net.connect: Propagate last error message from resolvers

Previously it would only say "unable to resolve server" for all DNS
problems. While "NoError in A lookup" might not make much sense to
users, it should help in debugging more than the previous generic error.

Friendlier errors will be future work.

3 files changed, 28 insertions, 5 deletions

diff --git a/net/connect.lua b/net/connect.lua
index d52d3901..51c36c7b 100644
--- a/net/connect.lua
+++ b/net/connect.lua
@@ -35,9 +35,9 @@ local function attempt_connection(p)
 	p.target_resolver:next(function (conn_type, ip, port, extra)
 		if not conn_type then
 			-- No more targets to try
-			p:log("debug", "No more connection targets to try");
+			p:log("debug", "No more connection targets to try", p.target_resolver.last_error);
 			if p.listeners.onfail then
-				p.listeners.onfail(p.data, p.last_error or "unable to resolve service");
+				p.listeners.onfail(p.data, p.target_resolver.last_error or p.last_error or "unable to resolve service");
 			end
 			return;
 		end
diff --git a/net/resolvers/basic.lua b/net/resolvers/basic.lua
index 34f1e1c7..b62440e1 100644
--- a/net/resolvers/basic.lua
+++ b/net/resolvers/basic.lua
@@ -52,12 +52,17 @@ function methods:next(cb)
 	local dns_resolver = adns.resolver();
 
 	if not self.extra or self.extra.use_ipv4 ~= false then
-		dns_resolver:lookup(function (answer)
+		dns_resolver:lookup(function (answer, err)
 			if answer then
 				secure = secure and answer.secure;
 				for _, record in ipairs(answer) do
 					table.insert(targets, { self.conn_type.."4", record.a, self.port, self.extra });
 				end
+				if answer.status then
+					self.last_error = answer.status .. " in A lookup";
+				end
+			else
+				self.last_error = err;
 			end
 			ready();
 		end, self.hostname, "A", "IN");
@@ -66,12 +71,17 @@ function methods:next(cb)
 	end
 
 	if not self.extra or self.extra.use_ipv6 ~= false then
-		dns_resolver:lookup(function (answer)
+		dns_resolver:lookup(function (answer, err)
 			if answer then
 				secure = secure and answer.secure;
 				for _, record in ipairs(answer) do
 					table.insert(targets, { self.conn_type.."6", record.aaaa, self.port, self.extra });
 				end
+				if answer.status then
+					self.last_error = answer.status .. " in AAAA lookup";
+				end
+			else
+				self.last_error = err;
 			end
 			ready();
 		end, self.hostname, "AAAA", "IN");
@@ -80,12 +90,17 @@ function methods:next(cb)
 	end
 
 	if self.extra and self.extra.use_dane == true then
-		dns_resolver:lookup(function (answer)
+		dns_resolver:lookup(function (answer, err)
 			if answer then
 				secure = secure and answer.secure;
 				for _, record in ipairs(answer) do
 					table.insert(tlsa, record.tlsa);
 				end
+				if answer.status then
+					self.last_error = answer.status .. " in TLSA lookup";
+				end
+			else
+				self.last_error = err;
 			end
 			ready();
 		end, ("_%d._tcp.%s"):format(self.port, self.hostname), "TLSA", "IN");
diff --git a/net/resolvers/service.lua b/net/resolvers/service.lua
index 204c8a7f..4988f423 100644
--- a/net/resolvers/service.lua
+++ b/net/resolvers/service.lua
@@ -20,6 +20,9 @@ function methods:next(cb)
 			self.resolver = basic.new(unpack(next_target, 1, 4));
 		end
 		self.resolver:next(function (...)
+			if self.resolver then
+				self.last_error = self.resolver.last_error;
+			end
 			if ... == nil then
 				self.resolver = nil;
 				self:next(cb);
@@ -57,12 +60,15 @@ 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 });
+				else
+					self.last_error = "zero SRV records found";
 				end
 				ready();
 				return;
 			end
 
 			if #answer == 1 and answer[1].srv.target == "." then -- No service here
+				self.last_error = "service explicitly unavailable";
 				ready();
 				return;
 			end
@@ -71,6 +77,8 @@ function methods:next(cb)
 			for _, record in ipairs(answer) do
 				table.insert(targets, { record.srv.target, record.srv.port, self.conn_type, self.extra });
 			end
+		else
+			self.last_error = err;
 		end
 		ready();
 	end, "_" .. self.service .. "._" .. self.conn_type .. "." .. self.hostname, "SRV", "IN");