[GITEA] Require Latex code to have a end sequence

- Currently the parser will look for `\[` and `$$` to detect when Latex
code starts, it will look for `\]` and `$$` respectively in order to
determine the end of the code. However if no end is found the parser
assumes the rest of the input is part of the Latex code.
- Adjust the parser's behavior to not allow the case to assume the rest
of the input is part of the Latex code and requires in order to
determine if some input is Latex code that the end sequence is also
specified.
- Example: `\[hello]` would no longer be detected as Latex code with
this patch.
- Added unit tests.
- Resolves https://codeberg.org/forgejo/forgejo/issues/1817

(cherry picked from commit 452aef1bb1)
(cherry picked from commit 8a857c24b0)
(cherry picked from commit acd1456db9)

modules/markup/markdown/markdown_test.go

@@ -510,6 +510,18 @@ func TestMathBlock(t *testing.T) {
 			"$$a$$",
 			`<pre class="code-block is-loading"><code class="chroma language-math display">a</code></pre>` + nl,
 		},
+		{
+			`\[a b\]`,
+			`<pre class="code-block is-loading"><code class="chroma language-math display">a b</code></pre>` + nl,
+		},
+		{
+			`\[a b]`,
+			`<p>[a b]</p>` + nl,
+		},
+		{
+			`$$a`,
+			`<p>$$a</p>` + nl,
+		},
 	}
 
 	for _, test := range testcases {

modules/markup/markdown/math/block_parser.go

@@ -55,10 +55,7 @@ func (b *blockParser) Open(parent ast.Node, reader text.Reader, pc parser.Contex
 		return node, parser.Close | parser.NoChildren
 	}
 
-	reader.Advance(segment.Len() - 1)
+	return nil, parser.NoChildren
-	segment.Start += 2
-	node.Lines().Append(segment)
-	return node, parser.NoChildren
 }
 
 // Continue parses the current line and returns a result of parsing.