Re-enable legacy diff notes on merge request diffs

This re-enables legacy diff notes on the merge request diffs
This feature was not workig correctly after the Vue refactor
LegacyDiffNotes have no `position`, instead they only have a `line_code`
As an extra, this also re-enables commenting on legacy diffs.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/48873

2 files changed, 23 insertions, 6 deletions

diff --git a/app/assets/javascripts/diffs/store/mutations.js b/app/assets/javascripts/diffs/store/mutations.js
index 6dc5bf16c65..b6818f905e5 100644
--- a/app/assets/javascripts/diffs/store/mutations.js
+++ b/app/assets/javascripts/diffs/store/mutations.js
@@ -90,7 +90,8 @@ export default {
     const firstDiscussion = discussions[0];
     const isDiffDiscussion = firstDiscussion.diff_discussion;
     const hasLineCode = firstDiscussion.line_code;
-    const isResolvable = firstDiscussion.resolvable;
+    const isResolvable =
+      firstDiscussion.resolvable || (!firstDiscussion.resolvable && !firstDiscussion.position);
     const diffPosition = diffPositionByLineCode[firstDiscussion.line_code];
 
     if (
diff --git a/app/assets/javascripts/diffs/store/utils.js b/app/assets/javascripts/diffs/store/utils.js
index b7e52a8f37f..d521e4584ad 100644
--- a/app/assets/javascripts/diffs/store/utils.js
+++ b/app/assets/javascripts/diffs/store/utils.js
@@ -4,6 +4,7 @@ import {
   LINE_POSITION_LEFT,
   LINE_POSITION_RIGHT,
   TEXT_DIFF_POSITION_TYPE,
+  LEGACY_DIFF_NOTE_TYPE,
   DIFF_NOTE_TYPE,
   NEW_LINE_TYPE,
   OLD_LINE_TYPE,
@@ -60,7 +61,7 @@ export function getNoteFormData(params) {
       noteable_type: noteableType,
       noteable_id: noteableData.id,
       commit_id: '',
-      type: DIFF_NOTE_TYPE,
+      type: diffFile.diffRefs.startSha ? DIFF_NOTE_TYPE : LEGACY_DIFF_NOTE_TYPE,
       line_code: noteTargetLine.lineCode,
     },
   };
@@ -230,7 +231,16 @@ export function getDiffPositionByLineCode(diffFiles) {
         const { lineCode, oldLine, newLine } = line;
 
         if (lineCode) {
-          acc[lineCode] = { baseSha, headSha, startSha, newPath, oldPath, oldLine, newLine };
+          acc[lineCode] = {
+            baseSha,
+            headSha,
+            startSha,
+            newPath,
+            oldPath,
+            oldLine,
+            newLine,
+            lineCode,
+          };
         }
       });
     }
@@ -242,8 +252,14 @@ export function getDiffPositionByLineCode(diffFiles) {
 // This method will check whether the discussion is still applicable
 // to the diff line in question regarding different versions of the MR
 export function isDiscussionApplicableToLine(discussion, diffPosition) {
-  const originalRefs = convertObjectPropsToCamelCase(discussion.original_position.formatter);
-  const refs = convertObjectPropsToCamelCase(discussion.position.formatter);
+  const { lineCode, ...diffPositionCopy } = diffPosition;
 
-  return _.isEqual(refs, diffPosition) || _.isEqual(originalRefs, diffPosition);
+  if (discussion.original_position && discussion.position) {
+    const originalRefs = convertObjectPropsToCamelCase(discussion.original_position.formatter);
+    const refs = convertObjectPropsToCamelCase(discussion.position.formatter);
+
+    return _.isEqual(refs, diffPositionCopy) || _.isEqual(originalRefs, diffPositionCopy);
+  }
+
+  return lineCode === discussion.line_code;
 }