Fix Syntax highlight for token change in added/deleted code (#12238)

* Fix Syntax highlight for token change in added/deleted code

For diffs we first syntax highlight the before and after line then use a 3rd party diff library to find the difference in them and create a substring based on that, which we then highlight a 2nd time to show the specific difference within a line that has changed. In a specific case if the diffrence also changes the chroma class it will split in the middle of the attr and cause broken HTML:

```
<span class="nx">oldtext<span>
<span class="k">var newtext<span>
```

Will then split on

```
<span class="
```

Where the difference starts, and produce something broken like:

```
<span class="<span class="removed-code">nx"oldtext</span></span

```

Fix that by detecting when it happens and putting the HTML back together properly before highlighting the added/deleted code.

Fixes #12235

* fix lint

* apply fix to all diff sections. Also handle case where insert/remove starts with a closing span

* Add a test for this new code

* remove comment

Co-authored-by: Lauris BH <lauris@nix.lv>

2 files changed, 48 insertions, 3 deletions

diff --git a/services/gitdiff/gitdiff.go b/services/gitdiff/gitdiff.go
index 7d08f63eb7..09a826500b 100644
--- a/services/gitdiff/gitdiff.go
+++ b/services/gitdiff/gitdiff.go
@@ -183,19 +183,56 @@ var (
 
 func diffToHTML(fileName string, diffs []diffmatchpatch.Diff, lineType DiffLineType) template.HTML {
 	buf := bytes.NewBuffer(nil)
-
+	var addSpan bool
 	for i := range diffs {
 		switch {
+		case diffs[i].Type == diffmatchpatch.DiffEqual:
+			// Looking for the case where our 3rd party diff library previously detected a string difference
+			// in the middle of a span class because we highlight them first. This happens when added/deleted code
+			// also changes the chroma class name. If found, just move the openining span code forward into the next section
+			if addSpan {
+				diffs[i].Text = "<span class=\"" + diffs[i].Text
+			}
+			if strings.HasSuffix(diffs[i].Text, "<span class=\"") {
+				addSpan = true
+				buf.WriteString(strings.TrimSuffix(diffs[i].Text, "<span class=\""))
+			} else {
+				addSpan = false
+				buf.WriteString(getLineContent(diffs[i].Text))
+			}
 		case diffs[i].Type == diffmatchpatch.DiffInsert && lineType == DiffLineAdd:
+			if addSpan {
+				addSpan = false
+				diffs[i].Text = "<span class=\"" + diffs[i].Text
+			}
+			// Print existing closing span first before opening added-code span so it doesn't unintentionally close it
+			if strings.HasPrefix(diffs[i].Text, "</span>") {
+				buf.WriteString("</span>")
+				diffs[i].Text = strings.TrimPrefix(diffs[i].Text, "</span>")
+			}
+			if strings.HasSuffix(diffs[i].Text, "<span class=\"") {
+				addSpan = true
+				diffs[i].Text = strings.TrimSuffix(diffs[i].Text, "<span class=\"")
+			}
 			buf.Write(addedCodePrefix)
 			buf.WriteString(getLineContent(diffs[i].Text))
 			buf.Write(codeTagSuffix)
 		case diffs[i].Type == diffmatchpatch.DiffDelete && lineType == DiffLineDel:
+			if addSpan {
+				addSpan = false
+				diffs[i].Text = "<span class=\"" + diffs[i].Text
+			}
+			if strings.HasPrefix(diffs[i].Text, "</span>") {
+				buf.WriteString("</span>")
+				diffs[i].Text = strings.TrimPrefix(diffs[i].Text, "</span>")
+			}
+			if strings.HasSuffix(diffs[i].Text, "<span class=\"") {
+				addSpan = true
+				diffs[i].Text = strings.TrimSuffix(diffs[i].Text, "<span class=\"")
+			}
 			buf.Write(removedCodePrefix)
 			buf.WriteString(getLineContent(diffs[i].Text))
 			buf.Write(codeTagSuffix)
-		case diffs[i].Type == diffmatchpatch.DiffEqual:
-			buf.WriteString(getLineContent(diffs[i].Text))
 		}
 	}
 	return template.HTML(buf.Bytes())
diff --git a/services/gitdiff/gitdiff_test.go b/services/gitdiff/gitdiff_test.go
index 0dc3f06155..dabb49654e 100644
--- a/services/gitdiff/gitdiff_test.go
+++ b/services/gitdiff/gitdiff_test.go
@@ -42,6 +42,14 @@ func TestDiffToHTML(t *testing.T) {
 		{Type: dmp.DiffInsert, Text: " baz"},
 		{Type: dmp.DiffEqual, Text: " biz"},
 	}, DiffLineDel))
+
+	assertEqual(t, "<span class=\"k\">if</span> <span class=\"p\">!</span><span class=\"nx\">nohl</span> <span class=\"o\">&amp;&amp;</span> <span class=\"added-code\"><span class=\"p\">(</span></span><span class=\"nx\">lexer</span> <span class=\"o\">!=</span> <span class=\"kc\">nil</span><span class=\"added-code\"> <span class=\"o\">||</span> <span class=\"nx\">r</span><span class=\"p\">.</span><span class=\"nx\">GuessLanguage</span><span class=\"p\">)</span></span> <span class=\"p\">{</span>", diffToHTML("", []dmp.Diff{
+		{Type: dmp.DiffEqual, Text: "<span class=\"k\">if</span> <span class=\"p\">!</span><span class=\"nx\">nohl</span> <span class=\"o\">&amp;&amp;</span> <span class=\""},
+		{Type: dmp.DiffInsert, Text: "p\">(</span><span class=\""},
+		{Type: dmp.DiffEqual, Text: "nx\">lexer</span> <span class=\"o\">!=</span> <span class=\"kc\">nil"},
+		{Type: dmp.DiffInsert, Text: "</span> <span class=\"o\">||</span> <span class=\"nx\">r</span><span class=\"p\">.</span><span class=\"nx\">GuessLanguage</span><span class=\"p\">)"},
+		{Type: dmp.DiffEqual, Text: "</span> <span class=\"p\">{</span>"},
+	}, DiffLineAdd))
 }
 
 func TestParsePatch(t *testing.T) {