tree bafb0664d0f3eca8be005eae20de5546a49e1297
parent 01f2af22af4f9ed9f1fff1ee0ff66946864b4730
author Patrick Hiesel <hiesel@google.com> 1573661299 -0800
committer Patrick Hiesel <hiesel@google.com> 1574121991 -0800

Replace reindexAfterRefUpdate setting with indexMergeable

Gerrit can index a boolean field called 'mergeable' that determines
if a change can be merged into the target ref. This bit can change
whenever the target ref advances. Gerrit therefore has logic to
reindex all open changes when the target ref advances.

Depending on the number of open changes, the frequency of updates of
the target ref and the size of the repo, this can be a very expensive
operation. For large installations, 'reindexAfterRefUpdate' was added
as a setting back in 2015 (I88ae7f4ad) to turn off automatic
reindexing.

This setting however, leads to inconsistent behavior: Gerrit stops
updating documents when the target ref advances, so the 'mergeable'
bit in the indexed document can be stale. For large repos, it is
most likely stale. Users can still query for 'is:mergeable' though and
Gerrit happily serves that stale bit in any query response.

It is worth noting, that all of this does not affect the UI as that
sends a separate, asynchronous request to compute mergeablitly when
needed and does not rely on the index.

This commit cleans this behavior up by replacing reindexAfterRefUpdate
with indexMergeable. After this commit, there are two modes of operation:
1) Gerrit indexes 'mergable' and keeps it up to date when the target ref
   advances. Gerrit allows queries for 'is:mergeable'.
2) Gerrit does not index 'mergeable' at all. Gerrit does not allow
   queries for 'is:mergeable'.

This way, users always get a consistent and correct result.

Change-Id: I053af1b99616920db7f0dda8f8ec770e8683df5c