fix: docker-gen and sprig coalesce are not identical

Author: buchdag

README.md

@@ -384,6 +384,7 @@ For example, this is a JSON version of an emitted RuntimeContainer struct:
 - [Functions from Go](https://pkg.go.dev/text/template#hdr-Functions)
 - [Functions from Sprig v3](https://masterminds.github.io/sprig/), except for those that have the same name as one of the following functions.
 - _`closest $array $value`_: Returns the longest matching substring in `$array` that matches `$value`
+- _`coalesce ...`_: Returns the first non-nil argument.
 - _`comment $delimiter $string`_: Returns `$string` with each line prefixed by `$delimiter` (helpful for debugging combined with Sprig `toPrettyJson`: `{{ toPrettyJson $ | comment "#" }}`).
 - _`contains $map $key`_: Returns `true` if `$map` contains `$key`. Takes maps from `string` to any type.
 - _`dir $path`_: Returns an array of filenames in the specified `$path`.
@@ -417,6 +418,7 @@ For example, this is a JSON version of an emitted RuntimeContainer struct:
 
 Sprig functions that have the same name as docker-gen function (but different behaviour) are made available with the `sprig` prefix:
 
+- _`sprigCoalesce ...`_: Alias for Sprig's [`coalesce`](https://masterminds.github.io/sprig/defaults.html).
 - _`sprigContains $string $string`_: Alias for Sprig's [`contains`](https://masterminds.github.io/sprig/strings.html).
 - _`sprigDir $path`_: Alias for Sprig's [`dir`](https://masterminds.github.io/sprig/paths.html).
 - _`sprigReplace $old $new $string`_: Alias for Sprig's [`replace`](https://masterminds.github.io/sprig/strings.html).

internal/template/functions.go

@@ -103,6 +103,16 @@ func dirList(path string) ([]string, error) {
 	return names, nil
 }
 
+// coalesce returns the first non nil argument
+func coalesce(input ...interface{}) interface{} {
+	for _, v := range input {
+		if v != nil {
+			return v
+		}
+	}
+	return nil
+}
+
 // when returns the trueValue when the condition is true and the falseValue otherwise
 func when(condition bool, trueValue, falseValue interface{}) interface{} {
 	if condition {

internal/template/functions_test.go

@@ -236,3 +236,11 @@ func TestDirList(t *testing.T) {
 	filesList, _ = dirList("/wrong/path")
 	assert.Equal(t, []string{}, filesList)
 }
+
+func TestCoalesce(t *testing.T) {
+	v := coalesce(nil, "second", "third")
+	assert.Equal(t, "second", v, "Expected second value")
+
+	v = coalesce(nil, nil, nil)
+	assert.Nil(t, v, "Expected nil value")
+}

internal/template/template.go

@@ -60,8 +60,9 @@ func newTemplate(name string) *template.Template {
 
 	sprigFuncMap := sprig.TxtFuncMap()
 
-	return tmpl.Funcs(sprigFuncMap).Funcs(template.FuncMap{
+	tmpl.Funcs(sprigFuncMap).Funcs(template.FuncMap{
 		"closest":                 arrayClosest,
+		"coalesce":                coalesce,
 		"comment":                 comment,
 		"contains":                contains,
 		"dir":                     dirList,
@@ -108,12 +109,15 @@ func newTemplate(name string) *template.Template {
 		"sha1":      sprigFuncMap["sha1sum"],
 
 		// aliases to sprig template functions masked by docker-gen functions with the same name
+		"sprigCoalesce": sprigFuncMap["coalesce"],
 		"sprigContains": sprigFuncMap["contains"],
 		"sprigDir":      sprigFuncMap["dir"],
 		"sprigReplace":  sprigFuncMap["replace"],
 		"sprigSplit":    sprigFuncMap["split"],
 		"sprigSplitn":   sprigFuncMap["splitn"],
 	})
+
+	return tmpl
 }
 
 func isBlank(str string) bool {