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template.go
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template.go
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// FIXME(thaJeztah): remove once we are a module; the go:build directive prevents go from downgrading language version to go1.16:
//go:build go1.19
package template
import (
"fmt"
"regexp"
"strings"
)
const (
delimiter = "\\$"
subst = "[_a-z][_a-z0-9]*(?::?[-?][^}]*)?"
)
var defaultPattern = regexp.MustCompile(fmt.Sprintf(
"%s(?i:(?P<escaped>%s)|(?P<named>%s)|{(?P<braced>%s)}|(?P<invalid>))",
delimiter, delimiter, subst, subst,
))
// DefaultSubstituteFuncs contains the default SubstituteFunc used by the docker cli
var DefaultSubstituteFuncs = []SubstituteFunc{
softDefault,
hardDefault,
requiredNonEmpty,
required,
}
// InvalidTemplateError is returned when a variable template is not in a valid
// format
type InvalidTemplateError struct {
Template string
}
func (e InvalidTemplateError) Error() string {
return fmt.Sprintf("Invalid template: %#v", e.Template)
}
// Mapping is a user-supplied function which maps from variable names to values.
// Returns the value as a string and a bool indicating whether
// the value is present, to distinguish between an empty string
// and the absence of a value.
type Mapping func(string) (string, bool)
// SubstituteFunc is a user-supplied function that apply substitution.
// Returns the value as a string, a bool indicating if the function could apply
// the substitution and an error.
type SubstituteFunc func(string, Mapping) (string, bool, error)
// SubstituteWith subsitute variables in the string with their values.
// It accepts additional substitute function.
func SubstituteWith(template string, mapping Mapping, pattern *regexp.Regexp, subsFuncs ...SubstituteFunc) (string, error) {
var err error
result := pattern.ReplaceAllStringFunc(template, func(substring string) string {
matches := pattern.FindStringSubmatch(substring)
groups := matchGroups(matches, pattern)
if escaped := groups["escaped"]; escaped != "" {
return escaped
}
substitution := groups["named"]
if substitution == "" {
substitution = groups["braced"]
}
if substitution == "" {
err = &InvalidTemplateError{Template: template}
return ""
}
for _, f := range subsFuncs {
var (
value string
applied bool
)
value, applied, err = f(substitution, mapping)
if err != nil {
return ""
}
if !applied {
continue
}
return value
}
value, _ := mapping(substitution)
return value
})
return result, err
}
// Substitute variables in the string with their values
func Substitute(template string, mapping Mapping) (string, error) {
return SubstituteWith(template, mapping, defaultPattern, 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]any, pattern *regexp.Regexp) map[string]string {
if pattern == nil {
pattern = defaultPattern
}
return recurseExtract(configDict, pattern)
}
func recurseExtract(value any, pattern *regexp.Regexp) map[string]string {
m := map[string]string{}
switch value := value.(type) {
case string:
if values, is := extractVariable(value, pattern); is {
for _, v := range values {
m[v.name] = v.value
}
}
case map[string]any:
for _, elem := range value {
submap := recurseExtract(elem, pattern)
for key, value := range submap {
m[key] = value
}
}
case []any:
for _, elem := range value {
if values, is := extractVariable(elem, pattern); is {
for _, v := range values {
m[v.name] = v.value
}
}
}
}
return m
}
type extractedValue struct {
name string
value string
}
func extractVariable(value any, pattern *regexp.Regexp) ([]extractedValue, bool) {
sValue, ok := value.(string)
if !ok {
return []extractedValue{}, false
}
matches := pattern.FindAllStringSubmatch(sValue, -1)
if len(matches) == 0 {
return []extractedValue{}, false
}
values := []extractedValue{}
for _, match := range matches {
groups := matchGroups(match, pattern)
if escaped := groups["escaped"]; escaped != "" {
continue
}
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, "-")
}
values = append(values, extractedValue{name: name, value: defaultValue})
}
return values, len(values) > 0
}
// Soft default (fall back if unset or empty)
func softDefault(substitution string, mapping Mapping) (string, bool, error) {
sep := ":-"
if !strings.Contains(substitution, sep) {
return "", false, nil
}
name, defaultValue := partition(substitution, sep)
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) {
sep := "-"
if !strings.Contains(substitution, sep) {
return "", false, nil
}
name, defaultValue := partition(substitution, sep)
value, ok := mapping(name)
if !ok {
return defaultValue, true, nil
}
return value, true, nil
}
func requiredNonEmpty(substitution string, mapping Mapping) (string, bool, error) {
return withRequired(substitution, mapping, ":?", func(v string) bool { return v != "" })
}
func required(substitution string, mapping Mapping) (string, bool, error) {
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, sep)
value, ok := mapping(name)
if !ok || !valid(value) {
return "", true, &InvalidTemplateError{
Template: fmt.Sprintf("required variable %s is missing a value: %s", name, errorMessage),
}
}
return value, true, nil
}
func matchGroups(matches []string, pattern *regexp.Regexp) map[string]string {
groups := make(map[string]string)
for i, name := range pattern.SubexpNames()[1:] {
groups[name] = matches[i+1]
}
return groups
}
// Split the string at the first occurrence of sep, and return the part before the separator,
// and the part after the separator.
//
// If the separator is not found, return the string itself, followed by an empty string.
func partition(s, sep string) (string, string) {
k, v, ok := strings.Cut(s, sep)
if !ok {
return s, ""
}
return k, v
}