readme: more tests (#1184)

* readme: more tests

Add dnssec and file plugin to the test readme. This requires creating a
bunch of files with the right content. Doing so already unconvered an
unconditional type assertion in DNSSEC. This PR will include the fix for
that as well.

Also extended the snippets in the file plugin README, so that they are
whole Corefile - showing more value and checking all corefile snippets.

Create outliner right now is the kubernetes plugin, because even setting
the right env vars will result in:

open /var/run/secrets/kubernetes.io/serviceaccount/token: no such file or directory":

Which we can't create for a test.

* lint

test/readme_test.go

@@ -14,7 +14,25 @@ import (
 	"github.com/mholt/caddy"
 )
 
-// TestReadme parses all README.md's of the plugins and checks if every example Corefile
+// As we use the filesystem as-is, these files need to exist ON DISK for the readme test to work. This is especially
+// useful for the *file* and *dnssec* plugins as their Corefiles are now tested as well. We create files in the
+// current dir for all these, meaning the example READMEs MUST use relative path in their READMEs.
+var contents = map[string]string{
+	"Kexample.org.+013+45330.key":     examplePub,
+	"Kexample.org.+013+45330.private": examplePriv,
+	"example.org.signed":              exampleOrg, // not signed, but does not matter for this test.
+}
+
+const (
+	examplePub = `example.org. IN DNSKEY 256 3 13 eNMYFZYb6e0oJOV47IPo5f/UHy7wY9aBebotvcKakIYLyyGscBmXJQhbKLt/LhrMNDE2Q96hQnI5PdTBeOLzhQ==
+`
+	examplePriv = `Private-key-format: v1.3
+Algorithm: 13 (ECDSAP256SHA256)
+PrivateKey: f03VplaIEA+KHI9uizlemUSbUJH86hPBPjmcUninPoM=
+`
+)
+
+// TestReadme parses all README.mds of the plugins and checks if every example Corefile
 // actually works. Each corefile snippet is only used if the language is set to 'corefile':
 //
 // ~~~ corefile
@@ -27,6 +45,9 @@ func TestReadme(t *testing.T) {
 	caddy.Quiet = true
 	dnsserver.Quiet = true
 
+	create(contents)
+	defer remove(contents)
+
 	log.SetOutput(ioutil.Discard)
 
 	middle := filepath.Join("..", "plugin")
@@ -99,3 +120,15 @@ func corefileFromReadme(readme string) ([]*Input, error) {
 	}
 	return input, nil
 }
+
+func create(c map[string]string) {
+	for name, content := range c {
+		ioutil.WriteFile(name, []byte(content), 0644)
+	}
+}
+
+func remove(c map[string]string) {
+	for name := range c {
+		os.Remove(name)
+	}
+}