From db872d0a2be7f5915c0dfba43db2914fdc501811 Mon Sep 17 00:00:00 2001
From: Tony Sun <tony.sun427@gmail.com>
Date: Mon, 14 Sep 2020 11:12:47 -0700
Subject: fix race condition (#3150)

This fixes a94e693f32672e4613bce0d80d0b9660f85275ea because a race
condition exisited where the 'DOWN' message could be received
before the compactor pid is spawned. Adding a synchronous call to
get the compactor pid guarantees that the couch_db_updater process
handling of finish_compaction has occurred.
---
 src/couch/src/couch_db.erl        | 9 +++++++++
 src/smoosh/src/smoosh_channel.erl | 2 +-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/src/couch/src/couch_db.erl b/src/couch/src/couch_db.erl
index e1d726dc9..390a198df 100644
--- a/src/couch/src/couch_db.erl
+++ b/src/couch/src/couch_db.erl
@@ -37,6 +37,7 @@
     get_committed_update_seq/1,
     get_compacted_seq/1,
     get_compactor_pid/1,
+    get_compactor_pid_sync/1,
     get_db_info/1,
     get_partition_info/2,
     get_del_doc_count/1,
@@ -572,6 +573,14 @@ get_compacted_seq(#db{}=Db) ->
 get_compactor_pid(#db{compactor_pid = Pid}) ->
     Pid.
 
+get_compactor_pid_sync(#db{main_pid=Pid}=Db) ->
+    case gen_server:call(Pid, compactor_pid, infinity) of
+        CPid when is_pid(CPid) ->
+            CPid;
+        _ ->
+            nil
+    end.
+
 get_db_info(Db) ->
     #db{
         name = Name,
diff --git a/src/smoosh/src/smoosh_channel.erl b/src/smoosh/src/smoosh_channel.erl
index d8a8d14a9..058f5518f 100644
--- a/src/smoosh/src/smoosh_channel.erl
+++ b/src/smoosh/src/smoosh_channel.erl
@@ -276,7 +276,7 @@ start_compact(State, Db) ->
     false ->
         DbPid = couch_db:get_pid(Db),
         Key = couch_db:name(Db),
-        case couch_db:get_compactor_pid(Db) of
+        case couch_db:get_compactor_pid_sync(Db) of
             nil ->
                 Ref = erlang:monitor(process, DbPid),
                 DbPid ! {'$gen_call', {self(), Ref}, start_compact},
-- 
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