fix: raise an error if MX lookup times out during sending

This avoids potentially sending mail to the A record when an MX query times out.

closes #2833

spec/lib/dns_resolver_spec.rb

@@ -3,7 +3,7 @@
 require "rails_helper"
 
 RSpec.describe DNSResolver do
-  subject(:resolver) { described_class.new }
+  subject(:resolver) { described_class.local }
 
   # Now, we could mock everything in here which would give us some comfort
   # but I do think that we'll benefit more from having a full E2E test here
@@ -14,15 +14,52 @@ RSpec.describe DNSResolver do
     it "returns a list of IP addresses" do
       expect(resolver.a("www.test.postalserver.io").sort).to eq ["1.2.3.4", "2.3.4.5"]
     end
+
     it "resolves a domain name containing an emoji" do
       expect(resolver.a("☺.test.postalserver.io").sort).to eq ["3.4.5.6"]
     end
+
+    it "returns an empty array when timeout is exceeded" do
+      allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+      expect(resolver.a("www.test.postalserver.io")).to eq []
+    end
+
+    context "when raise_timeout_errors is true" do
+      it "returns a list of IP addresses" do
+        expect(resolver.a("www.test.postalserver.io", raise_timeout_errors: true).sort).to eq ["1.2.3.4", "2.3.4.5"]
+      end
+
+      it "raises an error when the timeout is exceeded" do
+        allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+        expect do
+          resolver.a("www.test.postalserver.io", raise_timeout_errors: true)
+        end.to raise_error(Resolv::ResolvError, /timeout/)
+      end
+    end
   end
 
   describe "#aaaa" do
     it "returns a list of IP addresses" do
       expect(resolver.aaaa("www.test.postalserver.io").sort).to eq ["2a00:67a0:a::1", "2a00:67a0:a::2"]
     end
+
+    it "returns an empty array when timeout is exceeded" do
+      allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+      expect(resolver.aaaa("www.test.postalserver.io")).to eq []
+    end
+
+    context "when raise_timeout_errors is true" do
+      it "returns a list of IP addresses" do
+        expect(resolver.aaaa("www.test.postalserver.io", raise_timeout_errors: true).sort).to eq ["2a00:67a0:a::1", "2a00:67a0:a::2"]
+      end
+
+      it "raises an error when the timeout is exceeded" do
+        allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+        expect do
+          resolver.aaaa("www.test.postalserver.io", raise_timeout_errors: true)
+        end.to raise_error(Resolv::ResolvError, /timeout/)
+      end
+    end
   end
 
   describe "#txt" do
@@ -32,12 +69,51 @@ RSpec.describe DNSResolver do
         "another example"
       ]
     end
+
+    it "returns an empty array when timeout is exceeded" do
+      allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+      expect(resolver.txt("test.postalserver.io")).to eq []
+    end
+
+    context "when raise_timeout_errors is true" do
+      it "returns a list of TXT records" do
+        expect(resolver.txt("test.postalserver.io", raise_timeout_errors: true).sort).to eq [
+          "an example txt record",
+          "another example"
+        ]
+      end
+
+      it "raises an error when the timeout is exceeded" do
+        allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+        expect do
+          resolver.txt("test.postalserver.io", raise_timeout_errors: true)
+        end.to raise_error(Resolv::ResolvError, /timeout/)
+      end
+    end
   end
 
   describe "#cname" do
     it "returns a list of CNAME records" do
       expect(resolver.cname("cname.test.postalserver.io")).to eq ["www.test.postalserver.io"]
     end
+
+    it "returns an empty array when timeout is exceeded" do
+      allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+      expect(resolver.cname("cname.test.postalserver.io")).to eq []
+    end
+
+    context "when raise_timeout_errors is true" do
+      it "returns a list of CNAME records" do
+        expect(resolver.cname("cname.test.postalserver.io", raise_timeout_errors: true)).to eq ["www.test.postalserver.io"]
+      end
+
+      it "raises an error when the timeout is exceeded" do
+        allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+        expect do
+          resolver.cname("cname.test.postalserver.io", raise_timeout_errors: true)
+        end.to raise_error(Resolv::ResolvError, /timeout/)
+      end
+    end
   end
 
   describe "#mx" do
@@ -47,6 +123,27 @@ RSpec.describe DNSResolver do
         [20, "mx2.test.postalserver.io"]
       ]
     end
+
+    it "returns an empty array when timeout is exceeded" do
+      allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+      expect(resolver.mx("test.postalserver.io")).to eq []
+    end
+
+    context "when raise_timeout_errors is true" do
+      it "returns a list of MX records" do
+        expect(resolver.mx("test.postalserver.io", raise_timeout_errors: true)).to eq [
+          [10, "mx1.test.postalserver.io"],
+          [20, "mx2.test.postalserver.io"]
+        ]
+      end
+
+      it "raises an error when the timeout is exceeded" do
+        allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+        expect do
+          resolver.mx("test.postalserver.io", raise_timeout_errors: true)
+        end.to raise_error(Resolv::ResolvError, /timeout/)
+      end
+    end
   end
 
   describe "#effective_ns" do
@@ -56,12 +153,51 @@ RSpec.describe DNSResolver do
         "the-cake-is-a-lie.katapultdns.com"
       ]
     end
+
+    it "returns an empty array when timeout is exceeded" do
+      allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+      expect(resolver.effective_ns("postalserver.io")).to eq []
+    end
+
+    context "when raise_timeout_errors is true" do
+      it "returns a list of NS records" do
+        expect(resolver.effective_ns("postalserver.io", raise_timeout_errors: true).sort).to eq [
+          "prestigious-honeybadger.katapultdns.com",
+          "the-cake-is-a-lie.katapultdns.com"
+        ]
+      end
+
+      it "raises an error when the timeout is exceeded" do
+        allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+        expect do
+          resolver.effective_ns("postalserver.io", raise_timeout_errors: true)
+        end.to raise_error(Resolv::ResolvError, /timeout/)
+      end
+    end
   end
 
   describe "#ip_to_hostname" do
     it "returns the hostname for the given IP" do
       expect(resolver.ip_to_hostname("151.252.1.100")).to eq "ns1.katapultdns.com"
     end
+
+    it "returns the IP when the timeout is exceeded" do
+      allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+      expect(resolver.ip_to_hostname("151.252.1.100")).to eq "151.252.1.100"
+    end
+
+    context "when raise_timeout_errors is true" do
+      it "returns the hostname for the given IP" do
+        expect(resolver.ip_to_hostname("151.252.1.100", raise_timeout_errors: true)).to eq "ns1.katapultdns.com"
+      end
+
+      it "raises an error when the timeout is exceeded" do
+        allow(Postal::Config.dns).to receive(:timeout).and_return(0.00001)
+        expect do
+          resolver.ip_to_hostname("151.252.1.100", raise_timeout_errors: true)
+        end.to raise_error(Resolv::ResolvError, /timeout/)
+      end
+    end
   end
 
   describe ".for_domain" do
@@ -72,9 +208,40 @@ RSpec.describe DNSResolver do
   end
 
   describe ".local" do
-    it "returns a resolver with no nameservers" do
+    after do
+      # Remove all cached values for the local resolver
+      DNSResolver.instance_variable_set(:@local, nil)
+    end
+
+    it "returns a resolver with the local machine's resolvers" do
       resolver = described_class.local
-      expect(resolver.nameservers).to be nil
+      expect(resolver.nameservers).to be_a Array
+      expect(resolver.nameservers).to_not be_empty
+    end
+
+    context "when there is no resolv.conf" do
+      it "raises an error" do
+        allow(File).to receive(:file?).with("/etc/resolv.conf").and_return(false)
+        expect { described_class.local }.to raise_error(DNSResolver::LocalResolversUnavailableError,
+                                                        /no resolver config found at/i)
+      end
+    end
+
+    context "when no nameservers are found in resolv.conf" do
+      it "raises an error" do
+        allow(Resolv::DNS::Config).to receive(:parse_resolv_conf).with("/etc/resolv.conf").and_return({})
+        expect { described_class.local }.to raise_error(DNSResolver::LocalResolversUnavailableError,
+                                                        /could not find nameservers in/i)
+      end
+    end
+
+    context "when an empty array of nameserver is found in resolv.conf" do
+      it "raises an error" do
+        allow(Resolv::DNS::Config).to receive(:parse_resolv_conf).with("/etc/resolv.conf")
+                                                                 .and_return({ nameserver: [] })
+        expect { described_class.local }.to raise_error(DNSResolver::LocalResolversUnavailableError,
+                                                        /could not find nameservers in/i)
+      end
     end
   end