From 39d9efe4bd1e2249f31e5cad5c4734fd0deb2037 Mon Sep 17 00:00:00 2001
From: Tatsuhiro Tsujikawa <tatsuhiro.t@gmail.com>
Date: Thu, 4 Aug 2022 16:42:24 +0900
Subject: [PATCH] nghttpx: Randomize backend address backend robin order per
 thread (2nd Ed.)

This is the 2nd attempt to randomize backend address round robin
order.

The initial attempt of doing this was
fdcdb21c38ef7acf23853d47d1dc5bda69d5398f, which has been reverted via
470f06e5f6deab5284ffe93f15cc4ce8b16ee2ec because it broke the session
affinity.

This time, we shuffles seq fields and avoid reordering addrs vector.
---
 src/shrpx_worker.cc |  4 ++++
 src/util.h          | 20 ++++++++++++++++++++
 2 files changed, 24 insertions(+)

diff --git a/src/shrpx_worker.cc b/src/shrpx_worker.cc
index b04d19c8..2226bf43 100644
--- a/src/shrpx_worker.cc
+++ b/src/shrpx_worker.cc
@@ -360,6 +360,10 @@ void Worker::replace_downstream_config(
         addr.seq = seq++;
       }
 
+      util::shuffle(std::begin(shared_addr->addrs),
+                    std::end(shared_addr->addrs), randgen_,
+                    [](auto i, auto j) { std::swap((*i).seq, (*j).seq); });
+
       if (shared_addr->affinity.type == SessionAffinity::NONE) {
         std::map<StringRef, WeightGroup *> wgs;
         size_t num_wgs = 0;
diff --git a/src/util.h b/src/util.h
index bc271f43..f17154ec 100644
--- a/src/util.h
+++ b/src/util.h
@@ -879,6 +879,26 @@ void random_bytes(OutputIt first, OutputIt last, Generator &gen) {
   std::generate(first, last, [&dis, &gen]() { return dis(gen); });
 }
 
+// Shuffles the range [|first|, |last|] by calling swap function |fun|
+// for each pair.  |fun| takes 2 RandomIt iterators.  If |fun| is
+// noop, no modification is made.
+template <typename RandomIt, typename Generator, typename SwapFun>
+void shuffle(RandomIt first, RandomIt last, Generator &&gen, SwapFun fun) {
+  auto len = std::distance(first, last);
+  if (len < 2) {
+    return;
+  }
+
+  for (unsigned int i = 0; i < static_cast<unsigned int>(len - 1); ++i) {
+    auto dis = std::uniform_int_distribution<unsigned int>(i, len - 1);
+    auto j = dis(gen);
+    if (i == j) {
+      continue;
+    }
+    fun(first + i, first + j);
+  }
+}
+
 template <typename OutputIterator, typename CharT, size_t N>
 OutputIterator copy_lit(OutputIterator it, CharT (&s)[N]) {
   return std::copy_n(s, N - 1, it);