cli/compose/template: use lazyregexp to compile regexes on first use

This package needed an (internal) interface to abstract the lazy-regexp.
For this, I split the implementation from the exported implementation; this
also revealed that some functions are not used (at least not in our code
base), and we could consider deprecating these.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/compose/template/template.go

@@ -7,6 +7,8 @@ import (
 	"fmt"
 	"regexp"
 	"strings"
+
+	"github.com/docker/cli/internal/lazyregexp"
 )
 
 const (
@@ -14,11 +16,21 @@ const (
 	subst     = "[_a-z][_a-z0-9]*(?::?[-?][^}]*)?"
 )
 
-var defaultPattern = regexp.MustCompile(fmt.Sprintf(
+var defaultPattern = lazyregexp.New(fmt.Sprintf(
 	"%s(?i:(?P<escaped>%s)|(?P<named>%s)|{(?P<braced>%s)}|(?P<invalid>))",
 	delimiter, delimiter, subst, subst,
 ))
 
+// regexper is an internal interface to allow passing a [lazyregexp.Regexp]
+// in places where a custom ("regular") [regexp.Regexp] is accepted. It defines
+// only the methods we currently use.
+type regexper interface {
+	FindAllStringSubmatch(s string, n int) [][]string
+	FindStringSubmatch(s string) []string
+	ReplaceAllStringFunc(src string, repl func(string) string) string
+	SubexpNames() []string
+}
+
 // DefaultSubstituteFuncs contains the default SubstituteFunc used by the docker cli
 var DefaultSubstituteFuncs = []SubstituteFunc{
 	softDefault,
@@ -51,10 +63,16 @@ type SubstituteFunc func(string, Mapping) (string, bool, error)
 // SubstituteWith substitutes 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) {
+	return substituteWith(template, mapping, pattern, subsFuncs...)
+}
+
+// SubstituteWith substitutes variables in the string with their values.
+// It accepts additional substitute function.
+func substituteWith(template string, mapping Mapping, pattern regexper, subsFuncs ...SubstituteFunc) (string, error) {
 	var err error
 	result := pattern.ReplaceAllStringFunc(template, func(substring string) string {
 		matches := pattern.FindStringSubmatch(substring)
-		groups := matchGroups(matches, pattern)
+		groups := matchGroups(matches, defaultPattern)
 		if escaped := groups["escaped"]; escaped != "" {
 			return escaped
 		}
@@ -93,19 +111,23 @@ func SubstituteWith(template string, mapping Mapping, pattern *regexp.Regexp, su
 
 // Substitute variables in the string with their values
 func Substitute(template string, mapping Mapping) (string, error) {
-	return SubstituteWith(template, mapping, defaultPattern, DefaultSubstituteFuncs...)
+	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 {
+	return extractVariables(configDict, pattern)
+}
+
+func extractVariables(configDict map[string]any, pattern regexper) map[string]string {
 	if pattern == nil {
 		pattern = defaultPattern
 	}
 	return recurseExtract(configDict, pattern)
 }
 
-func recurseExtract(value any, pattern *regexp.Regexp) map[string]string {
+func recurseExtract(value any, pattern regexper) map[string]string {
 	m := map[string]string{}
 
 	switch val := value.(type) {
@@ -141,7 +163,7 @@ type extractedValue struct {
 	value string
 }
 
-func extractVariable(value any, pattern *regexp.Regexp) ([]extractedValue, bool) {
+func extractVariable(value any, pattern regexper) ([]extractedValue, bool) {
 	sValue, ok := value.(string)
 	if !ok {
 		return []extractedValue{}, false
@@ -227,7 +249,7 @@ func withRequired(substitution string, mapping Mapping, sep string, valid func(s
 	return value, true, nil
 }
 
-func matchGroups(matches []string, pattern *regexp.Regexp) map[string]string {
+func matchGroups(matches []string, pattern regexper) map[string]string {
 	groups := make(map[string]string)
 	for i, name := range pattern.SubexpNames()[1:] {
 		groups[name] = matches[i+1]

cli/compose/template/template_test.go

@@ -169,15 +169,15 @@ func TestSubstituteWithCustomFunc(t *testing.T) {
 		return value, true, nil
 	}
 
-	result, err := SubstituteWith("ok ${FOO}", defaultMapping, defaultPattern, errIsMissing)
+	result, err := substituteWith("ok ${FOO}", defaultMapping, defaultPattern, errIsMissing)
 	assert.NilError(t, err)
 	assert.Check(t, is.Equal("ok first", result))
 
-	result, err = SubstituteWith("ok ${BAR}", defaultMapping, defaultPattern, errIsMissing)
+	result, err = substituteWith("ok ${BAR}", defaultMapping, defaultPattern, errIsMissing)
 	assert.NilError(t, err)
 	assert.Check(t, is.Equal("ok ", result))
 
-	_, err = SubstituteWith("ok ${NOTHERE}", defaultMapping, defaultPattern, errIsMissing)
+	_, err = substituteWith("ok ${NOTHERE}", defaultMapping, defaultPattern, errIsMissing)
 	assert.Check(t, is.ErrorContains(err, "required variable"))
 }
 
@@ -278,7 +278,7 @@ func TestExtractVariables(t *testing.T) {
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			actual := ExtractVariables(tc.dict, defaultPattern)
+			actual := extractVariables(tc.dict, defaultPattern)
 			assert.Check(t, is.DeepEqual(actual, tc.expected))
 		})
 	}