rnewson commented on a change in pull request #1176: Implement pluggable 
authentication and session support for replicator
URL: https://github.com/apache/couchdb/pull/1176#discussion_r170285923
 
 

 ##########
 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
+%
+%   http://www.apache.org/licenses/LICENSE-2.0
+%
+% 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.
+
+
+-module(couch_replicator_auth_session).
+
+
+-behaviour(couch_replicator_auth).
+-behaviour(gen_server).
+
+
+-export([
+    initialize/1,
+    update_headers/2,
+    handle_response/4,
+    cleanup/1
+]).
+
+-export([
+    init/1,
+    terminate/2,
+    handle_call/3,
+    handle_cast/2,
+    handle_info/2,
+    code_change/3,
+    format_status/2
+]).
+
+
+-include_lib("ibrowse/include/ibrowse.hrl").
+-include_lib("couch_replicator/include/couch_replicator_api_wrap.hrl").
+
+
+-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),
+    {Headers1, {Pid, Epoch1}}.
+
+
+-spec handle_response(term(), code(), headers(), term()) ->
+    {continue | retry, term()}.
+handle_response({Pid, Epoch}, Code, Headers, Body) ->
+    Args =  {handle_response, Code, Headers, Body, Epoch},
+    {Retry, Epoch1} = gen_server:call(Pid, Args, infinity),
+    {Retry, {Pid, Epoch1}}.
+
+
+-spec cleanup(term()) -> ok.
+cleanup({Pid, _Epoch}) ->
+    gen_server:call(Pid, stop, infinity).
+
+
+%% Definitions
+
+-define(MIN_UPDATE_INTERVAL, 5).
+
+
+%% gen_server state
+
+-record(state, {
+    epoch = 0 :: non_neg_integer(),
+    cookie :: string() | undefined,
+    user :: string() | undefined,
+    pass :: string() | undefined,
+    httpdb_timeout :: integer(),
+    httpdb_pool :: pid(),
+    httpdb_ibrowse_options = [] :: list(),
+    session_url :: string(),
+    next_refresh = infinity :: infinity |  non_neg_integer(),
+    refresh_tstamp = 0 :: non_neg_integer()
+}).
+
+
+%% gen_server functions
+
+init([User, Pass, HttpDb]) ->
+    State = #state{
+        user = User,
+        pass = Pass,
+        session_url = get_session_url(HttpDb#httpdb.url),
+        httpdb_pool = HttpDb#httpdb.httpc_pool,
+        httpdb_timeout = HttpDb#httpdb.timeout,
+        httpdb_ibrowse_options = HttpDb#httpdb.ibrowse_options
+    },
+    case refresh(State) of
+        {ok, UpdatedState} ->
+            {ok, UpdatedState};
+        {error, {session_not_supported, _, _}} ->
+            ignore;
+        {error, Error} ->
+            {stop, Error}
+    end.
+
+
+terminate(_Reason, _State) ->
+    ok.
+
+
+handle_call({update_headers, Headers, _Epoch}, _From, State) ->
+    case maybe_refresh(State) of
+        {ok, State1} ->
+            Cookie = "AuthSession=" ++ State1#state.cookie,
+            Headers1 = [{"Cookie", Cookie} | Headers],
+            {reply, {Headers1, State1#state.epoch}, State1};
+        {error, Error} ->
+            LogMsg = "~p: Stopping session auth plugin because of error ~p",
+            couch_log:error(LogMsg, [?MODULE, Error]),
+            {stop, Error, State}
+    end;
+
+handle_call({handle_response, Code, Headers, _, Epoch}, _From, State) ->
+    {Retry, State1} = process_response(Code, Headers, Epoch, State),
+    {reply, {Retry, State1#state.epoch}, State1};
+
+handle_call(stop, _From, State) ->
+    {stop, normal, ok, State}.
+
+
+handle_cast(Msg, State) ->
+    couch_log:error("~p: Received un-expected cast ~p", [?MODULE, Msg]),
+    {noreply, State}.
+
+
+handle_info(Msg, State) ->
+    couch_log:error("~p : Received un-expected message ~p", [?MODULE, Msg]),
+    {noreply, State}.
+
+
+code_change(_OldVsn, State, _Extra) ->
+    {ok, State}.
+
+
+format_status(_Opt, [_PDict, State]) ->
+    [
+        {epoch, State#state.epoch},
+        {user, State#state.user},
+        {session_url, State#state.session_url},
+        {refresh_tstamp, State#state.refresh_tstamp}
+    ].
+
+
+%% Private helper functions
+
+-spec remove_creds(#httpdb{}) ->
+    {ok, string(), string(), #httpdb{}} | {error, term()}.
+remove_creds(#httpdb{url = Url, headers = Headers} = HttpDb) ->
+    {{HeadersUser, HeadersPass}, HeadersNoCreds} =
+            couch_replicator_utils:remove_basic_auth_from_headers(Headers),
+    case remove_creds_from_url(Url) of
+        {ok, UrlUser, UrlPass, UrlNoCreds} ->
+            case pick_creds({UrlUser, UrlPass}, {HeadersUser, HeadersPass}) of
+                {ok, User, Pass} ->
+                    HttpDb1 = HttpDb#httpdb{
+                        url = UrlNoCreds,
+                        headers = HeadersNoCreds
+                    },
+                    {ok, User, Pass, HttpDb1};
+                {error, Error} ->
+                    {error, Error}
+            end;
+        {error, Error} ->
+            {error, Error}
+    end.
+
+
+% Credentials could be specified in the url and/or in the headers.
+%  * If no credentials specified return error.
+%  * If specified in url but not in headers, pick url creds.
+%  * Otherwise pick headers creds.
+%
+-spec pick_creds(creds(), creds()) ->
+    {ok, string(), string()} | {error, missing_credentials}.
+pick_creds({undefined, _}, {undefined, _}) ->
+    {error, missing_credentials};
+pick_creds({UrlUser, UrlPass}, {undefined, _}) ->
+    {ok, UrlUser, UrlPass};
+pick_creds({_, _}, {HeadersUser, HeadersPass}) ->
+    {ok, HeadersUser, HeadersPass}.
+
+
+-spec remove_creds_from_url(string()) ->
+    {ok, string() | undefined, string() | undefined, string()} |
+    {error, term()}.
+remove_creds_from_url(Url) ->
+    case ibrowse_lib:parse_url(Url) of
+        {error, Error} ->
+            {error, Error};
+        #url{username = undefined, password = undefined} ->
+            {ok, undefined, undefined, Url};
+        #url{protocol = Proto, username = User, password = Pass} ->
+            % Excise user and pass parts from the url. Try to keep the host,
+            % port and path as they were in the original.
+            Prefix = lists:concat([Proto, "://", User, ":", Pass, "@"]),
+            Suffix = lists:sublist(Url, length(Prefix) + 1, length(Url) + 1),
+            NoCreds = lists:concat([Proto, "://", Suffix]),
 
 Review comment:
   ah, bleh. ok.

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