enhance: fall back to DEFAULT_TEMPLATE.md when style-specific template is missing (#38803)

Closes #38801

Introduce `DEFAULT_TEMPLATE.md` as the default message for all merge
styles.

https://gitea.com/gitea/docs/pulls/491

By the way, fix incorrect os.Expand usage for merge message & repo
template

---------

Co-authored-by: waterWang <waterWang@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

modules/templates/vars/vars.go

@@ -5,14 +5,17 @@ package vars
 
 import (
 	"fmt"
+	"regexp"
 	"strings"
+	"sync"
 	"unicode"
 	"unicode/utf8"
 )
 
-// Expand replaces all variables like {var} by `vars` map, it always returns the expanded string regardless of errors
-// if error occurs, the error part doesn't change and is returned as it is.
-func Expand(template string, vars map[string]string) (string, error) {
+// ExpandCurlyBrace replaces all variables like {var} by `vars` map,
+// it always returns the expanded string regardless of errors.
+// if error occurs (wrong syntax, missing variable), the error part doesn't change and is returned as it is.
+func ExpandCurlyBrace(template string, vars map[string]string) (string, error) {
 	// in the future, if necessary, we can introduce some escape-char,
 	// for example: it will use `#' as a reversed char, templates will use `{#{}` to do escape and output char '{'.
 	var buf strings.Builder
@@ -71,3 +74,26 @@ func Expand(template string, vars map[string]string) (string, error) {
 
 	return buf.String(), err
 }
+
+var globalVars = sync.OnceValue(func() (ret struct {
+	regexpShellLike *regexp.Regexp
+},
+) {
+	ret.regexpShellLike = regexp.MustCompile(`(\$\{[a-zA-Z_]\w*\}|\$[a-zA-Z_]\w*)`)
+	return ret
+})
+
+// ExpandShellLike works like os.Expand, the difference is that this function keeps the non-existing keys
+func ExpandShellLike(template string, vars map[string]string) string {
+	re := globalVars().regexpShellLike
+	return re.ReplaceAllStringFunc(template, func(s string) string {
+		key := s[1:]
+		if strings.HasPrefix(key, "{") && strings.HasSuffix(key, "}") {
+			key = key[1 : len(key)-1]
+		}
+		if val, ok := vars[key]; ok {
+			return val
+		}
+		return s
+	})
+}

modules/templates/vars/vars_test.go

@@ -9,8 +9,8 @@ import (
 	"github.com/stretchr/testify/assert"
 )
 
-func TestExpandVars(t *testing.T) {
-	kases := []struct {
+func TestExpandCurlyBrace(t *testing.T) {
+	cases := []struct {
 		tmpl  string
 		data  map[string]string
 		out   string
@@ -57,11 +57,11 @@ func TestExpandVars(t *testing.T) {
 		},
 	}
 
-	for _, kase := range kases {
-		t.Run(kase.tmpl, func(t *testing.T) {
-			res, err := Expand(kase.tmpl, kase.data)
-			assert.Equal(t, kase.out, res)
-			if kase.error {
+	for _, c := range cases {
+		t.Run(c.tmpl, func(t *testing.T) {
+			res, err := ExpandCurlyBrace(c.tmpl, c.data)
+			assert.Equal(t, c.out, res)
+			if c.error {
 				assert.Error(t, err)
 			} else {
 				assert.NoError(t, err)
@@ -69,3 +69,19 @@ func TestExpandVars(t *testing.T) {
 		})
 	}
 }
+
+func TestExpandShellLike(t *testing.T) {
+	cases := []struct {
+		tmpl string
+		data map[string]string
+		out  string
+	}{
+		{tmpl: "$key ${key} $other ${other}", data: map[string]string{"key": "val"}, out: "val val $other ${other}"},
+		{tmpl: "$ key ${key }", data: map[string]string{"key": "val"}, out: "$ key ${key }"},
+	}
+
+	for _, c := range cases {
+		out := ExpandShellLike(c.tmpl, c.data)
+		assert.Equal(t, c.out, out, "tmpl: %s", c.tmpl)
+	}
+}