Prevent a terrible race condition

Looking into #649 I realized there's a pretty terrible race condition if
an index is compacted quickly followed by an index update. Since we
don't check the index updater message it would be possible for us to
swap out a compaction change, followed by immediately resetting to the
new state from the index updater. This would be bad as we'd possibly end
up with a situation where our long lived index would be operating on a
file that no longer existed on disk.

1 files changed, 31 insertions, 18 deletions

diff --git a/src/couch_index/src/couch_index.erl b/src/couch_index/src/couch_index.erl
index 9da928dac..604d5038c 100644
--- a/src/couch_index/src/couch_index.erl
+++ b/src/couch_index/src/couch_index.erl
@@ -230,24 +230,37 @@ handle_cast({new_state, NewIdxState}, State) ->
         mod=Mod,
         idx_state=OldIdxState
     } = State,
-    assert_signature_match(Mod, OldIdxState, NewIdxState),
-    CurrSeq = Mod:get(update_seq, NewIdxState),
-    Args = [
-        Mod:get(db_name, NewIdxState),
-        Mod:get(idx_name, NewIdxState),
-        CurrSeq
-    ],
-    couch_log:debug("Updated index for db: ~s idx: ~s seq: ~B", Args),
-    Rest = send_replies(State#st.waiters, CurrSeq, NewIdxState),
-    case State#st.committed of
-        true -> erlang:send_after(commit_delay(), self(), commit);
-        false -> ok
-    end,
-    {noreply, State#st{
-        idx_state=NewIdxState,
-        waiters=Rest,
-        committed=false
-    }};
+    OldFd = Mod:get(fd, OldIdxState),
+    NewFd = Mod:get(fd, NewIdxState),
+    case NewFd == OldFd of
+        true ->
+            assert_signature_match(Mod, OldIdxState, NewIdxState),
+            CurrSeq = Mod:get(update_seq, NewIdxState),
+            Args = [
+                Mod:get(db_name, NewIdxState),
+                Mod:get(idx_name, NewIdxState),
+                CurrSeq
+            ],
+            couch_log:debug("Updated index for db: ~s idx: ~s seq: ~B", Args),
+            Rest = send_replies(State#st.waiters, CurrSeq, NewIdxState),
+            case State#st.committed of
+                true -> erlang:send_after(commit_delay(), self(), commit);
+                false -> ok
+            end,
+            {noreply, State#st{
+                idx_state=NewIdxState,
+                waiters=Rest,
+                committed=false
+            }};
+        false ->
+            Fmt = "Ignoring update from old indexer for db: ~s idx: ~s",
+            Args = [
+                Mod:get(db_name, NewIdxState),
+                Mod:get(idx_name, NewIdxState)
+            ],
+            couch_log:warning(Fmt, Args),
+            {noreply, State}
+    end;
 handle_cast({update_error, Error}, State) ->
     send_all(State#st.waiters, Error),
     {noreply, State#st{waiters=[]}};