file|
+            diff.opening_external_diff do |inner_file|
+              expect(outer_file).to eq(inner_file)
+            end
+
+            outer_file
+          end
+
+        diff.opening_external_diff do |outer_file_b|
+          expect(outer_file_a).not_to eq(outer_file_b)
+        end
+      end
+    end
+  end
 end
diff --git a/spec/support/helpers/stub_configuration.rb b/spec/support/helpers/stub_configuration.rb
index 2851cd9733c..ff21bbe28ca 100644
--- a/spec/support/helpers/stub_configuration.rb
+++ b/spec/support/helpers/stub_configuration.rb
@@ -56,6 +56,10 @@ module StubConfiguration
     allow(Gitlab.config.lfs).to receive_messages(to_settings(messages))
   end
 
+  def stub_external_diffs_setting(messages)
+    allow(Gitlab.config.external_diffs).to receive_messages(to_settings(messages))
+  end
+
   def stub_artifacts_setting(messages)
     allow(Gitlab.config.artifacts).to receive_messages(to_settings(messages))
   end
diff --git a/spec/support/helpers/stub_object_storage.rb b/spec/support/helpers/stub_object_storage.rb
index 58b5c6a6435..e0c50e533a6 100644
--- a/spec/support/helpers/stub_object_storage.rb
+++ b/spec/support/helpers/stub_object_storage.rb
@@ -42,6 +42,13 @@ module StubObjectStorage
                                  **params)
   end
 
+  def stub_external_diffs_object_storage(uploader = described_class, **params)
+    stub_object_storage_uploader(config: Gitlab.config.external_diffs.object_store,
+                                 uploader: uploader,
+                                 remote_directory: 'external_diffs',
+                                 **params)
+  end
+
   def stub_lfs_object_storage(**params)
     stub_object_storage_uploader(config: Gitlab.config.lfs.object_store,
                                  uploader: LfsObjectUploader,
diff --git a/spec/uploaders/external_diff_uploader_spec.rb b/spec/uploaders/external_diff_uploader_spec.rb
new file mode 100644
index 00000000000..1c959770dc4
--- /dev/null
+++ b/spec/uploaders/external_diff_uploader_spec.rb
@@ -0,0 +1,67 @@
+require 'spec_helper'
+
+describe ExternalDiffUploader do
+  let(:diff) { create(:merge_request).merge_request_diff }
+  let(:path) { Gitlab.config.external_diffs.storage_path }
+
+  subject(:uploader) { described_class.new(diff, :external_diff) }
+
+  it_behaves_like "builds correct paths",
+                  store_dir: %r[merge_request_diffs/mr-\d+],
+                  cache_dir: %r[/external-diffs/tmp/cache],
+                  work_dir: %r[/external-diffs/tmp/work]
+
+  context "object store is REMOTE" do
+    before do
+      stub_external_diffs_object_storage
+    end
+
+    include_context 'with storage', described_class::Store::REMOTE
+
+    it_behaves_like "builds correct paths",
+                    store_dir: %r[merge_request_diffs/mr-\d+]
+  end
+
+  describe 'migration to object storage' do
+    context 'with object storage disabled' do
+      it "is skipped" do
+        expect(ObjectStorage::BackgroundMoveWorker).not_to receive(:perform_async)
+
+        diff
+      end
+    end
+
+    context 'with object storage enabled' do
+      before do
+        stub_external_diffs_setting(enabled: true)
+        stub_external_diffs_object_storage(background_upload: true)
+      end
+
+      it 'is scheduled to run after creation' do
+        expect(ObjectStorage::BackgroundMoveWorker).to receive(:perform_async).with(described_class.name, 'MergeRequestDiff', :external_diff, kind_of(Numeric))
+
+        diff
+      end
+    end
+  end
+
+  describe 'remote file' do
+    context 'with object storage enabled' do
+      before do
+        stub_external_diffs_setting(enabled: true)
+        stub_external_diffs_object_storage
+
+        diff.update!(external_diff_store: described_class::Store::REMOTE)
+      end
+
+      it 'can store file remotely' do
+        allow(ObjectStorage::BackgroundMoveWorker).to receive(:perform_async)
+
+        diff
+
+        expect(diff.external_diff_store).to eq(described_class::Store::REMOTE)
+        expect(diff.external_diff.path).not_to be_blank
+      end
+    end
+  end
+end