From d26811f5e570192b2dd5fb66d3d789b66362508c Mon Sep 17 00:00:00 2001
From: Matthew Wild <mwild1@gmail.com>
Date: Thu, 17 Mar 2022 18:20:26 +0000
Subject: net.resolvers.service: Honour record 'weight' when picking SRV
 targets

#NotHappyEyeballs
---
 net/resolvers/service.lua | 81 +++++++++++++++++++++++++++++++++++++++--------
 1 file changed, 68 insertions(+), 13 deletions(-)

(limited to 'net')

diff --git a/net/resolvers/service.lua b/net/resolvers/service.lua
index 3810cac8..a7ce76a3 100644
--- a/net/resolvers/service.lua
+++ b/net/resolvers/service.lua
@@ -2,23 +2,78 @@ local adns = require "net.adns";
 local basic = require "net.resolvers.basic";
 local inet_pton = require "util.net".pton;
 local idna_to_ascii = require "util.encodings".idna.to_ascii;
-local unpack = table.unpack or unpack; -- luacheck: ignore 113
 
 local methods = {};
 local resolver_mt = { __index = methods };
 
+local function new_target_selector(rrset)
+	local rr_count = rrset and #rrset;
+	if not rr_count or rr_count == 0 then
+		rrset = nil;
+	else
+		table.sort(rrset, function (a, b) return a.srv.priority < b.srv.priority end);
+	end
+	local rrset_pos = 1;
+	local priority_bucket, bucket_total_weight, bucket_len, bucket_used;
+	return function ()
+		if not rrset then return; end
+
+		if not priority_bucket or bucket_used >= bucket_len then
+			if rrset_pos > rr_count then return; end -- Used up all records
+
+			-- Going to start on a new priority now. Gather up all the next
+			-- records with the same priority and add them to priority_bucket
+			priority_bucket, bucket_total_weight, bucket_len, bucket_used = {}, 0, 0, 0;
+			local current_priority;
+			repeat
+				local curr_record = rrset[rrset_pos].srv;
+				if not current_priority then
+					current_priority = curr_record.priority;
+				elseif current_priority ~= curr_record.priority then
+					break;
+				end
+				table.insert(priority_bucket, curr_record);
+				bucket_total_weight = bucket_total_weight + curr_record.weight;
+				bucket_len = bucket_len + 1;
+				rrset_pos = rrset_pos + 1;
+			until rrset_pos > rr_count;
+		end
+
+		bucket_used = bucket_used + 1;
+		local n, running_total = math.random(0, bucket_total_weight), 0;
+		local target_record;
+		for i = 1, bucket_len do
+			local candidate = priority_bucket[i];
+			if candidate then
+				running_total = running_total + candidate.weight;
+				if running_total >= n then
+					target_record = candidate;
+					bucket_total_weight = bucket_total_weight - candidate.weight;
+					priority_bucket[i] = nil;
+					break;
+				end
+			end
+		end
+		return target_record;
+	end;
+end
+
 -- Find the next target to connect to, and
 -- pass it to cb()
 function methods:next(cb)
-	if self.targets then
-		if not self.resolver then
-			if #self.targets == 0 then
+	if self.resolver or self._get_next_target then
+		if not self.resolver then -- Do we have a basic resolver currently?
+			-- We don't, so fetch a new SRV target, create a new basic resolver for it
+			local next_srv_target = self._get_next_target and self._get_next_target();
+			if not next_srv_target then
+				-- No more SRV targets left
 				cb(nil);
 				return;
 			end
-			local next_target = table.remove(self.targets, 1);
-			self.resolver = basic.new(unpack(next_target, 1, 4));
+			-- Create a new basic resolver for this SRV target
+			self.resolver = basic.new(next_srv_target.target, next_srv_target.port, self.conn_type, self.extra);
 		end
+		-- Look up the next (basic) target from the current target's resolver
 		self.resolver:next(function (...)
 			if self.resolver then
 				self.last_error = self.resolver.last_error;
@@ -31,6 +86,9 @@ function methods:next(cb)
 			end
 		end);
 		return;
+	elseif self.in_progress then
+		cb(nil);
+		return;
 	end
 
 	if not self.hostname then
@@ -39,9 +97,9 @@ function methods:next(cb)
 		return;
 	end
 
-	local targets = {};
+	self.in_progress = true;
+
 	local function ready()
-		self.targets = targets;
 		self:next(cb);
 	end
 
@@ -63,7 +121,7 @@ function methods:next(cb)
 
 			if #answer == 0 then
 				if self.extra and self.extra.default_port then
-					table.insert(targets, { self.hostname, self.extra.default_port, self.conn_type, self.extra });
+					self.resolver = basic.new(self.hostname, self.extra.default_port, self.conn_type, self.extra);
 				else
 					self.last_error = "zero SRV records found";
 				end
@@ -77,10 +135,7 @@ function methods:next(cb)
 				return;
 			end
 
-			table.sort(answer, function (a, b) return a.srv.priority < b.srv.priority end);
-			for _, record in ipairs(answer) do
-				table.insert(targets, { record.srv.target, record.srv.port, self.conn_type, self.extra });
-			end
+			self._get_next_target = new_target_selector(answer);
 		else
 			self.last_error = err;
 		end
-- 
cgit v1.2.3