From f035313b164ec60c72ac13c7c4cbcc387e1b8e27 Mon Sep 17 00:00:00 2001
From: Pavel Emelyanov <xemul@scylladb.com>
Date: Fri, 26 Nov 2021 15:29:57 +0300
Subject: [PATCH] generic_server: Gentle iterator

Add the ability to iterate over the list of connections in a "gentle"
manner, i.e. -- preempting the loop when required.

Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>
---
 generic_server.cc | 17 +++++++++++++++++
 generic_server.hh |  9 +++++++++
 2 files changed, 26 insertions(+)

diff --git a/generic_server.cc b/generic_server.cc
index 15b33bf192..e7198e1002 100644
--- a/generic_server.cc
+++ b/generic_server.cc
@@ -11,6 +11,7 @@
 #include "to_string.hh"
 
 #include <seastar/core/when_all.hh>
+#include <seastar/core/loop.hh>
 #include <seastar/core/reactor.hh>
 
 namespace generic_server {
@@ -30,10 +31,26 @@ connection::~connection()
 {
     --_server._current_connections;
     server::connections_list_t::iterator iter = _server._connections_list.iterator_to(*this);
+    for (auto&& gi : _server._gentle_iterators) {
+        if (gi.iter == iter) {
+            gi.iter++;
+        }
+    }
     _server._connections_list.erase(iter);
     _server.maybe_stop();
 }
 
+future<> server::for_each_gently(noncopyable_function<void(const connection&)> fn) {
+    _gentle_iterators.emplace_front(*this);
+    std::list<gentle_iterator>::iterator gi = _gentle_iterators.begin();
+    return seastar::do_until([ gi ] { return gi->iter == gi->end; },
+        [ gi, fn = std::move(fn) ] {
+            fn(*(gi->iter++));
+            return make_ready_future<>();
+        }
+    ).finally([ this, gi ] { _gentle_iterators.erase(gi); });
+}
+
 static bool is_broken_pipe_or_connection_reset(std::exception_ptr ep) {
     try {
         std::rethrow_exception(ep);
diff --git a/generic_server.hh b/generic_server.hh
index 3d922b1c0c..fa06ec29e0 100644
--- a/generic_server.hh
+++ b/generic_server.hh
@@ -82,6 +82,13 @@ protected:
     future<> _stopped = _all_connections_stopped.get_future();
     using connections_list_t = boost::intrusive::list<connection>;
     connections_list_t _connections_list;
+    struct gentle_iterator {
+        connections_list_t::const_iterator iter, end;
+        gentle_iterator(const server& s) : iter(s._connections_list.begin()), end(s._connections_list.end()) {}
+        gentle_iterator(const gentle_iterator&) = delete;
+        gentle_iterator(gentle_iterator&&) = delete;
+    };
+    std::list<gentle_iterator> _gentle_iterators;
     std::vector<server_socket> _listeners;
 
 public:
@@ -102,6 +109,8 @@ protected:
 
     virtual future<> unadvertise_connection(shared_ptr<connection> conn);
 
+    future<> for_each_gently(noncopyable_function<void(const connection&)>);
+
     void maybe_stop();
 };