Migration throws warning when constants key is not defined in file migration source

Problem/Motivation

[warning] Trying to access array offset on value of type null File.php:105
[warning] Undefined array key "constants" File.php:105

Steps to reproduce

Create a files migration without constants defined in the source.
There are cases where you do not need source_base_path in the migration.
Like when you are migrating files from Drupal 7 to Drupal 9, You can use stage file proxy to refer files to the production server or you can have files already copied into your new sites where there is no need for re downloading of files.

uuid: fe9b8ff3-8713-4ded-9ccd-9644d540156b
langcode: en
status: true
dependencies: {  }
id: upgrade_d7_file
class: Drupal\migrate\Plugin\Migration
field_plugin_method: null
cck_plugin_method: null
migration_tags:
  - 'Drupal 7'
  - Content
migration_group: migrate_drupal_7
label: 'Public files'
source:
  plugin: d7_file
  scheme: public
  constants:
    source_base_path: 'https://example.com/'
process:
  fid:
    -
      plugin: get
      source: fid
  filename:
    -
      plugin: get
      source: filename
  source_full_path:
    -
      plugin: concat
      delimiter: /
      source:
        - constants/source_base_path
        - filepath
    -
      plugin: urlencode
  uri:
    -
      plugin: file_copy
      source:
        - '@source_full_path'
        - uri
  filemime:
    -
      plugin: get
      source: filemime
  status:
    -
      plugin: get
      source: status
  created:
    -
      plugin: get
      source: timestamp
  changed:
    -
      plugin: get
      source: timestamp
  uid:
    -
      plugin: get
      source: uid
destination:
  plugin: 'entity:file'
migration_dependencies:
  required: {  }
  optional: {  }

Above migration doesnt throw warning. Where as below migration throws warning.

uuid: fe9b8ff3-8713-4ded-9ccd-9644d540156b
langcode: en
status: true
dependencies: {  }
id: upgrade_d7_file
class: Drupal\migrate\Plugin\Migration
field_plugin_method: null
cck_plugin_method: null
migration_tags:
  - 'Drupal 7'
  - Content
migration_group: migrate_drupal_7
label: 'Public files'
source:
  plugin: d7_file
  scheme: public
process:
  fid:
    -
      plugin: get
      source: fid
  filename:
    -
      plugin: get
      source: filename
  uri:
    -
      plugin: get
      source: uri
  filemime:
    -
      plugin: get
      source: filemime
  status:
    -
      plugin: get
      source: status
  created:
    -
      plugin: get
      source: timestamp
  changed:
    -
      plugin: get
      source: timestamp
  uid:
    -
      plugin: get
      source: uid
destination:
  plugin: 'entity:file'
migration_dependencies:
  required: {  }
  optional: {  }

Proposed resolution

Add check for the config defined or not in prepare row of file migration plugin.

  public function prepareRow(Row $row) {
    // Compute the filepath property, which is a physical representation of
    // the URI relative to the Drupal root.
    $path = str_replace(['public:/', 'private:/'], [$this->publicPath, $this->privatePath], $row->getSourceProperty('uri'));
    // At this point, $path could be an absolute path or a relative path,
    // depending on how the scheme's variable was set. So we need to shear out
    // the source_base_path in order to make them all relative.
    $path = preg_replace('#' . preg_quote($this->configuration['constants']['source_base_path']) . '#', '', $path, 1);
    $row->setSourceProperty('filepath', $path);
    return parent::prepareRow($row);
  }

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet