docker stack: allow '=' separator in extra_hosts

extra_hosts in the compose file format allows '=' as a separator, and brackets
around IP addresses, the engine API doesn't.

So, transform the values when reading a compose file for 'docker stack'.

Signed-off-by: Rob Murray <rob.murray@docker.com>

cli/compose/loader/loader.go

@@ -328,7 +328,7 @@ func createTransformHook(additionalTransformers ...Transformer) mapstructure.Dec
 		reflect.TypeOf(types.MappingWithEquals{}):                transformMappingOrListFunc("=", true),
 		reflect.TypeOf(types.Labels{}):                           transformMappingOrListFunc("=", false),
 		reflect.TypeOf(types.MappingWithColon{}):                 transformMappingOrListFunc(":", false),
-		reflect.TypeOf(types.HostsList{}):                        transformListOrMappingFunc(":", false),
+		reflect.TypeOf(types.HostsList{}):                        transformHostsList,
 		reflect.TypeOf(types.ServiceVolumeConfig{}):              transformServiceVolumeConfig,
 		reflect.TypeOf(types.BuildConfig{}):                      transformBuildConfig,
 		reflect.TypeOf(types.Duration(0)):                        transformStringToDuration,
@@ -808,28 +808,58 @@ var transformStringList TransformerFunc = func(data any) (any, error) {
 	}
 }
 
-func transformMappingOrListFunc(sep string, allowNil bool) TransformerFunc {
-	return func(data any) (any, error) {
-		return transformMappingOrList(data, sep, allowNil), nil
-	}
-}
+var transformHostsList TransformerFunc = func(data any) (any, error) {
+	// Transform pairs of host and IP addr that may be represented as a map, or a list
+	// of '=' or ':' separated strings, into a list of ':' separated strings.
+	l := transformListOrMapping(data, ":", false, []string{"=", ":"})
 
-func transformListOrMappingFunc(sep string, allowNil bool) TransformerFunc {
-	return func(data any) (any, error) {
-		return transformListOrMapping(data, sep, allowNil), nil
+	// Remove brackets from IP addresses if present (for example "[::1]" -> "::1").
+	hl := l.([]string)
+	result := make([]string, 0, len(hl))
+	for _, hip := range hl {
+		host, ip, _ := strings.Cut(hip, ":")
+		if len(ip) > 2 && ip[0] == '[' && ip[len(ip)-1] == ']' {
+			ip = ip[1 : len(ip)-1]
+		}
+		result = append(result, fmt.Sprintf("%s:%s", host, ip))
 	}
+	return result, nil
 }
 
-func transformListOrMapping(listOrMapping any, sep string, allowNil bool) any {
+func transformListOrMapping(listOrMapping any, sep string, allowNil bool, allowSeps []string) any {
 	switch value := listOrMapping.(type) {
 	case map[string]any:
 		return toStringList(value, sep, allowNil)
 	case []any:
-		return listOrMapping
+		result := make([]string, 0, len(value))
+		for _, entry := range value {
+			for i, allowSep := range allowSeps {
+				entry := fmt.Sprint(entry)
+				k, v, ok := strings.Cut(entry, allowSep)
+				if ok {
+					// Entry uses this allowed separator. Add it to the result, using
+					// sep as a separator.
+					result = append(result, fmt.Sprintf("%s%s%s", k, sep, v))
+					break
+				} else if i == len(allowSeps)-1 {
+					// No more separators to try, keep the entry if allowNil.
+					if allowNil {
+						result = append(result, k)
+					}
+				}
+			}
+		}
+		return result
 	}
 	panic(errors.Errorf("expected a map or a list, got %T: %#v", listOrMapping, listOrMapping))
 }
 
+func transformMappingOrListFunc(sep string, allowNil bool) TransformerFunc {
+	return func(data any) (any, error) {
+		return transformMappingOrList(data, sep, allowNil), nil
+	}
+}
+
 func transformMappingOrList(mappingOrList any, sep string, allowNil bool) any {
 	switch values := mappingOrList.(type) {
 	case map[string]any:

cli/compose/loader/loader_test.go

@@ -1302,12 +1302,14 @@ services:
     extra_hosts:
       "zulu": "162.242.195.82"
       "alpha": "50.31.209.229"
+      "beta": "[fd20:f8a7:6e5b::2]"
       "host.docker.internal": "host-gateway"
 `)
 	assert.NilError(t, err)
 
 	expected := types.HostsList{
 		"alpha:50.31.209.229",
+		"beta:fd20:f8a7:6e5b::2",
 		"host.docker.internal:host-gateway",
 		"zulu:162.242.195.82",
 	}
@@ -1324,16 +1326,23 @@ services:
     image: busybox
     extra_hosts:
       - "zulu:162.242.195.82"
+      - "whiskey=162.242.195.83"
       - "alpha:50.31.209.229"
       - "zulu:ff02::1"
-      - "host.docker.internal:host-gateway"
+      - "whiskey=ff02::2"
+      - "foxtrot=[ff02::3]"
+      - "host.docker.internal=host-gateway"
+      - "noaddress"
 `)
 	assert.NilError(t, err)
 
 	expected := types.HostsList{
 		"zulu:162.242.195.82",
+		"whiskey:162.242.195.83",
 		"alpha:50.31.209.229",
 		"zulu:ff02::1",
+		"whiskey:ff02::2",
+		"foxtrot:ff02::3",
 		"host.docker.internal:host-gateway",
 	}