From 165531bcf223f1c05c4c2eaef7cd2f2943c10584 Mon Sep 17 00:00:00 2001
From: Adam Kocoloski <kocolosk@apache.org>
Date: Sat, 15 Aug 2009 05:11:45 +0000
Subject: better failure modes in replication.  See COUCHDB-193, COUCHDB-416

If you try to replicate a DB to itself, the replication will proceed, but no
checkpoints will be saved, and the logs will say

"checkpoint failure: conflict (are you replicating to yourself?)"

If you try to specify a non-existent DB as source or target, replication will
fail immediately with a 404.  The response body will indicate which DB could
not be opened.


git-svn-id: https://svn.apache.org/repos/asf/couchdb/trunk@804436 13f79535-47bb-0310-9956-ffa450edef68
---
 src/couchdb/couch_httpd_misc_handlers.erl |  5 ++++-
 src/couchdb/couch_rep.erl                 | 24 +++++++++++++++++-------
 src/couchdb/couch_rep_httpc.erl           |  2 ++
 3 files changed, 23 insertions(+), 8 deletions(-)

diff --git a/src/couchdb/couch_httpd_misc_handlers.erl b/src/couchdb/couch_httpd_misc_handlers.erl
index c4f28308..e72f9d2f 100644
--- a/src/couchdb/couch_httpd_misc_handlers.erl
+++ b/src/couchdb/couch_httpd_misc_handlers.erl
@@ -79,13 +79,16 @@ handle_task_status_req(Req) ->
 
 handle_replicate_req(#httpd{method='POST'}=Req) ->
     PostBody = couch_httpd:json_body_obj(Req),
-    case couch_rep:replicate(PostBody, Req#httpd.user_ctx) of
+    try couch_rep:replicate(PostBody, Req#httpd.user_ctx) of
     {ok, {JsonResults}} ->
         send_json(Req, {[{ok, true} | JsonResults]});
     {error, {Type, Details}} ->
         send_json(Req, 500, {[{error, Type}, {reason, Details}]});
     {error, Reason} ->
         send_json(Req, 500, {[{error, Reason}]})
+    catch
+    throw:{db_not_found, Msg} ->
+        send_json(Req, 404, {[{error, db_not_found}, {reason, Msg}]})
     end;
 handle_replicate_req(Req) ->
     send_method_not_allowed(Req, "POST").
diff --git a/src/couchdb/couch_rep.erl b/src/couchdb/couch_rep.erl
index c5a07685..cde0a85e 100644
--- a/src/couchdb/couch_rep.erl
+++ b/src/couchdb/couch_rep.erl
@@ -78,7 +78,11 @@ replicate(PostBody, UserCtx) ->
         replicate(PostBody, UserCtx)
     end.
 
-init([RepId, {PostProps}, UserCtx] = InitArgs) ->
+init(InitArgs) ->
+    try do_init(InitArgs)
+    catch throw:{db_not_found, DbUrl} -> {stop, {db_not_found, DbUrl}} end.
+
+do_init([RepId, {PostProps}, UserCtx] = InitArgs) ->
     process_flag(trap_exit, true),
 
     SourceProps = proplists:get_value(<<"source">>, PostProps),
@@ -177,7 +181,6 @@ handle_info({'EXIT', Pid, Reason}, State) ->
     {stop, Reason, State}.
 
 terminate(normal, State) ->
-    % ?LOG_DEBUG("replication terminating normally", []),
     #state{
         checkpoint_history = CheckpointHistory,
         committed_seq = NewSeq,
@@ -252,7 +255,9 @@ start_replication_server(Replicator) ->
         end;
     {error, {already_started, Pid}} ->
         ?LOG_DEBUG("replication ~p already running at ~p", [RepId, Pid]),
-        Pid
+        Pid;
+    {error, {{db_not_found, DbUrl}, _}} ->
+        throw({db_not_found, <<"could not open ", DbUrl/binary>>})
     end.
 
 compare_replication_logs(SrcDoc, TgtDoc) ->
@@ -389,7 +394,7 @@ open_db({Props}, _UserCtx) ->
     },
     case couch_rep_httpc:db_exists(Db) of
     true -> Db;
-    false -> throw({db_not_found, Url})
+    false -> throw({db_not_found, ?l2b(Url)})
     end;
 open_db(<<"http://",_/binary>>=Url, _) ->
     open_db({[{<<"url">>,Url}]}, []);
@@ -446,17 +451,22 @@ do_checkpoint(State) ->
         {<<"source_last_seq">>, RecordSeqNum},
         {<<"history">>, lists:sublist([NewHistoryEntry | OldHistory], 50)}
     ]},
-    % ?LOG_DEBUG("updating src doc ~p", [SourceLog]),
+
+    try
     {SrcRevPos,SrcRevId} =
         update_doc(Source, SourceLog#doc{body=NewRepHistory}, []),
-    % ?LOG_DEBUG("updating tgt doc ~p", [TargetLog]),
     {TgtRevPos,TgtRevId} =
         update_doc(Target, TargetLog#doc{body=NewRepHistory}, []),
     State#state{
         checkpoint_history = NewRepHistory,
         source_log = SourceLog#doc{revs={SrcRevPos, [SrcRevId]}},
         target_log = TargetLog#doc{revs={TgtRevPos, [TgtRevId]}}
-    }.
+    }
+    catch throw:conflict ->
+    ?LOG_ERROR("checkpoint failure: conflict (are you replicating to yourself?)",
+        []),
+    State
+    end.
 
 commit_to_both(Source, Target) ->
     % commit the src async
diff --git a/src/couchdb/couch_rep_httpc.erl b/src/couchdb/couch_rep_httpc.erl
index 3e3b5f05..3d0696e4 100644
--- a/src/couchdb/couch_rep_httpc.erl
+++ b/src/couchdb/couch_rep_httpc.erl
@@ -96,6 +96,8 @@ process_response({ok, Status, Headers, Body}, Req) ->
         MochiHeaders = mochiweb_headers:make(Headers),
         RedirectUrl = mochiweb_headers:get_value("Location", MochiHeaders),
         do_request(Req#http_db{url = RedirectUrl});
+    Code =:= 409 ->
+        throw(conflict);
     Code >= 400, Code < 500 ->
         ?JSON_DECODE(maybe_decompress(Headers, Body));
     Code =:= 500; Code =:= 502 ->
-- 
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