fix: joinUrl no longer collapses the URL scheme (#2915)

internal/templater/funcs.go

@@ -3,8 +3,8 @@ package templater
 import (
 	"maps"
 	"math/rand/v2"
+	"net/url"
 	"os"
-	"path"
 	"path/filepath"
 	"runtime"
 	"strings"
@@ -103,8 +103,13 @@ func joinEnv(elem ...string) string {
 	return strings.Join(elem, string(os.PathListSeparator))
 }
 
-func joinUrl(elem ...string) string {
-	return path.Join(elem...)
+func joinUrl(elem ...string) (string, error) {
+	if len(elem) == 0 {
+		return "", nil
+	}
+	// Use net/url.JoinPath rather than path.Join: the latter runs path.Clean,
+	// which collapses the "//" in a URL scheme (e.g. "http://" -> "http:/").
+	return url.JoinPath(elem[0], elem[1:]...)
 }
 
 func merge(base map[string]any, v ...map[string]any) map[string]any {

internal/templater/funcs_test.go

@@ -0,0 +1,31 @@
+package templater
+
+import "testing"
+
+func TestJoinUrl(t *testing.T) {
+	t.Parallel()
+
+	for _, tt := range []struct {
+		name string
+		elem []string
+		want string
+	}{
+		// The URL scheme's "//" must be preserved, not collapsed by path.Clean.
+		{"scheme preserved", []string{"http://localhost", "path1", "path2"}, "http://localhost/path1/path2"},
+		{"https with base path", []string{"https://example.com/api", "v1", "users"}, "https://example.com/api/v1/users"},
+		{"trailing slash on base", []string{"http://localhost/", "path"}, "http://localhost/path"},
+		{"single element", []string{"http://localhost"}, "http://localhost/"},
+	} {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			got, err := joinUrl(tt.elem...)
+			if err != nil {
+				t.Fatalf("joinUrl(%q) unexpected error: %v", tt.elem, err)
+			}
+			if got != tt.want {
+				t.Errorf("joinUrl(%q) = %q; want %q", tt.elem, got, tt.want)
+			}
+		})
+	}
+}