JIRA Assignee dropdown doesn't return expected number of assignable users.

Summary

JIRA Assignee dropdown uses REST API: /rest/api/latest/user/assignable/search
Searching assignable user though REST API: /rest/api/latest/user/assignable/search?username=&project=<PROG> doesn't return expected number of assignable users.

This is regression of new functionality introduced in JIRA 7.2: in-memory Lucene index (Implemented as IndexedUserDao.search, see JRASERVER-65126, JRASERVER-60532)

Environment

• JIRA 7.2.x
• Large number of users: 1k+

Steps to Reproduce

1. Configure some small amount of user to be assignable for a specific project, eg: 3
2. Trigger the REST call with expected maxResults: _/rest/api/latest/user/assignable/search?project=<PROG>&maxResults=3
3. Trigger the REST call with higher maxResults: _/rest/api/latest/user/assignable/search?project=<PROG>&maxResults=4
   • Actual maxResults value depends on specific data set

Expected Results

• REST call returns all expected assignable users.
• Returned result set doesn't depend on maxResults
• Returned result set doesn't contain duplicates

Actual Results

• REST doesn't return expected number of assignable users, less then expected
• Result set size depends on maxResults
• Sometimes returned result set contains duplicates

Notes

None

Workaround

It might cause performance degradation for large JIRA instances. Please be careful and rollback the change if it has significant performance impact.
To disable new in-memory Lucene index (JRASERVER-65126), JIRA will use SQL to search users after that:

1. Add the following line to <JIRA_HOME>/jira-config.properties (create file if doesn't exist):

   jira.luceneUserSearcher = false

2. Restart JIRA