add feature include files into template, parsing several templates files for use of standard go 'template' function.

Author: aywan

README.md

@@ -208,6 +208,8 @@ e75a60548dc9 = 1  # a key can be either container name (nginx) or ID
 
 The templates used by docker-gen are written using the Go [text/template](http://golang.org/pkg/text/template/) language. In addition to the [built-in functions](http://golang.org/pkg/text/template/#hdr-Functions) supplied by Go, docker-gen uses [sprig](https://masterminds.github.io/sprig/) and some additional functions to make it simpler (or possible) to generate your desired output.
 
+For parsing several templates, split path with `;` (for example `template = "nginx.tmpl;header.tmpl"`). This makes possible to use go nested templates through standard `template` function.
+
 #### Emit Structure
 
 Within the templates, the object emitted by docker-gen will be a structure consisting of following Go structs:
@@ -365,6 +367,7 @@ For example, this is a JSON version of an emitted RuntimeContainer struct:
 * *`groupByKeys $containers $fieldPath`*: Returns the same as `groupBy` but only returns the keys of the map.
 * *`groupByMulti $containers $fieldPath $sep`*: Like `groupBy`, but the string value specified by `$fieldPath` is first split by `$sep` into a list of strings. A container whose `$fieldPath` value contains a list of strings will show up in the map output under each of those strings.
 * *`groupByLabel $containers $label`*: Returns the same as `groupBy` but grouping by the given label's value.
+* *`include $file`*: Returns content of `$file`, and empty string if file reading error.
 * *`intersect $slice1 $slice2`*: Returns the strings that exist in both string slices.
 * *`json $value`*: Returns the JSON representation of `$value` as a `string`.
 * *`keys $map`*: Returns the keys from `$map`. If `$map` is `nil`, a `nil` is returned. If `$map` is not a `map`, an error will be thrown.

internal/template/functions.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"io/ioutil"
 	"log"
+	"os"
 	"reflect"
 	"strings"
 )
@@ -31,6 +32,14 @@ func keys(input interface{}) (interface{}, error) {
 	return k, nil
 }
 
+func include(file string) string {
+	data, err := os.ReadFile(file)
+	if err != nil {
+		return ""
+	}
+	return string(data)
+}
+
 func intersect(l1, l2 []string) []string {
 	m := make(map[string]bool)
 	m2 := make(map[string]bool)

internal/template/functions_test.go

@@ -78,6 +78,16 @@ func TestKeysNil(t *testing.T) {
 	}
 }
 
+func TestInclude(t *testing.T) {
+	data := include("some_random_file")
+	assert.Equal(t, "", data)
+
+	_ = os.WriteFile("/tmp/docker-gen-test-temp-file", []byte("some string"), 0o777)
+	data = include("/tmp/docker-gen-test-temp-file")
+	assert.Equal(t, "some string", data)
+	_ = os.Remove("/tmp/docker-gen-test-temp-file")
+}
+
 func TestIntersect(t *testing.T) {
 	i := intersect([]string{"foo.fo.com", "bar.com"}, []string{"foo.bar.com"})
 	assert.Len(t, i, 0, "Expected no match")

internal/template/template.go

@@ -54,6 +54,7 @@ func newTemplate(name string) *template.Template {
 		"groupByMulti":           groupByMulti,
 		"groupByLabel":           groupByLabel,
 		"json":                   marshalJson,
+		"include":                include,
 		"intersect":              intersect,
 		"keys":                   keys,
 		"replace":                strings.Replace,
@@ -207,13 +208,14 @@ func GenerateFile(config config.Config, containers context.Context) bool {
 }
 
 func executeTemplate(templatePath string, containers context.Context) []byte {
-	tmpl, err := newTemplate(filepath.Base(templatePath)).ParseFiles(templatePath)
+	templatePathList := strings.Split(templatePath, ";")
+	tmpl, err := newTemplate(filepath.Base(templatePath)).ParseFiles(templatePathList...)
 	if err != nil {
 		log.Fatalf("Unable to parse template: %s", err)
 	}
 
 	buf := new(bytes.Buffer)
-	err = tmpl.ExecuteTemplate(buf, filepath.Base(templatePath), &containers)
+	err = tmpl.ExecuteTemplate(buf, filepath.Base(templatePathList[0]), &containers)
 	if err != nil {
 		log.Fatalf("Template error: %s\n", err)
 	}