bug #13683 [PROCESS] make sure /dev/tty is readable (staabm)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #13683).

Discussion
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[PROCESS] make sure /dev/tty is readable

When using Process from Web-SAPI it is likely that the webserver user
doesn't has rights to use /dev/tty

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #13261
| License       | MIT
| Doc PR        | -

Commits
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935afe6 [PROCESS] make sure /dev/tty is readable

Author: fabpot

src/Symfony/Component/Process/Process.php

@@ -792,6 +792,9 @@ public function setTty($tty)
         if ('\\' === DIRECTORY_SEPARATOR && $tty) {
             throw new RuntimeException('TTY mode is not supported on Windows platform.');
         }
+        if ($tty && (!file_exists('/dev/tty') || !is_readable('/dev/tty'))) {
+            throw new RuntimeException('TTY mode requires /dev/tty to be readable.');
+        }
 
         $this->tty = (bool) $tty;