This is just a first evaluation to understand if Diffusion public identities can be used to expand Calendar invitees, so to have less "Private User".
Data protection considerations: who import the calendar ICS data already knows the emails in the file. Who import should just do not know verified emails in general. Diffusion identities - instead - are already a public information.
Specifications to respect the above considerations:
- for imported events, NEVER create any new Identity
- for imported events, just try to match an identity
- if the Diffusion application is disabled, do not try to perform any lookup there
Current structure of the Diffusion identity entity:
```lang=sql
CREATE TABLE `repository_identity` (
`id` int(10) unsigned NOT NULL AUTO_INCREMENT,
`phid` varbinary(64) NOT NULL,
`dateCreated` int(10) unsigned NOT NULL,
`dateModified` int(10) unsigned NOT NULL,
`automaticGuessedUserPHID` varbinary(64) DEFAULT NULL,
`manuallySetUserPHID` varbinary(64) DEFAULT NULL,
`currentEffectiveUserPHID` varbinary(64) DEFAULT NULL,
`identityNameHash` binary(12) NOT NULL,
`identityNameRaw` longblob NOT NULL,
`identityNameEncoding` varchar(16) DEFAULT NULL,
`authorPHID` varbinary(64) NOT NULL,
`emailAddress` varchar(255) CHARACTER SET utf8mb4 COLLATE utf8mb4_unicode_ci DEFAULT NULL,
PRIMARY KEY (`id`),
UNIQUE KEY `key_phid` (`phid`),
UNIQUE KEY `key_identity` (`identityNameHash`),
KEY `key_email` (`emailAddress`(64))
) ENGINE=InnoDB AUTO_INCREMENT=3617 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin
```
Way to look for identities (untested - incomplete):
```php
$email = $email_found_from_imported_ics_file;
$identity = id(new PhabricatorRepositoryIdentityQuery)
->withEmailAddresses(array($email))
->setLimit(1)
->executeOne();
// order by manuallySetUserPHID NOT NULL, automaticGuessedUserPHID NOT NULL
```