The comment "language" column in D7 might be empty after migrating from D6, this is invalid in D8

I have only glanced at this patch and this jumped out at me.

+++ b/core/modules/comment/src/Plugin/migrate/source/d7/Comment.php
@@ -44,6 +45,12 @@ public function prepareRow(Row $row) {
+    // If this comment was upgraded from Drupal 6 or older, it's possible that
+    // no language code was specified.
+    if ($row->getSourceProperty('language') === '') {

Instead of passing an empty string in the pipeline we normally use the default_value process plugin to set the value in the pipeline. For example as is done in the node migrations.

  langcode:
    plugin: default_value
    source: language
    default_value: "und"

Well I reckon that NULL, 0 and '0' would be 'bad' data and it would be perfectly reasonable to attempt to migrate the row with a langcode of 'und'.

This will also need a migration test. I mean update the fixture with a new comment and make assertions in d7/MigrateCommentTest.

I think this fallback should be needed not only in case of bad data but for case when no such language exists (I faced with orphaned comments in language which were deleted years ago)

updated with first found #3044158: Orphaned comments Error: Call to a member function getOwner() but it was very common case for d5/d6 era

I had to update Drupal\Tests\migrate_drupal_ui\Functional\d7\Upgrade7Test since we have one more comments.
Also removed the use Drupal\Core\Language\LanguageInterface; line from Drupal\comment\Plugin\migrate\source\d7\Commentfor test-only patch.

Addressed #10, but not with the default_value plugin: I still prefer using LanguageInterface::LANGCODE_NOT_SPECIFIED instead of hard-coding und in the migration.

I'll review this during the coming week.

@huzooka and @Wim Leers, thanks for work here, this was easy to review.

This look really good and there is failing patch as well. The test and changes to the fixture are doing exactly what needs to be done.

1. +++ b/core/modules/comment/src/Plugin/migrate/source/d7/Comment.php
   @@ -44,6 +45,12 @@ public function prepareRow(Row $row) {
   +    // If this comment was upgraded from Drupal 6 or older, it's possible that
   

   This is just setting a default if the language is not defined and should be done in the process pipeline, not the source plugin. Doing so in the source plugin adds code that requires testing and maintenance. We can avoid that by using the pipeline.

   And I think a comment in the pipeline would be helpful as well.

   #  Comments migrated from Drupal 6 to Drupal 7 may not have a language.
      langcode:
       plugin: default_value
       source: language
       default_value: "und"

2. +++ b/core/modules/comment/tests/src/Kernel/Migrate/d7/MigrateCommentTest.php
   @@ -105,6 +105,27 @@ public function testMigration() {
   +    // Comment migrated from Drupal 6 to Drupal 7 may have empty language
   +    // property. This is invalid in Drupal 8. Test that these comments will have
   +    // valid language after the migration.
   

   Then with part of this comment moved to the pipeline this can be shorter. Maybe just
   'Tests a comment migrated from Drupal 6 to Drupal 7 that did not have a language.' or something like that

3. +++ b/core/modules/comment/tests/src/Kernel/Migrate/d7/MigrateCommentTest.php
   index dc99489563..1231a7a377 100644
   --- a/core/modules/comment/tests/src/Kernel/Plugin/migrate/source/d7/CommentTest.php
   

   Changes not needed if the default value is set in the process pipepline.

Updating tags

Addressed everything from #24.

I removed the previous additions of the comment source plugin test Drupal\Tests\comment\Kernel\Plugin\migrate\source\d7\CommentTest, and set the default 'und' value in the process pipeline.

@quietone, @Wim Leers, @andypost: thank you all!

Committed and pushed 642b7653d4 to 9.0.x and 17ef9e3e9a to 8.9.x and fc579acaa2 to 8.8.x. Thanks!

Backported to 8.8.x as a migrate bugfix.