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jiangphcn pushed a commit to branch 
COUCHDB-3326-clustered-purge-pr4-on-compact-plugin
in repository https://gitbox.apache.org/repos/asf/couchdb.git

commit 830240b6efd5808cdb964fdb05e584758fcc9f2f
Author: Paul J. Davis <[email protected]>
AuthorDate: Wed May 30 14:34:44 2018 -0500

    Create new on_compact trigger
    
    This trigger allows any storage engine that makes use of compaction to
    notify that compaction is starting. This is preparatory work for
    clustered indexes so that existing indexes are allowed to ensure they
    have a clustered purge local doc before compaction runs.
    
    COUCHDB-3326
    
    Co-Authored-By: jiangphcn <[email protected]>
---
 src/couch/src/couch_bt_engine_compactor.erl |  2 ++
 src/couch/src/couch_db_engine.erl           | 31 ++++++++++++++++++++++++++++-
 src/couch/src/couch_db_plugin.erl           |  5 +++++
 3 files changed, 37 insertions(+), 1 deletion(-)

diff --git a/src/couch/src/couch_bt_engine_compactor.erl 
b/src/couch/src/couch_bt_engine_compactor.erl
index 4e52064..2c5b78e 100644
--- a/src/couch/src/couch_bt_engine_compactor.erl
+++ b/src/couch/src/couch_bt_engine_compactor.erl
@@ -44,6 +44,8 @@ start(#st{} = St, DbName, Options, Parent) ->
     } = St,
     couch_log:debug("Compaction process spawned for db \"~s\"", [DbName]),
 
+    couch_db_engine:trigger_on_compact(DbName),
+
     {ok, NewSt, DName, DFd, MFd, Retry} =
             open_compaction_files(Header, FilePath, Options),
     erlang:monitor(process, MFd),
diff --git a/src/couch/src/couch_db_engine.erl 
b/src/couch/src/couch_db_engine.erl
index 4974201..2fe0b0d 100644
--- a/src/couch/src/couch_db_engine.erl
+++ b/src/couch/src/couch_db_engine.erl
@@ -627,7 +627,8 @@
     count_changes_since/2,
 
     start_compaction/1,
-    finish_compaction/2
+    finish_compaction/2,
+    trigger_on_compact/1
 ]).
 
 
@@ -891,3 +892,31 @@ finish_compaction(Db, CompactInfo) ->
     end,
     ok = gen_server:call(couch_server, {db_updated, NewDb}, infinity),
     {ok, NewDb}.
+
+
+trigger_on_compact(DbName) ->
+    {ok, DDocs} = get_ddocs(DbName),
+    couch_db_plugin:on_compact(DbName, DDocs).
+
+
+get_ddocs(<<"shards/", _/binary>> = DbName) ->
+    {_, Ref} = spawn_monitor(fun() ->
+        exit(fabric:design_docs(mem3:dbname(DbName)))
+    end),
+    receive
+        {'DOWN', Ref, _, _, {ok, JsonDDocs}} ->
+            {ok, lists:map(fun(JsonDDoc) ->
+                couch_doc:from_json_obj(JsonDDoc)
+            end, JsonDDocs)};
+        {'DOWN', Ref, _, _, Else} ->
+            Else
+    end;
+get_ddocs(DbName) ->
+    couch_util:with_db(DbName, fun(Db) ->
+        FoldFun = fun(FDI, Acc) ->
+            Doc = couch_db:open_doc_int(Db, FDI, []),
+            {ok, [Doc | Acc]}
+        end,
+        {ok, Docs} = couch_db:fold_design_docs(Db, FoldFun, [], []),
+        {ok, lists:reverse(Docs)}
+    end).
diff --git a/src/couch/src/couch_db_plugin.erl 
b/src/couch/src/couch_db_plugin.erl
index 740b812..8163256 100644
--- a/src/couch/src/couch_db_plugin.erl
+++ b/src/couch/src/couch_db_plugin.erl
@@ -18,6 +18,7 @@
     after_doc_read/2,
     validate_docid/1,
     check_is_admin/1,
+    on_compact/2,
     on_delete/2
 ]).
 
@@ -56,6 +57,10 @@ check_is_admin(Db) ->
     %% callbacks return true only if it specifically allow the given Id
     couch_epi:any(Handle, ?SERVICE_ID, check_is_admin, [Db], []).
 
+on_compact(DbName, DDocs) ->
+    Handle = couch_epi:get_handle(?SERVICE_ID),
+    couch_epi:apply(Handle, ?SERVICE_ID, on_compact, [DbName, DDocs], []).
+
 on_delete(DbName, Options) ->
     Handle = couch_epi:get_handle(?SERVICE_ID),
     couch_epi:apply(Handle, ?SERVICE_ID, on_delete, [DbName, Options], []).

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