Allow users with existing WordPress accounts to sign in

An error occurs if a user with an existing WordPress account tries to
sign in using OpenID Connect. This patch fixes this problem by adding
the OpenID Connect meta data to the existing user's account after
successful authorization.

includes/openid-connect-generic-client-wrapper.php

@@ -412,6 +412,11 @@ class OpenID_Connect_Generic_Client_Wrapper {
 			$username = $this->get_username_from_claim( $user_claim );
 		}
 
+		// Before trying to create the user, first check if a user with the same email already exists
+		if( $uid = email_exists( $email ) ) {
+			return $this->update_existing_user( $uid, $subject_identity );
+		}
+		
 		// allow other plugins / themes to determine authorization 
 		// of new accounts based on the returned user claim
 		$create_user = apply_filters( 'openid-connect-generic-user-creation-test', TRUE, $user_claim );
@@ -443,4 +448,25 @@ class OpenID_Connect_Generic_Client_Wrapper {
 		
 		return $user;
 	}
+	
+	
+	/**
+	 * Update an existing user with OpenID Connect meta data
+	 * 
+	 * @param $uid
+	 * @param $subject_identity
+	 *
+	 * @return \WP_Error | \WP_User
+	 */
+	function update_existing_user( $uid, $subject_identity ) {
+		// add the OpenID Connect meta data 
+		add_user_meta( $uid, 'openid-connect-generic-user', TRUE, TRUE );
+		add_user_meta( $uid, 'openid-connect-generic-subject-identity', (string) $subject_identity, TRUE );
+		
+		// allow plugins / themes to take action on user update
+		do_action( 'openid-connect-generic-user-update', $uid );
+		
+		// return our updated user
+		return get_user_by( 'id', $uid );
+	}
 }