bug #24660 Escape trailing \ in QuestionHelper autocompletion (kamazee)

This PR was merged into the 2.7 branch.

Discussion
----------

Escape trailing \ in QuestionHelper autocompletion

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #24652
| License       | MIT

Fixes #24652
Trailing backslash, being unescaped, used to escape closing formatting
tag and, thus, formatting tag appeared in autocompletion

Output of the added test without the fix:
```
./phpunit --filter testAutocompleteWithTrailingBackslash src/Symfony/Component/Console/Tests/Helper/QuestionHelperTest.php
#!/usr/bin/env php
PHPUnit 6.0.13 by Sebastian Bergmann and contributors.

Testing Symfony\Component\Console\Tests\Helper\QuestionHelperTest
F                                                                   1 / 1 (100%)

Time: 4.28 seconds, Memory: 6.00MB

There was 1 failure:

1) Symfony\Component\Console\Tests\Helper\QuestionHelperTest::testAutocompleteWithTrailingBackslash
Failed asserting that two strings are equal.
--- Expected
+++ Actual
@@ @@
-'ExampleNamespace\'
+'ExampleNamespace</hl>'
```

`OutputFormatter::escapeTrailingBackslash` is marked as `@internal`; however, this seems to be a valid use without exposing outside of the component. `OutputFormatter::escape`, which is not internal, doesn't fit here because escaping tags in autocompletion options is a deeper topic: there might be a valid use for tags in autocompletion and even if not, taking out the possibility to use them might be considered a BC break (even though it hasn't been advertised anywhere).

Commits
-------

0c0f1da Escape trailing \ in QuestionHelper autocompletion

src/Symfony/Component/Console/Helper/QuestionHelper.php

@@ -11,6 +11,7 @@
 
 namespace Symfony\Component\Console\Helper;
 
+use Symfony\Component\Console\Formatter\OutputFormatter;
 use Symfony\Component\Console\Input\InputInterface;
 use Symfony\Component\Console\Output\ConsoleOutputInterface;
 use Symfony\Component\Console\Output\OutputInterface;
@@ -303,7 +304,7 @@ private function autocomplete(OutputInterface $output, Question $question, $inpu
                 // Save cursor position
                 $output->write("\0337");
                 // Write highlighted text
-                $output->write('<hl>'.substr($matches[$ofs], $i).'</hl>');
+                $output->write('<hl>'.OutputFormatter::escapeTrailingBackslash(substr($matches[$ofs], $i)).'</hl>');
                 // Restore cursor position
                 $output->write("\0338");
             }

src/Symfony/Component/Console/Tests/Helper/QuestionHelperTest.php

@@ -156,6 +156,46 @@ public function testAskWithAutocompleteWithNonSequentialKeys()
         $this->assertEquals('AsseticBundle', $dialog->ask($this->createInputInterfaceMock(), $this->createOutputInterface(), $question));
     }
 
+    public function testAutocompleteWithTrailingBackslash()
+    {
+        if (!$this->hasSttyAvailable()) {
+            $this->markTestSkipped('`stty` is required to test autocomplete functionality');
+        }
+
+        $inputStream = $this->getInputStream('E');
+
+        $dialog = new QuestionHelper();
+        $dialog->setInputStream($inputStream);
+        $helperSet = new HelperSet(array(new FormatterHelper()));
+        $dialog->setHelperSet($helperSet);
+
+        $question = new Question('');
+        $expectedCompletion = 'ExampleNamespace\\';
+        $question->setAutocompleterValues(array($expectedCompletion));
+
+        $output = $this->createOutputInterface();
+        $dialog->ask($this->createInputInterfaceMock(), $output, $question);
+
+        $outputStream = $output->getStream();
+        rewind($outputStream);
+        $actualOutput = stream_get_contents($outputStream);
+
+        // Shell control (esc) sequences are not so important: we only care that
+        // <hl> tag is interpreted correctly and replaced
+        $irrelevantEscSequences = array(
+            "\0337" => '', // Save cursor position
+            "\0338" => '', // Restore cursor position
+            "\033[K" => '', // Clear line from cursor till the end
+        );
+
+        $importantActualOutput = strtr($actualOutput, $irrelevantEscSequences);
+
+        // Remove colors (e.g. "\033[30m", "\033[31;41m")
+        $importantActualOutput = preg_replace('/\033\[\d+(;\d+)?m/', '', $importantActualOutput);
+
+        $this->assertEquals($expectedCompletion, $importantActualOutput);
+    }
+
     public function testAskHiddenResponse()
     {
         if ('\\' === DIRECTORY_SEPARATOR) {