Drupal redirects to homepage on batch page.

I've noticed weird issue which causing the redirect to home page while running batch script (such as indexing the site).

I've tracked the problem to request_path() which returns empty string despite I'm being on /batch?op=start&id=10916 page, as result I'm redirected to the home page which is not expected.

I'm using PHP built-in server which is probably the reason of such problem (ran by: php -S localhost:1234).

In request_path() I've tracked the problem to the following code which "thinks" this is a home page request:

    // If the path equals the script filename, either because 'index.php' was
    // explicitly provided in the URL, or because the server added it to
    // $_SERVER['REQUEST_URI'] even when it wasn't provided in the URL (some
    // versions of Microsoft IIS do this), the front page should be served.
    if ($path == basename($_SERVER['PHP_SELF'])) {
      $path = '';
    }

These are my results of some testing the values by the following line:

    var_dump('PHP_SELF = ' . $_SERVER['PHP_SELF'], 'basename = ' . basename($_SERVER['PHP_SELF']), '$path = ' . $path); exit;

For example:

Page: /batch?op=start&id=10916

string 'PHP_SELF = /index.php/batch' (length=27)
string 'basename = batch' (length=16)
string '$path = batch' (length=13)

Problem: Since PHP_SELF basename equals to batch, Drupal thinks this is the file, therefore removing the correct value of $path

Page: update.php

string 'PHP_SELF = /update.php' (length=22)
string 'basename = update.php' (length=21)
string '$path = update.php' (length=18)

This is correct.

Page: /user

string 'PHP_SELF = /index.php/user' (length=26)
string 'basename = user' (length=15)
string '$path = user' (length=12)

This is not correct and will result in homepage page.

Page: /admin/reports
string 'PHP_SELF = /index.php/admin/reports' (length=35)
string 'basename = reports' (length=18)
string '$path = admin/reports' (length=21)

This won't redirect to homepage.

I think the fix should be similar as proposed in #223496: Use SCRIPT_FILENAME instead of PHP_SELF 7 years ago, so to use $_SERVER['SCRIPT_FILENAME'] instead.

So basename of SCRIPT_FILENAME will return: index.php, not batch, in case of batch case.