trackItemInsert() from _search_api_entity_datasource_bundle_change() can cause "Duplicate entry" in search_api_item, if previous operation did not finish cleanly.

Oh I think I really have the explanation.

Create a search index 'my_index'.
Enable indexing for node type 'some_type' (but not 'other_type').
Create some nodes and index them.

-> Now the search_api_item table contains entries for 'some_type'.

And now..
Create node type 'other_type'.
Enable indexing of 'other_type', disable indexing of 'some_type'.

-> Now the search_api_item table now still contains entries for 'some_type'.

Convert nodes from 'some_type' to 'other_type'.

-> Now search_api will try to insert the same items as 'other_type', because it thinks that entries for 'some_type' do not exist.

Here is a possible fix. Maybe I will produce a patch..

abstract class SearchApiAbstractDataSourceController implements SearchApiDataSourceControllerInterface {
  [..]

  /**
   * {@inheritdoc}
   */
  public function trackItemInsert(array $item_ids, array $indexes) {
    if (!$this->table || $item_ids === array()) {
      return;
    }

    $index_ids = [];
    foreach ($indexes as $index) {
      $this->checkIndex($index);
      // Prevent the array key to be converted to integer.
      $index_ids['_' . $index->id] = $index->id;
    }

    // Since large amounts of items can overstrain the database, only add items
    // in chunks.
    foreach (array_chunk($item_ids, 1000) as $chunk) {

      $q = db_select($this->table, 't');
      $q->condition($this->itemIdColumn, $chunk);
      $q->condition($this->indexIdColumn, $index_ids);
      $q->addField('t', $this->itemIdColumn, 'itemId');
      $q->addField('t', $this->indexIdColumn, 'indexId');
      $q->addField('t', $this->changedColumn, 'changed');

      // PHP array keys can be unreliable in some cases..
      $existing = [];
      foreach ($q->execute() as $row) {
        // Prevent the array keys to be converted to integer.
        $existing['_' . $row->itemId]['_' . $row->indexId] = TRUE;
      }

      $insert = db_insert($this->table)
        ->fields(array($this->itemIdColumn, $this->indexIdColumn, $this->changedColumn));

      foreach ($chunk as $item_id) {
        foreach ($index_ids as $index_id) {
          if (!empty($existing['_' . $item_id]['_' . $index_id])) {
            $q = db_update($this->table);
            $q->condition($this->indexIdColumn, $index_id);
            $q->condition($this->itemIdColumn, $item_id);
            $q->fields([$this->changedColumn => 1]);
            $q->execute();
          }
          else {
            $insert->values(array(
              $this->itemIdColumn => $item_id,
              $this->indexIdColumn => $index_id,
              $this->changedColumn => 1,
            ));
          }
        }
      }
      $insert->execute();
    }
  }

"Prevent the array key to be converted to integer."
Why am I doing this?

I think to remember some experiment where different strings would become the same array key, like this:

assert($a !== $b && [$a => TRUE] !== [$b => TRUE]);

Now I cannot find any examples anymore to reproduce this.

EDIT
https://3v4l.org/2XYtc -> behaves as it should.
Maybe this problem only exists when converting from float.

I'm pretty sure we forbid editing of the indexed bundles after creating the index, for exactly this reason? How are you doing that?

This is a complicated project and I don't know exactly why this happened.
My assumption is that it was coming from a feature module somewhere.
Maybe the "forbid editing" only applies to the form, but not to the feature. This would mean the feature needs a hook_update_N() that wipes the search index?

Or if we want to fix this in search_api:
One solution could be to separate the conf variable from the actual state.
- The variable tells us which bundles *should* be indexed.
- A value in cache or elsewhere, possibly even in the search index itself (?), tells us which bundles were actually indexed last time.

Or maybe this:
Catch the exception, tell the user to clear the search index. (if this helps)

I just tried: The bundle settings can be easily changed with features, by changing feature code and doing drush fr.