nickva commented on a change in pull request #1176: Implement pluggable 
authentication and session support for replicator

 File path: src/couch_replicator/src/couch_replicator_auth_session.erl
 @@ -0,0 +1,545 @@
+% Licensed under the Apache License, Version 2.0 (the "License"); you may not
+% use this file except in compliance with the License. You may obtain a copy of
+% the License at
+% Unless required by applicable law or agreed to in writing, software
+% distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+% WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+% License for the specific language governing permissions and limitations under
+% the License.
+% This is the replicator session auth plugin. It implements session based
+% authentication for the replicator. The only public API are the functions from
+% the couch_replicator_auth behaviour. Most of the logic and state is in the
+% gen_server. An instance of a gen_server could be spawned for the source and
+% target endpoints of each replication jobs.
+% The workflow is roughly this:
+%  * On initialization, try to get a cookie in `refresh/1` If an error occurs,
+%    the crash. If `_session` endpoint fails with a 404 (not found), return
+%    `ignore` assuming session authentication is not support or we simply hit a
+%    non-CouchDb server.
+%  * Before each request, auth framework calls `update_headers` API function.
+%    Before updating the headers and returning, check if need to refresh again.
+%    The check looks `next_refresh` time. If that time is set (not `infinity`)
+%    and just expired, then obtain a new cookie, then update headers and
+%    return.
+%  * After each request, auth framework calls `handle_response` function. If
+%    request was successful check if a new cookie was sent by the server in the
+%    `Set-Cookie` header. If it was then then that becomes the current cookie.
+%  * If last request has an auth failure, check if request used a stale cookie
+%    In this case nothing is done, and the client is told to retry. Next time
+%    it updates its headers befor the request it should pick up the latest
+%    cookie.
+%  * If last request failed and cookie was the latest known cookie, schedule a
+%    refresh and tell client to retry. However, if the cookie was just updated,
+%    tell the client to continue such that it will handle the auth failure on
+%    its own via a set of retries with exponential backoffs. This is it to
+%    ensure if something goes wrong and one of the endpoints issues invalid
+%    cookies, replicator won't be stuck in a busy loop refreshing them.
+    initialize/1,
+    update_headers/2,
+    handle_response/4,
+    cleanup/1
+    init/1,
+    terminate/2,
+    handle_call/3,
+    handle_cast/2,
+    handle_info/2,
+    code_change/3,
+    format_status/2
+-type headers() :: [{string(), string()}].
+-type code() :: non_neg_integer().
+-type creds() :: {string() | undefined, string() | undefined}.
+% Behavior API callbacks
+-spec initialize(#httpdb{}) -> {ok, #httpdb{}, term()} | ignore.
+initialize(#httpdb{} = HttpDb) ->
+    case remove_creds(HttpDb) of
+        {ok, User, Pass, HttpDb1} ->
+            case gen_server:start_link(?MODULE, [User, Pass, HttpDb1], []) of
+                {ok, Pid} ->
+                    {ok, HttpDb1, {Pid, 0}};
+                ignore ->
+                    ignore;
+                {error, Error} ->
+                    {error, Error}
+            end;
+        {error, missing_credentials} ->
+            ignore;
+        {error, Error} ->
+            {error, Error}
+    end.
+-spec update_headers(term(), headers()) -> {headers(), term()}.
+update_headers({Pid, Epoch}, Headers) ->
+    Args = {update_headers, Headers, Epoch},
+    {Headers1, Epoch1} = gen_server:call(Pid, Args, infinity),
 Review comment:
   Because part of updating headers could be refreshing the cookie, which is an 
IO operation. Even if it calls during `process_response` gen_server could be 
talking to a session endpoint refreshing a cookie. HTTP requests inherit  the 
timeout value used in the replication doc / config settings so those will be 
bounded by that.

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