From fedf37da2d9edc92340cc9d92e6affa346ab01f6 Mon Sep 17 00:00:00 2001
From: Robert Newson <rnewson@apache.org>
Date: Sun, 16 Jan 2022 15:58:55 +0000
Subject: [PATCH] Non-zero instance_start_time

Set instance_start_time to the creation time of the database to
restore the ability for the replicator to detect a db recreation event
during a replication. Without this, a replication can fail to notice
the db was deleted and recreated, write the checkpoint document for
the in-progress replication and keep going, whereas what should happen
is the replication should reset from sequence 0.
---
 src/chttpd/src/chttpd_db.erl                      |  3 ++-
 .../src/couch_replicator_scheduler_job.erl        |  9 +++------
 src/fabric/src/fabric_db_info.erl                 |  5 +++--
 src/mem3/src/mem3.erl                             | 15 +++++++++++++++
 4 files changed, 23 insertions(+), 9 deletions(-)

diff --git a/src/chttpd/src/chttpd_db.erl b/src/chttpd/src/chttpd_db.erl
index 875df6e0005..cfae8ad5f45 100644
--- a/src/chttpd/src/chttpd_db.erl
+++ b/src/chttpd/src/chttpd_db.erl
@@ -554,12 +554,13 @@ db_req(
     %% for missing databases that'd return error 404 from chttpd
     %% get_security used to prefer shards on the same node over other nodes
     fabric:get_security(DbName, [{user_ctx, Ctx}]),
+    CreationTime = mem3:shard_creation_time(DbName),
     send_json(
         Req,
         201,
         {[
             {ok, true},
-            {instance_start_time, <<"0">>}
+            {instance_start_time, CreationTime}
         ]}
     );
 db_req(#httpd{path_parts = [_, <<"_ensure_full_commit">>]} = Req, _Db) ->
diff --git a/src/couch_replicator/src/couch_replicator_scheduler_job.erl b/src/couch_replicator/src/couch_replicator_scheduler_job.erl
index f300dd44e93..6d9711790d9 100644
--- a/src/couch_replicator/src/couch_replicator_scheduler_job.erl
+++ b/src/couch_replicator/src/couch_replicator_scheduler_job.erl
@@ -846,18 +846,15 @@ do_checkpoint(State) ->
             end;
         {SrcInstanceStartTime, _NewTgtInstanceStartTime} ->
             {checkpoint_commit_failure, <<
-                "Target database out of sync. "
-                "Try to increase max_dbs_open at the target's server."
+                "instance_start_time on target database has changed since last checkpoint."
             >>};
         {_NewSrcInstanceStartTime, TgtInstanceStartTime} ->
             {checkpoint_commit_failure, <<
-                "Source database out of sync. "
-                "Try to increase max_dbs_open at the source's server."
+                "instance_start_time on source database has changed since last checkpoint."
             >>};
         {_NewSrcInstanceStartTime, _NewTgtInstanceStartTime} ->
             {checkpoint_commit_failure, <<
-                "Source and target databases out of "
-                "sync. Try to increase max_dbs_open at both servers."
+                "instance_start_time on source and target database has changed since last checkpoint."
             >>}
     end.
 
diff --git a/src/fabric/src/fabric_db_info.erl b/src/fabric/src/fabric_db_info.erl
index 2366420c894..5461404c508 100644
--- a/src/fabric/src/fabric_db_info.erl
+++ b/src/fabric/src/fabric_db_info.erl
@@ -19,6 +19,7 @@
 
 go(DbName) ->
     Shards = mem3:shards(DbName),
+    CreationTime = mem3:shard_creation_time(DbName),
     Workers = fabric_util:submit_jobs(Shards, get_db_info, []),
     RexiMon = fabric_util:create_monitors(Shards),
     Fun = fun handle_message/3,
@@ -28,7 +29,7 @@ go(DbName) ->
     try
         case fabric_util:recv(Workers, #shard.ref, Fun, Acc0) of
             {ok, Acc} ->
-                {ok, Acc};
+                {ok, [{instance_start_time, CreationTime} | Acc]};
             {timeout, {WorkersDict, _, _}} ->
                 DefunctWorkers = fabric_util:remove_done_workers(
                     WorkersDict,
@@ -117,7 +118,7 @@ merge_results(Info) ->
             (_K, _V, Acc) ->
                 Acc
         end,
-        [{instance_start_time, <<"0">>}],
+        [],
         Dict
     ).
 
diff --git a/src/mem3/src/mem3.erl b/src/mem3/src/mem3.erl
index 7151a3ec119..5a985b7f827 100644
--- a/src/mem3/src/mem3.erl
+++ b/src/mem3/src/mem3.erl
@@ -33,6 +33,7 @@
 -export([get_placement/1]).
 -export([ping/1, ping/2]).
 -export([db_is_current/1]).
+-export([shard_creation_time/1]).
 
 %% For mem3 use only.
 -export([name/1, node/1, range/1, engine/1]).
@@ -198,6 +199,20 @@ shard_suffix(DbName0) when is_binary(DbName0) ->
 shard_suffix(Db) ->
     shard_suffix(couch_db:name(Db)).
 
+shard_creation_time(DbName0) ->
+    Shard = hd(shards(DbName0)),
+    case Shard#shard.name of
+        <<"shards/", _:8/binary, "-", _:8/binary, "/", DbName/binary>> ->
+            case filename:extension(DbName) of
+                <<".", Time/binary>> ->
+                    Time;
+                _ ->
+                    <<"0">>
+            end;
+        _ ->
+            <<"0">>
+    end.
+
 fold_shards(Fun, Acc) ->
     mem3_shards:fold(Fun, Acc).