🐞 Don't panic() invoking a customized TXT

We made a mistake: we blindly invoked a function that was sometimes
`nil`. Specifically, if we had a customized domain (e.g. `ns.sslip.io`)
that didn't have a TXT record (a function), we'd try to invoke it
anyway. Bad move.

Now we ensure the function is there before we try to invoke it.

This is a curious affirmation of installing metrics: if we hadn't seen
that the server had been restarted because uptime was too low, we
wouldn't have caught this bug.

Drive-by: we made the lengths of TXT records of `version.status.sslip.io`
exactly match what we replace them with during the linking phase. We
hope that this fixes the wrong-line-numbers we see in the `panic()`
messages.

[fixes #14]

bosh-release/src/sslip.io-dns-server/integration_test.go

@@ -111,8 +111,8 @@ var _ = Describe("sslip.io-dns-server", func() {
 				`TypeSRV example.com. \? nil, SOA example.com. briancunnie.gmail.com. 2022011900 900 900 1800 180\n$`),
 			Entry(`TXT for version.status.sslip.io is the version number of the xip software (which gets overwritten during linking)`,
 				"@127.0.0.1 version.status.sslip.io txt +short",
-				`\A"dev"\n"today"\n"xxx"\n\z`,
+				`\A"0.0.0"\n"0001/01/01-99:99:99-0800"\n"cafexxx"\n\z`,
-				`TypeTXT version.status.sslip.io. \? \["dev"\], \["today"\], \["xxx"\]`),
+				`TypeTXT version.status.sslip.io. \? \["0.0.0"\], \["0001/01/01-99:99:99-0800"\], \["cafexxx"\]`),
 			Entry(`TXT is the querier's IPv4 address and the domain "ip.sslip.io"`,
 				"@127.0.0.1 ip.sslip.io txt +short",
 				`127.0.0.1`,

bosh-release/src/sslip.io-dns-server/xip/xip.go

@@ -105,9 +105,9 @@ var (
 	dkim2, _ = dnsmessage.NewName("protonmail2.domainkey.dw4gykv5i2brtkjglrf34wf6kbxpa5hgtmg2xqopinhgxn5axo73a.domains.proton.ch.")
 	dkim3, _ = dnsmessage.NewName("protonmail3.domainkey.dw4gykv5i2brtkjglrf34wf6kbxpa5hgtmg2xqopinhgxn5axo73a.domains.proton.ch.")
 
-	VersionSemantic = "dev"
+	VersionSemantic = "0.0.0"
-	VersionDate     = "today"
+	VersionDate     = "0001/01/01-99:99:99-0800"
-	VersionGitHash  = "xxx"
+	VersionGitHash  = "cafexxx"
 
 	TxtKvCustomizations = KvCustomizations{}
 	Customizations      = DomainCustomizations{
@@ -748,8 +748,10 @@ func (x Xip) TXTResources(fqdn string) ([]dnsmessage.TXTResource, error) {
 	if domain, ok := Customizations[strings.ToLower(fqdn)]; ok {
 		// Customizations[strings.ToLower(fqdn)] returns a _function_,
 		// we call that function, which has the same return signature as this method
+		if domain.TXT != nil {
 			return domain.TXT(x)
 		}
+	}
 	return nil, nil
 }
 

bosh-release/src/sslip.io-dns-server/xip/xip_test.go

@@ -161,6 +161,13 @@ var _ = Describe("Xip", func() {
 				Expect(txts[0].TXT[0]).To(MatchRegexp("^1.1.1.1$"))
 			})
 		})
+		When(`a customized domain without a TXT entry is queried`, func() {
+			It("returns no records (and doesn't panic, either)", func() {
+				txts, err := x.TXTResources("ns.sslip.io.")
+				Expect(err).To(Not(HaveOccurred()))
+				Expect(len(txts)).To(Equal(0))
+			})
+		})
 		When(`the domain "k-v.io is queried"`, func() {
 			txtTests := func() {
 				DescribeTable(`the domain "k-v.io" is queried for TXT records`,

spec/check-dns_spec.rb

@@ -109,19 +109,19 @@ describe domain do
       expect(`dig @#{whois_nameserver} TXT ip.#{domain} +short`).to match(/^"([0-9]+\.[0-9]+\.[0-9]+\.[0-9+])|(([0-9a-fA-F]*:){2,7}[0-9a-fA-F]*)"$/)
     end
 
-    it "sets a key-value @#{whois_nameserver} sslipio-spec.kv.sslip.io" do
+    it "sets a key-value @#{whois_nameserver} sslipio-spec.k-v.io" do
       expect(`dig @#{whois_nameserver} put.MyKey.sslipio-spec.kv.#{domain} TXT +short`).to match(/^"MyKey"$/)
     end
 
-    it "gets a key-value @#{whois_nameserver} sslipio-spec.kv.sslip.io" do
+    it "gets a key-value @#{whois_nameserver} sslipio-spec.k-v.io" do
       expect(`dig @#{whois_nameserver} sslipio-spec.kv.#{domain} TXT +short`).to match(/^"MyKey"$/)
     end
 
-    it "deletes a key-value @#{whois_nameserver} sslipio-spec.kv.sslip.io" do
+    it "deletes a key-value @#{whois_nameserver} sslipio-spec.k-v.io" do
       expect(`dig @#{whois_nameserver} delete.sslipio-spec.kv.#{domain} TXT +short`).to match(/^"MyKey"$/)
     end
 
-    it "gets a key-value @#{whois_nameserver} sslipio-spec.kv.sslip.io" do
+    it "gets a key-value @#{whois_nameserver} sslipio-spec.k-v.io" do
       expect(`dig @#{whois_nameserver} sslipio-spec.kv.#{domain} TXT +short`).to match(/^$/)
     end
   end