Add a new `ExtractVariables` function to `compose/template` package

It allows to get easily all the variables defined in a
composefile (the `map[string]interface{}` representation that
`loader.ParseYAML` returns at least) and their default value too.

This commit also does some small function extract on substitution
funcs to reduce a tiny bit duplication.

Signed-off-by: Vincent Demeester <vincent@sbr.pm>

cli/compose/template/template.go

@@ -93,25 +93,91 @@ func Substitute(template string, mapping Mapping) (string, error) {
 	return SubstituteWith(template, mapping, pattern, DefaultSubstituteFuncs...)
 }
 
+// ExtractVariables returns a map of all the variables defined in the specified
+// composefile (dict representation) and their default value if any.
+func ExtractVariables(configDict map[string]interface{}) map[string]string {
+	return recurseExtract(configDict)
+}
+
+func recurseExtract(value interface{}) map[string]string {
+	m := map[string]string{}
+
+	switch value := value.(type) {
+	case string:
+		if v, is := extractVariable(value); is {
+			m[v.name] = v.value
+		}
+	case map[string]interface{}:
+		for _, elem := range value {
+			submap := recurseExtract(elem)
+			for key, value := range submap {
+				m[key] = value
+			}
+		}
+
+	case []interface{}:
+		for _, elem := range value {
+			if v, is := extractVariable(elem); is {
+				m[v.name] = v.value
+			}
+		}
+	}
+
+	return m
+}
+
+type extractedValue struct {
+	name  string
+	value string
+}
+
+func extractVariable(value interface{}) (extractedValue, bool) {
+	sValue, ok := value.(string)
+	if !ok {
+		return extractedValue{}, false
+	}
+	matches := pattern.FindStringSubmatch(sValue)
+	if len(matches) == 0 {
+		return extractedValue{}, false
+	}
+	groups := matchGroups(matches)
+	if escaped := groups["escaped"]; escaped != "" {
+		return extractedValue{}, false
+	}
+	val := groups["named"]
+	if val == "" {
+		val = groups["braced"]
+	}
+	name := val
+	var defaultValue string
+	switch {
+	case strings.Contains(val, ":?"):
+		name, _ = partition(val, ":?")
+	case strings.Contains(val, "?"):
+		name, _ = partition(val, "?")
+	case strings.Contains(val, ":-"):
+		name, defaultValue = partition(val, ":-")
+	case strings.Contains(val, "-"):
+		name, defaultValue = partition(val, "-")
+	}
+	return extractedValue{name: name, value: defaultValue}, true
+}
+
 // Soft default (fall back if unset or empty)
 func softDefault(substitution string, mapping Mapping) (string, bool, error) {
-	if !strings.Contains(substitution, ":-") {
+	return withDefault(substitution, mapping, "-:")
-		return "", false, nil
-	}
-	name, defaultValue := partition(substitution, ":-")
-	value, ok := mapping(name)
-	if !ok || value == "" {
-		return defaultValue, true, nil
-	}
-	return value, true, nil
 }
 
 // Hard default (fall back if-and-only-if empty)
 func hardDefault(substitution string, mapping Mapping) (string, bool, error) {
-	if !strings.Contains(substitution, "-") {
+	return withDefault(substitution, mapping, "-")
+}
+
+func withDefault(substitution string, mapping Mapping, sep string) (string, bool, error) {
+	if !strings.Contains(substitution, sep) {
 		return "", false, nil
 	}
-	name, defaultValue := partition(substitution, "-")
+	name, defaultValue := partition(substitution, sep)
 	value, ok := mapping(name)
 	if !ok {
 		return defaultValue, true, nil
@@ -120,26 +186,20 @@ func hardDefault(substitution string, mapping Mapping) (string, bool, error) {
 }
 
 func requiredNonEmpty(substitution string, mapping Mapping) (string, bool, error) {
-	if !strings.Contains(substitution, ":?") {
+	return withRequired(substitution, mapping, ":?", func(v string) bool { return v != "" })
-		return "", false, nil
-	}
-	name, errorMessage := partition(substitution, ":?")
-	value, ok := mapping(name)
-	if !ok || value == "" {
-		return "", true, &InvalidTemplateError{
-			Template: fmt.Sprintf("required variable %s is missing a value: %s", name, errorMessage),
-		}
-	}
-	return value, true, nil
 }
 
 func required(substitution string, mapping Mapping) (string, bool, error) {
-	if !strings.Contains(substitution, "?") {
+	return withRequired(substitution, mapping, "?", func(_ string) bool { return true })
+}
+
+func withRequired(substitution string, mapping Mapping, sep string, valid func(string) bool) (string, bool, error) {
+	if !strings.Contains(substitution, sep) {
 		return "", false, nil
 	}
-	name, errorMessage := partition(substitution, "?")
+	name, errorMessage := partition(substitution, sep)
 	value, ok := mapping(name)
-	if !ok {
+	if !ok || !valid(value) {
 		return "", true, &InvalidTemplateError{
 			Template: fmt.Sprintf("required variable %s is missing a value: %s", name, errorMessage),
 		}

cli/compose/template/template_test.go

@@ -172,3 +172,91 @@ func TestSubstituteWithCustomFunc(t *testing.T) {
 	_, err = SubstituteWith("ok ${NOTHERE}", defaultMapping, pattern, errIsMissing)
 	assert.Check(t, is.ErrorContains(err, "required variable"))
 }
+
+func TestExtractVariables(t *testing.T) {
+	testCases := []struct {
+		dict     map[string]interface{}
+		expected map[string]string
+	}{
+		{
+			dict:     map[string]interface{}{},
+			expected: map[string]string{},
+		},
+		{
+			dict: map[string]interface{}{
+				"foo": "bar",
+			},
+			expected: map[string]string{},
+		},
+		{
+			dict: map[string]interface{}{
+				"foo": "$bar",
+			},
+			expected: map[string]string{
+				"bar": "",
+			},
+		},
+		{
+			dict: map[string]interface{}{
+				"foo": "${bar}",
+			},
+			expected: map[string]string{
+				"bar": "",
+			},
+		},
+		{
+			dict: map[string]interface{}{
+				"foo": "${bar?:foo}",
+			},
+			expected: map[string]string{
+				"bar": "",
+			},
+		},
+		{
+			dict: map[string]interface{}{
+				"foo": "${bar?foo}",
+			},
+			expected: map[string]string{
+				"bar": "",
+			},
+		},
+		{
+			dict: map[string]interface{}{
+				"foo": "${bar:-foo}",
+			},
+			expected: map[string]string{
+				"bar": "foo",
+			},
+		},
+		{
+			dict: map[string]interface{}{
+				"foo": "${bar-foo}",
+			},
+			expected: map[string]string{
+				"bar": "foo",
+			},
+		},
+		{
+			dict: map[string]interface{}{
+				"foo": "${bar:-foo}",
+				"bar": map[string]interface{}{
+					"foo": "${fruit:-banana}",
+					"bar": "vegetable",
+				},
+				"baz": []interface{}{
+					"foo",
+					"$toto",
+				},
+			},
+			expected: map[string]string{
+				"bar":   "foo",
+				"fruit": "banana",
+				"toto":  "",
+			},
+		},
+	}
+	for _, tc := range testCases {
+		actual := ExtractVariables(tc.dict)
+		assert.Check(t, is.DeepEqual(actual, tc.expected))
+	}
+}