Add inline support for middleware/hosts (#1072)

This fix add inline support for middleware/hosts so that
it is possible to specify hosts file insides the Corefile:
```
hosts inline example.org {
    10.0.0.1 example.org
    fallthrough
}
```

This fix fixes 999.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

plugin/hosts/README.md

@@ -11,6 +11,7 @@ available hosts files that block access to advertising servers.
 
 ~~~
 hosts [FILE [ZONES...]] {
+    [INLINE]
     fallthrough
 }
 ~~~
@@ -18,7 +19,10 @@ hosts [FILE [ZONES...]] {
 * **FILE** the hosts file to read and parse. If the path is relative the path from the *root*
   directive will be prepended to it. Defaults to /etc/hosts if omitted
 * **ZONES** zones it should be authoritative for. If empty, the zones from the configuration block
-    are used.
+   are used.
+* **INLINE** the hosts file contents inlined in Corefile. If there are any lines before fallthrough
+   then all of them will be treated as the additional content for hosts file. The specified hosts
+   file path will still be read but entries will be overrided.
 * `fallthrough` If zone matches and no record can be generated, pass request to the next plugin.
 
 ## Examples
@@ -43,3 +47,12 @@ hosts example.hosts example.org example.net {
     fallthrough
 }
 ~~~
+
+Load hosts file inlined in Corefile.
+
+~~~
+hosts example.hosts example.org {
+    10.0.0.1 example.org
+    fallthrough
+}
+~~~

plugin/hosts/hostsfile.go

@@ -53,6 +53,11 @@ type Hostsfile struct {
 	// We don't support old-classful IP address notation.
 	byAddr map[string][]string
 
+	// inline saves the hosts file is inlined in Corefile
+	// We need a copy here as we want to use inline to override
+	// the default /etc/hosts
+	inline []string
+
 	expire time.Time
 	path   string
 	mtime  time.Time
@@ -74,6 +79,10 @@ func (h *Hostsfile) ReadHosts() {
 
 	var file *os.File
 	if file, _ = os.Open(h.path); file == nil {
+		// If this is the first time then we will try to parse inline
+		if len(h.byAddr) == 0 && len(h.inline) > 0 {
+			h.Parse(nil)
+		}
 		return
 	}
 	defer file.Close()
@@ -92,7 +101,12 @@ func (h *Hostsfile) Parse(file io.Reader) {
 	hsv6 := make(map[string][]net.IP)
 	is := make(map[string][]string)
 
-	scanner := bufio.NewScanner(file)
+	var readers []io.Reader
+	if file != nil {
+		readers = append(readers, file)
+	}
+	readers = append(readers, strings.NewReader(strings.Join(h.inline, "\n")))
+	scanner := bufio.NewScanner(io.MultiReader(readers...))
 	for scanner.Scan() {
 		line := scanner.Bytes()
 		if i := bytes.Index(line, []byte{'#'}); i >= 0 {

plugin/hosts/setup.go

@@ -4,6 +4,7 @@ import (
 	"log"
 	"os"
 	"path"
+	"strings"
 
 	"github.com/coredns/coredns/core/dnsserver"
 	"github.com/coredns/coredns/plugin"
@@ -80,6 +81,11 @@ func hostsParse(c *caddy.Controller) (Hosts, error) {
 				}
 				return h, c.ArgErr()
 			default:
+				if !h.Fallthrough {
+					line := strings.Join(append([]string{c.Val()}, c.RemainingArgs()...), " ")
+					h.inline = append(h.inline, line)
+					continue
+				}
 				return h, c.Errf("unknown property '%s'", c.Val())
 			}
 		}

plugin/hosts/setup_test.go

@@ -84,3 +84,77 @@ func TestHostsParse(t *testing.T) {
 		}
 	}
 }
+
+func TestHostsInlineParse(t *testing.T) {
+	tests := []struct {
+		inputFileRules      string
+		shouldErr           bool
+		expectedbyAddr      map[string][]string
+		expectedFallthrough bool
+	}{
+		{
+			`hosts highly_unlikely_to_exist_hosts_file example.org {
+                                10.0.0.1 example.org
+                                fallthrough
+                        }`,
+			false,
+			map[string][]string{
+				`10.0.0.1`: {
+					`example.org.`,
+				},
+			},
+			true,
+		},
+		{
+			`hosts highly_unlikely_to_exist_hosts_file example.org {
+                                10.0.0.1 example.org
+                        }`,
+			false,
+			map[string][]string{
+				`10.0.0.1`: {
+					`example.org.`,
+				},
+			},
+			false,
+		},
+		{
+			`hosts highly_unlikely_to_exist_hosts_file example.org {
+                                fallthrough
+                                10.0.0.1 example.org
+                        }`,
+			true,
+			map[string][]string{},
+			true,
+		},
+	}
+
+	for i, test := range tests {
+		c := caddy.NewTestController("dns", test.inputFileRules)
+		h, err := hostsParse(c)
+
+		if err == nil && test.shouldErr {
+			t.Fatalf("Test %d expected errors, but got no error", i)
+		} else if err != nil && !test.shouldErr {
+			t.Fatalf("Test %d expected no errors, but got '%v'", i, err)
+		} else if !test.shouldErr {
+			if h.Fallthrough != test.expectedFallthrough {
+				t.Fatalf("Test %d expected fallthrough of %v, got %v", i, test.expectedFallthrough, h.Fallthrough)
+			}
+			for k, expectedVal := range test.expectedbyAddr {
+				if val, ok := h.byAddr[k]; !ok {
+					t.Fatalf("Test %d expected %v, got no entry", i, k)
+				} else {
+					if len(expectedVal) != len(val) {
+						t.Fatalf("Test %d expected %v records for %v, got %v", i, len(expectedVal), k, len(val))
+					}
+					for j := range expectedVal {
+						if expectedVal[j] != val[j] {
+							t.Fatalf("Test %d expected %v for %v, got %v", i, expectedVal[j], j, val[j])
+						}
+					}
+				}
+			}
+		}
+	}
+
+}