[couchdb] 01/02: Prevent a terrible race condition

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davisp pushed a commit to branch 649-fix-mrview-changes-since-test
in repository https://gitbox.apache.org/repos/asf/couchdb.git

commit d3e06625015cac765620ec11eb3fea518fb14016
Author: Paul J. Davis <pa...@gmail.com>
AuthorDate: Wed Jul 12 15:50:17 2017 -0500

    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.
---
 src/couch_index/src/couch_index.erl | 49 +++++++++++++++++++++++--------------
 1 file changed, 31 insertions(+), 18 deletions(-)

diff --git a/src/couch_index/src/couch_index.erl b/src/couch_index/src/couch_index.erl
index 9da928d..604d503 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=[]}};

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