#3503613 Check if project already contains _WEB_ROOT directory

Closes #3503613

includes/include.drupalci.main.yml

@@ -195,8 +195,8 @@
     [[ $_TARGET_DB_TYPE == "mariadb" ]] && export SIMPLETEST_DB=mysql://$MYSQL_USER:$MYSQL_PASSWORD@database/$MYSQL_DATABASE
 
 .setup-webserver: &setup-webserver
-  - mkdir -p $BROWSERTEST_OUTPUT_DIRECTORY
-  - mkdir -p $CI_PROJECT_DIR/$_WEB_ROOT/sites/default/files/simpletest
+  - mkdir -pv $BROWSERTEST_OUTPUT_DIRECTORY
+  - mkdir -pv $CI_PROJECT_DIR/$_WEB_ROOT/sites/default/files/simpletest
   - chmod -R 755 $BROWSERTEST_OUTPUT_DIRECTORY
   - chmod -R 755 $CI_PROJECT_DIR/$_WEB_ROOT/sites/default/files/simpletest
   - ln -s $CI_PROJECT_DIR/$_WEB_ROOT /var/www/html
@@ -461,6 +461,12 @@ stages:
     - *show-ci-variables
     - *show-environment-variables
     - *add-composer-drupal-lenient
+    # Check if the project already has an existing directory which will clash with $_WEB_ROOT. If it does, fail the job.
+    - test -d "$_WEB_ROOT" && echo -e "$DIVIDER\nThis project contains a top-level directory named '$_WEB_ROOT'
+      which is the same as the variable _WEB_ROOT. This variable defines the location for installing Drupal core
+      and the clash may cause some jobs to fail or give misleading results.\n \nTo resolve this, you can either
+      rename the directory within your project or set a different custom value for the _WEB_ROOT variable. The
+      name can be anything other than an existing directory in your project.$DIVIDER" && exit 1;
     # Save the initial list of top-level files and folders. This will be just the project's own, nothing more.
     # Use -A to include the hidden files but not . and ..
     - export PROJECT_FILES=$(ls -A)