When trying to build the index of the current wiki from the Administration > Applications > Search section, the page keeps loading undefinetly. Opening a new tab to the same page will have the same endless loading effect.
The problem lies in the fix for the issue
XWIKI-7321 that opens an index writer (and an associated lock), but never closes it when it is done deleting documents. This leaves the lock open and future requests will fail to obtain the lock.
This does not affect searches, but it will prevent new/updated documents from being indexed since any index writing related operation is in deadlock until you restart the wiki.
Also, this does not affect reindexing the "Entire farm", just reindexing a specific wiki.
1. Stop the wiki
2. Delete the "<permanentDirectory>/lucene" folder containing the index
3. Restart the wiki.
The initial indexing will get you an up to date index, but as soon as you want to update the index manually, you will have to repeat steps 1-3).