config: remove unknown variable elements from the config

config/raw_config.go

@@ -0,0 +1,46 @@
+package config
+
+// UnknownVariableValue is a sentinel value that can be used
+// to denote that the value of a variable is unknown at this time.
+// RawConfig uses this information to build up data about
+// unknown keys.
+const UnknownVariableValue = "74D93920-ED26-11E3-AC10-0800200C9A66"
+
+// RawConfig is a structure that holds a piece of configuration
+// where te overall structure is unknown since it will be used
+// to configure a plugin or some other similar external component.
+//
+// RawConfigs can be interpolated with variables that come from
+// other resources, user variables, etc.
+//
+// RawConfig supports a query-like interface to request
+// information from deep within the structure.
+type RawConfig struct {
+	Raw       map[string]interface{}
+	Variables map[string]InterpolatedVariable
+}
+
+// Interpolate uses the given mapping of variable values and uses
+// those as the values to replace any variables in this raw
+// configuration.
+//
+// Any prior calls to Interpolate are replaced with this one.
+//
+// If a variable key is missing, this will panic.
+func (r *RawConfig) Interpolate(map[string]string) {
+}
+
+// Config returns the entire configuration with the variables
+// interpolated from any call to Interpolate.
+//
+// If any interpolated variables are unknown (value set to
+// UnknownVariableValue), the first non-container (map, slice, etc.) element
+// will be removed from the config. The keys of unknown variables
+// can be found using the UnknownKeys function.
+//
+// By pruning out unknown keys from the configuration, the raw
+// structure will always successfully decode into its ultimate
+// structure using something like mapstructure.
+func (r *RawConfig) Config() map[string]interface{} {
+	return nil
+}

config/raw_config_test.go

@@ -0,0 +1,3 @@
+package config
+
+

config/variable.go

@@ -103,6 +103,8 @@ type variableReplaceWalker struct {
 
 	loc    reflectwalk.Location
 	m, mk  reflect.Value
+	cs     []reflect.Value
+	csData interface{}
 }
 
 func (w *variableReplaceWalker) Enter(loc reflectwalk.Location) error {
@@ -112,16 +114,24 @@ func (w *variableReplaceWalker) Enter(loc reflectwalk.Location) error {
 
 func (w *variableReplaceWalker) Exit(loc reflectwalk.Location) error {
 	w.loc = reflectwalk.None
+
+	switch loc {
+	case reflectwalk.Map:
+		w.cs = w.cs[:len(w.cs)-1]
+	}
+
 	return nil
 }
 
-func (w *variableReplaceWalker) Map(reflect.Value) error {
+func (w *variableReplaceWalker) Map(m reflect.Value) error {
+	w.cs = append(w.cs, m)
 	return nil
 }
 
 func (w *variableReplaceWalker) MapElem(m, k, v reflect.Value) error {
 	w.m = m
 	w.mk = k
+	w.csData = k
 	return nil
 }
 
@@ -157,6 +167,13 @@ func (w *variableReplaceWalker) Primitive(v reflect.Value) error {
 			panic("no value for variable key: " + key)
 		}
 
+		// If this is an unknown variable, then we remove it from
+		// the configuration.
+		if value == UnknownVariableValue {
+			w.removeCurrent()
+			return nil
+		}
+
 		// Replace
 		result = strings.Replace(result, match[0], value, -1)
 	}
@@ -173,3 +190,16 @@ func (w *variableReplaceWalker) Primitive(v reflect.Value) error {
 
 	return nil
 }
+
+func (w *variableReplaceWalker) removeCurrent() {
+	c := w.cs[len(w.cs)-1]
+	switch c.Kind() {
+	case reflect.Map:
+		// Zero value so that we delete the map key
+		var val reflect.Value
+
+		// Get the key and delete it
+		k := w.csData.(reflect.Value)
+		c.SetMapIndex(k, val)
+	}
+}

config/variable_test.go

@@ -107,6 +107,7 @@ func TestVariableReplaceWalker(t *testing.T) {
 	w := &variableReplaceWalker{
 		Values: map[string]string{
 			"var.bar":     "bar",
+			"var.unknown": UnknownVariableValue,
 		},
 	}
 
@@ -138,6 +139,33 @@ func TestVariableReplaceWalker(t *testing.T) {
 				},
 			},
 		},
+		{
+			map[string]interface{}{
+				"foo": map[string]interface{}{
+					"foo": []string{"${var.bar}"},
+				},
+			},
+			map[string]interface{}{
+				"foo": map[string]interface{}{
+					"foo": []string{"bar"},
+				},
+			},
+		},
+		{
+			map[string]interface{}{
+				"foo": "bar",
+				"bar": "hello${var.unknown}world",
+			},
+			map[string]interface{}{
+				"foo": "bar",
+			},
+		},
+		{
+			map[string]interface{}{
+				"foo": []string{"foo", "${var.unknown}", "bar"},
+			},
+			map[string]interface{}{},
+		},
 	}
 
 	for i, tc := range cases {