From 4acbbe47a4e3482e22fcae6fa321b00ce988b37a Mon Sep 17 00:00:00 2001
From: Robert Newson <rnewson@apache.org>
Date: Fri, 24 Sep 2021 14:03:35 +0100
Subject: support maximum changes_duration configuration option

---
 rel/overlay/etc/default.ini       |  5 ++++
 src/chttpd/src/chttpd_changes.erl | 61 +++++++++++++++++++++++++--------------
 2 files changed, 45 insertions(+), 21 deletions(-)

diff --git a/rel/overlay/etc/default.ini b/rel/overlay/etc/default.ini
index 6085e5134..1b8441f66 100644
--- a/rel/overlay/etc/default.ini
+++ b/rel/overlay/etc/default.ini
@@ -208,6 +208,11 @@ bind_address = 127.0.0.1
 ;
 ; Bulk docs transaction batch size in bytes
 ;update_docs_batch_size = 2500000
+;
+; The upper bound, in milliseconds, that a continuous changes feed will return
+; database results for. The response ends on receipt of the first change to occur
+; after the duration period has lapsed.
+;changes_duration = infinity
 
 [couch_httpd_auth]
 ; WARNING! This only affects the node-local port (5986 by default).
diff --git a/src/chttpd/src/chttpd_changes.erl b/src/chttpd/src/chttpd_changes.erl
index dbe3e954a..56ad716b2 100644
--- a/src/chttpd/src/chttpd_changes.erl
+++ b/src/chttpd/src/chttpd_changes.erl
@@ -31,7 +31,7 @@
 
 %% export so we can use fully qualified call to facilitate hot-code upgrade
 -export([
-    keep_sending_changes/3
+    keep_sending_changes/4
 ]).
 
 -record(changes_acc, {
@@ -107,7 +107,8 @@ handle_db_changes(Args0, Req, Db0) ->
                     keep_sending_changes(
                         Args#changes_args{dir = fwd},
                         Acc0,
-                        true
+                        true,
+                        os:timestamp()
                     )
                 after
                     fabric2_events:stop_listener(Listener),
@@ -573,7 +574,7 @@ send_lookup_changes(FullDocInfos, StartSeq, Dir, Db, Fun, Acc0) ->
             {ok, FinalAcc}
     end.
 
-keep_sending_changes(Args, Acc0, FirstRound) ->
+keep_sending_changes(Args, Acc0, FirstRound, T0) ->
     #changes_args{
         feed = ResponseType,
         limit = Limit,
@@ -604,24 +605,32 @@ keep_sending_changes(Args, Acc0, FirstRound) ->
                 true ->
                     case wait_updated(Timeout, TimeoutFun, UserAcc3) of
                         {updated, UserAcc4} ->
-                            UserCtx = fabric2_db:get_user_ctx(Db),
-                            DbOptions1 = [{user_ctx, UserCtx} | DbOptions],
-                            case fabric2_db:open(fabric2_db:name(Db), DbOptions1) of
-                                {ok, Db2} ->
-                                    ?MODULE:keep_sending_changes(
-                                        Args#changes_args{limit = NewLimit},
-                                        ChangesAcc#changes_acc{
-                                            db = Db2,
-                                            user_acc = UserAcc4,
-                                            seq = EndSeq,
-                                            prepend = Prepend2,
-                                            timeout = Timeout,
-                                            timeout_fun = TimeoutFun
-                                        },
-                                        false
-                                    );
-                                _Else ->
-                                    end_sending_changes(Callback, UserAcc3, EndSeq)
+                            AccumulatedTime = timer:now_diff(os:timestamp(), T0) div 1000,
+                            Max = changes_duration(),
+                            case AccumulatedTime > Max of
+                                true ->
+                                    end_sending_changes(Callback, UserAcc4, EndSeq);
+                                false ->
+                                    UserCtx = fabric2_db:get_user_ctx(Db),
+                                    DbOptions1 = [{user_ctx, UserCtx} | DbOptions],
+                                    case fabric2_db:open(fabric2_db:name(Db), DbOptions1) of
+                                        {ok, Db2} ->
+                                            ?MODULE:keep_sending_changes(
+                                                Args#changes_args{limit = NewLimit},
+                                                ChangesAcc#changes_acc{
+                                                    db = Db2,
+                                                    user_acc = UserAcc4,
+                                                    seq = EndSeq,
+                                                    prepend = Prepend2,
+                                                    timeout = Timeout,
+                                                    timeout_fun = TimeoutFun
+                                                },
+                                                false,
+                                                T0
+                                            );
+                                        _Else ->
+                                            end_sending_changes(Callback, UserAcc3, EndSeq)
+                                    end
                             end;
                         {stop, UserAcc4} ->
                             end_sending_changes(Callback, UserAcc4, EndSeq)
@@ -629,6 +638,16 @@ keep_sending_changes(Args, Acc0, FirstRound) ->
             end
     end.
 
+
+changes_duration() ->
+    %% preserving original (3.x) configuration segment;
+    case config:get("fabric", "changes_duration", "infinity") of
+        "infinity" ->
+            infinity;
+        MaxStr ->
+            list_to_integer(MaxStr)
+    end.
+
 notify_waiting_for_updates(Callback, UserAcc) ->
     Callback(waiting_for_updates, UserAcc).
 
-- 
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