Catalog error when search-index rebuild a harvest source

[FuhuXia]

We can run following command to reindex a package
ckan search-index rebuild [dataset_name]
When running the command on a Catalog harvest source, it generates an SearchIndexError, then the harvest source disappears from the UI.

How to reproduce

1. Create a data.json harvest source named, e.g. test-json-source, in a test organization on Catalog ( replicated on sandbox and staging)
2. Run a harvest job
3. SSH into harvester instance, run the command ckan search-index rebuild test-json-source

Expected behavior

Command runs successfully.

Actual behavior

It generates a SearchIndexError.
Harvest source test-json-source is removed from its organization.

Solr responded with an error (HTTP 400): [Reason: Error parsing JSON field value.
Unexpected OBJECT_START at [3098], field=status]

[FuhuXia]

The cause of this issue is a bug found in pysolr. It only happens to harvest source that contains at least one harvest job run, making the pkg_dict sent to solr a multi-level nested JSON object.

{
	'owner_org': u'068066e8-4d6a-427d-b8b7-d4c63bdc733f',
	'maintainer': None, 
	...
	'status': {
		'job_count': 1,
		'total_datasets': 3L,
		'last_job': {
			'status': u'Finished', 
			'finished': '2021-02-23 19:52:38.606360',
			...
			'stats': {
				'deleted': 0,
				'updated': 0,
				...
			}
		}
	}
}

For the fresh harvest source with 0 harvest job, the last_job is None, we don't see this issue. It only happens to catalog.data.gov fcs branch where we upgraded pysolr version from CKAN default 3.6.0 to 3.9.0 for performance reason.

Since rebuild command is actually delete and add again, the record is deleted from solr index and not added back, the harvest source disappears from the UI.

Tested on 3.6.0, 3.7.0, 3.8.0, no such issue.