Fall back to the raw diff if base64 encoding fails

A small number of merge request diff files have `binary: true` set
inappropriately. During the external diffs migration on GitLab.com,
approximately 30 in every million (0.003%) fail because the `diffs`
column is the plain diff, rather than the base64-encoded diff.

It would be nicer to correct the data, but a quick and easy fix is to
fall back to the raw column data if the diff is unreadable as base64.

This is particularly safe because the data must *also* be processed as
a diff before being shown to the user, and two characters very commonly
seen in diffs - `-` and `@` - are not valid base64 characters.

This change will fix display and migration of these broken diffs, and
as a happy side effect, the migration to external storage, along with
this change, will also fix the `binary` column for us, so the error
will correct over time.

app/models/merge_request_diff_file.rb

@@ -25,6 +25,16 @@ class MergeRequestDiffFile < ApplicationRecord
         super
       end
 
-    binary? ? content.unpack1('m0') : content
+    return content unless binary?
+
+    # If the data isn't valid base64, return it as-is, since it's almost certain
+    # to be a valid diff. Parsing it as a diff will fail if it's something else.
+    #
+    # https://gitlab.com/gitlab-org/gitlab/-/issues/240921
+    begin
+      content.unpack1('m0')
+    rescue ArgumentError
+      content
+    end
   end
 end

changelogs/unreleased/240921-fix-merge-request-diff-file-invalid-base64.yml

+---
+title: Fix reading some merge request diffs
+merge_request: 40598
+author:
+type: fixed

spec/models/merge_request_diff_file_spec.rb

@@ -25,6 +25,14 @@ RSpec.describe MergeRequestDiffFile do
         it 'unpacks from base 64' do
           expect(subject.diff).to eq(unpacked)
         end
+
+        context 'invalid base64' do
+          let(:packed) { '---/dev/null' }
+
+          it 'returns the raw diff' do
+            expect(subject.diff).to eq(packed)
+          end
+        end
       end
 
       context 'when the diff is not marked as binary' do