Create new on_compact triggerCOUCHDB-3326-clustered-purge-pr4-on-compact-plugin

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 <jiangph@cn.ibm.com>

3 files changed, 37 insertions, 1 deletions

diff --git a/src/couch/src/couch_bt_engine_compactor.erl b/src/couch/src/couch_bt_engine_compactor.erl
index 4e52064b4..2c5b78e0b 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 49742012d..2fe0b0d9d 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 740b8121b..816325699 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], []).