metadata: Allow simple string keys

If a file is a simple string (as opposed to a map) then write it with the
default perms. This makes for slightly terser metadata when creating a simple
datafile.

Signed-off-by: Ian Campbell <ian.campbell@docker.com>

docs/metadata.md

@@ -29,10 +29,7 @@ For example, the following userdata file:
         }
     },
     "foo" : {
-        "bar" : {
-            "perm": "0644",
-            "content": "foobar"
-        },
+        "bar" : "foobar",
         "baz" : {
             "perm": "0600",
             "content": "bar"
@@ -47,6 +44,16 @@ will generate the following files:
 /var/config/foo/baz
 ```
 
+Each file can either be:
+
+- a simple string (as for `foo/bar` above) in which case the file will
+  be created with the given contents and read/write (but not execute)
+  permissions for user and read permissions for group and everyone else (in octal format `0644`).
+- a map (as for `ssh/sshd_config` and `foo/baz` above) with the
+  following mandatory keys:
+  - `content`: the contents of the file.
+  - `perm`: the permissions to create the file with.
+
 This hierarchy can then be used by individual containers, who can bind
 mount the config sub-directory into their namespace where it is
 needed.

pkg/metadata/main.go

@@ -167,11 +167,11 @@ func processUserData(data []byte) error {
 			continue
 		}
 		for f, i := range files {
-			fi, ok := i.(map[string]interface{})
-			if !ok {
-				log.Printf("Couldn't convert JSON for items: %s", i)
-				continue
-			}
+			p := uint64(0644)
+			var c string
+
+			switch fi := i.(type) {
+			case map[string]interface{}:
 				if _, ok := fi["perm"]; !ok {
 					log.Printf("No permission provided %s:%s", f, fi)
 					continue
@@ -180,12 +180,18 @@ func processUserData(data []byte) error {
 					log.Printf("No content provided %s:%s", f, fi)
 					continue
 				}
-			c := fi["content"].(string)
-			p, err := strconv.ParseUint(fi["perm"].(string), 8, 32)
-			if err != nil {
+				c = fi["content"].(string)
+				if p, err = strconv.ParseUint(fi["perm"].(string), 8, 32); err != nil {
 					log.Printf("Failed to parse permission %s: %s", fi, err)
 					continue
 				}
+			case string:
+				c = fi
+			default:
+				log.Printf("Couldn't convert JSON for items: %s", i)
+				continue
+			}
+
 			if err := ioutil.WriteFile(path.Join(dir, f), []byte(c), os.FileMode(p)); err != nil {
 				log.Printf("Failed to write %s/%s: %s", dir, f, err)
 				continue