Fix: catastrophic backtracking in astUtils linebreak regex (fixes #7893…

…) (#7898)

* Fix: catastrophic backtracking in astUtils linebreak regex (fixes #7893)

This fixes an issue where `astUtils.getLocationFromRangeIndex` and `astUtils.getRangeIndexFromLocation` were using a regular expression susceptible to catastrophic backtracking. The match would take quadratic time in the length of the last line of the file. Since the file in #7893 contains a 1.5 million character source map URL on the last line, rules like `no-multiple-empty-lines` would hang when using ast-utils to split the file into lines.

This issue only applies to files without trailing newlines, and is only noticable when the last line of the file contains more than 30000 characters or so. Since only a few rules use these `astUtils` functions, this would only appear when either `no-useless-escape` or `no-multiple-empty-lines` reports an error for the file.

Simplified example: Node 7.4.0 hangs when evaluating this expression.

```js
/[^\n]*\n/.test('A'.repeat(1000000))
```

* Add explanatory comments

lib/ast-utils.js

@@ -290,8 +290,23 @@ const lineIndexCache = new WeakMap();
 function getLineIndices(sourceCode) {
 
     if (!lineIndexCache.has(sourceCode)) {
-        const lineIndices = (sourceCode.text.match(/[^\r\n\u2028\u2029]*(\r\n|\r|\n|\u2028|\u2029)/g) || [])
-            .reduce((indices, line) => indices.concat(indices[indices.length - 1] + line.length), [0]);
+        const lineIndices = [0];
+        const lineEndingPattern = /\r\n|[\r\n\u2028\u2029]/g;
+        let match;
+
+        /*
+         * Previously, this function was implemented using a regex that
+         * matched a sequence of non-linebreak characters followed by a
+         * linebreak, then adding the lengths of the matches. However,
+         * this caused a catastrophic backtracking issue when the end
+         * of a file contained a large number of non-newline characters.
+         * To avoid this, the current implementation just matches newlines
+         * and uses match.index to get the correct line start indices.
+         */
+
+        while ((match = lineEndingPattern.exec(sourceCode.text))) {
+            lineIndices.push(match.index + match[0].length);
+        }
 
         // Store the sourceCode object in a WeakMap to avoid iterating over all of the lines every time a sourceCode object is passed in.
         lineIndexCache.set(sourceCode, lineIndices);

tests/lib/rules/no-multiple-empty-lines.js

@@ -309,6 +309,13 @@ ruleTester.run("no-multiple-empty-lines", rule, {
             errors: [getExpectedError(1)],
             options: [{ max: 1 }],
             parserOptions: { ecmaVersion: 6 }
+        },
+        {
+
+            // https://github.com/eslint/eslint/issues/7893
+            code: `a\n\n\n\n${"a".repeat(1e5)}`,
+            output: `a\n\n\n${"a".repeat(1e5)}`,
+            errors: [getExpectedError(2)]
         }
     ]
 });