REST api failed if the assignee is invalid user(non-existent user)

Problem Description:

LDAP connector or (Crowd application with LDAP connector) has been one of customer favourite configuration. However, there are times that the user would be leave the company and some administrator would remove that inactive user from LDAP directory. Thus, it caused that the user to be removed the directory within crowd or JIRA. However, the removed user would still be remain as the assignee of some issues although the user and the associated membership were completely removed from related directory.

We can still update a ticket which the assignee was the removed user through the GUI edit operation or inline edit. However, when trying to update or transition through REST API or REST client, the update operation failed.

Step to Reproduce:

1. Create a user(eg. testuser) in LDAP server
2. Setup a read only LDAP directory in JIRA (it can be achieve through crowd connected to Read Only LDAP connector) then synchronize the directory to allow the user added into JIRA
3. Ensure the user has the assignable permission. For easier replication, grant user all the groups permission.
4. Create a test issue and assign to the testuser
5. Remove the user from LDAP server
6. Perform the any update cURL command or update an issue through REST client and the error will be show up.

cURL command triggered:

curl -D- -u admin:admin -X POST --data @"Desktop\data.json" -H "Content-Type: application/json" http://localhost:8527/rest/api/2/issue/DEMO-15/transitions?expand=transitions.fields

Error thrown in the command prompt:

HTTP/1.1 400 Bad Request
Server: Apache-Coyote/1.1
X-AREQUESTID: 1330x1630x1
Set-Cookie: JSESSIONID=DBB357E7C30BC89A15A1944475C59D13; Path=/; HttpOnly
X-Seraph-LoginReason: OK
Set-Cookie: atlassian.xsrf.token=BQ3E-463Q-AVK1-5MLS|0c3a3da6067e31a4881f77b6ac2c58dbe39f9a37|lin; Path=/
X-ASESSIONID: 30rkz
X-AUSERNAME: admin
Cache-Control: no-cache, no-store, no-transform
Content-Type: application/json;charset=UTF-8
Transfer-Encoding: chunked
Date: Wed, 27 Mar 2013 14:10:00 GMT
Connection: close

{"errorMessages":[],"errors":{"assignee":"User 'testuser' does not exist."}}

Error thrown in the REST Client through IDE when run the project:

Exception in thread "main" java.lang.NullPointerException
	at RestTry.main(RestTry.java:40)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
	at java.lang.reflect.Method.invoke(Method.java:597)
	at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)

Workaround

Create the user back into the LDAP server without any group membership and synchronize the user back to JIRA. However, make sure the username still remain the same as the name it was removed earlier.