fix(bind): interpret bind defined as type []map[string]string (#302)

As per the type definition of Layer, bind is of type Bind which is a
struct with fields Source and Dest.

But when we do UnmarshalYAML for type Bind, we are assuming that we
would always get the Bind as a []string and not
[]map[interface{}]interface{} (when making a call to
getStringOrStringSlice).

This PR fixes the getStringOrStringSlice to consider Bind defined
as a []map[string]string and returns appropriate bind string
in that case.

Fixes #301

Signed-off-by: Darshil Parikh <darshil.parikh@gmail.com>

test/binds.bats

@@ -0,0 +1,65 @@
+load helpers
+
+function setup() {
+    stacker_setup
+}
+
+function teardown() {
+    cleanup
+}
+
+@test "bind as string slice" {
+    cat > stacker.yaml <<"EOF"
+bind-test:
+    from:
+        type: oci
+        url: ${{CENTOS_OCI}}
+    binds:
+        - ${{bind_path}} -> /root/tree1/foo
+    run: |
+        touch /root/tree1/foo/bar
+EOF
+    mkdir -p tree1/foo
+
+    # since we are creating directory as
+    # real root and then `touch`-ing a file
+    # where in user NS, need to have rw persmission
+    # for others
+    chmod +666 tree1/foo
+
+    bind_path=$(realpath tree1/foo)
+
+    out=$(stacker build --substitute bind_path=${bind_path} --substitute CENTOS_OCI=$CENTOS_OCI)
+
+    [[ "${out}" =~ ^(.*filesystem bind-test built successfully)$ ]]
+
+    stat tree1/foo/bar
+}
+
+@test "bind as struct" {
+    cat > stacker.yaml <<"EOF"
+bind-test:
+    from:
+        type: oci
+        url: ${{CENTOS_OCI}}
+    binds:
+        - Source: ${{bind_path}}
+          Dest: /root/tree1/foo
+    run: |
+        touch /root/tree1/foo/bar
+EOF
+    mkdir -p tree1/foo
+
+    # since we are creating directory as
+    # real root and then `touch`-ing a file
+    # where in user NS, need to have rw persmission
+    # for others
+    chmod +666 tree1/foo
+
+    bind_path=$(realpath tree1/foo)
+
+    out=$(stacker build --substitute bind_path=$bind_path  --substitute CENTOS_OCI=$CENTOS_OCI)
+    [[ "${out}" =~ ^(.*filesystem bind-test built successfully)$ ]]
+
+    stat tree1/foo/bar
+}

types/layer.go

@@ -52,22 +52,67 @@ type OverlayDir struct {
 
 type OverlayDirs []OverlayDir
 
-func getStringOrStringSlice(iface interface{}, xform func(string) ([]string, error)) ([]string, error) {
+func validateDataAsBind(i interface{}) (map[interface{}]interface{}, error) {
+	bindMap, ok := i.(map[interface{}]interface{})
+	if !ok {
+		return nil, errors.Errorf("unable to cast into map[interface{}]interface{}: %T", i)
+	}
+
+	// validations
+	bindSource, ok := bindMap["Source"]
+	if !ok {
+		return nil, errors.Errorf("bind source missing: %v", i)
+	}
+
+	_, ok = bindSource.(string)
+	if !ok {
+		return nil, errors.Errorf("unknown bind source type, expected string: %T", i)
+	}
+
+	bindDest, ok := bindMap["Dest"]
+	if !ok {
+		return nil, errors.Errorf("bind dest missing: %v", i)
+	}
+
+	_, ok = bindDest.(string)
+	if !ok {
+		return nil, errors.Errorf("unknown bind dest type, expected string: %T", i)
+	}
+
+	if bindSource == "" || bindDest == "" {
+		return nil, errors.Errorf("empty source or dest: %v", i)
+	}
+
+	return bindMap, nil
+}
+
+func getStringOrStringSlice(data interface{}, xform func(string) ([]string, error)) ([]string, error) {
 	// The user didn't supply run: at all, so let's not do anything.
-	if iface == nil {
+	if data == nil {
 		return []string{}, nil
 	}
 
 	// This is how the yaml decoder decodes it if it's:
 	// run:
 	//     - foo
 	//     - bar
-	ifs, ok := iface.([]interface{})
+	ifs, ok := data.([]interface{})
 	if ok {
 		strs := []string{}
 		for _, i := range ifs {
-			s, ok := i.(string)
-			if !ok {
+			s := ""
+			switch v := i.(type) {
+			case string:
+				s = v
+			case interface{}:
+				bindMap, err := validateDataAsBind(i)
+				if err != nil {
+					return nil, err
+				}
+
+				// validations passed, return as string in form: source -> dest
+				s = fmt.Sprintf("%s -> %s", bindMap["Source"], bindMap["Dest"])
+			default:
 				return nil, errors.Errorf("unknown run array type: %T", i)
 			}
 
@@ -80,20 +125,20 @@ func getStringOrStringSlice(iface interface{}, xform func(string) ([]string, err
 	// run: |
 	//     echo hello world
 	//     echo goodbye cruel world
-	line, ok := iface.(string)
+	line, ok := data.(string)
 	if ok {
 		return xform(line)
 	}
 
 	// This is how it is after we do our find replace and re-set it; as a
 	// convenience (so we don't have to re-wrap it in interface{}), let's
 	// handle []string
-	strs, ok := iface.([]string)
+	strs, ok := data.([]string)
 	if ok {
 		return strs, nil
 	}
 
-	return nil, errors.Errorf("unknown directive type: %T", iface)
+	return nil, errors.Errorf("unknown directive type: %T", data)
 }
 
 // StringList allows this type to be parsed from the yaml parser as either a