Chore: Reproduce nested blocks failure (refs #181)

The current implementation assumes the `blocks` module-level variable
will only be used once per physical file. If the file contains a nested
Markdown code block, the processor will be recursively run on the code
block's virtual file.

tests/lib/plugin.js

@@ -245,6 +245,45 @@ describe("plugin", () => {
         assert.strictEqual(report.results[0].messages[4].column, 2);
     });
 
+    // https://github.com/eslint/eslint-plugin-markdown/issues/181
+    it("should work when called on nested code blocks in the same file", () => {
+
+        /*
+         * As of this writing, the nested code block, though it uses the same
+         * Markdown processor, must use a different extension or ESLint will not
+         * re-apply the processor on the nested code block. To work around that,
+         * a file named `test.md` contains a nested `markdown` code block in
+         * this test.
+         *
+         * https://github.com/eslint/eslint/pull/14227/files#r602802758
+         */
+        const code = [
+            "<!-- test.md -->",
+            "",
+            "````markdown",
+            "<!-- test.md/0_0.markdown -->",
+            "",
+            "This test only repros if the MD files have a different number of lines before code blocks.",
+            "",
+            "```js",
+            "// test.md/0_0.markdown/0_0.js",
+            "console.log('single quotes')",
+            "```",
+            "````"
+        ].join("\n");
+        const recursiveCli = initCLI("eslintrc.json", {
+            extensions: [".js", ".markdown", ".md"]
+        });
+        const report = recursiveCli.executeOnText(code, "test.md");
+
+        assert.strictEqual(report.results.length, 1);
+        assert.strictEqual(report.results[0].messages.length, 2);
+        assert.strictEqual(report.results[0].messages[0].message, "Unexpected console statement.");
+        assert.strictEqual(report.results[0].messages[0].line, 8);
+        assert.strictEqual(report.results[0].messages[1].message, "Strings must use doublequote.");
+        assert.strictEqual(report.results[0].messages[1].line, 8);
+    });
+
     describe("configuration comments", () => {
 
         it("apply only to the code block immediately following", () => {