Allow for variables in the configuration parser filename (#34)

* try this first

* fix typo and casing

server/config_parser.go

@@ -3,11 +3,33 @@ package server
 import (
 	"runtime"
 
-	"github.com/gammazero/workerpool"
+	"fmt"
+	"strings"
 
+	"github.com/gammazero/workerpool"
 	"github.com/pelican-dev/wings/internal/ufs"
 )
 
+// Helper function to replace variables in the file path of the configuration parser
+func replaceParserConfigPathVariables(filename string, envvars map[string]interface{}) string {
+	// Check if filename contains at least one '{' and one '}'
+	// This is here for performance as 99% of the eggs configuration parsers do not have variables in its path
+	if !strings.Contains(filename, "{") || !strings.Contains(filename, "}") {
+		return filename
+	}
+
+	// Replace "{{" with "${" and "}}" with "}"
+	filename = strings.ReplaceAll(filename, "{{", "${")
+	filename = strings.ReplaceAll(filename, "}}", "}")
+
+	// replaces ${varname} with varval
+	for varname, varval := range envvars {
+		filename = strings.ReplaceAll(filename, fmt.Sprintf("${%s}", varname), fmt.Sprint(varval))
+	}
+
+	return filename
+}
+
 // UpdateConfigurationFiles updates all the defined configuration files for
 // a server automatically to ensure that they always use the specified values.
 func (s *Server) UpdateConfigurationFiles() {
@@ -20,7 +42,8 @@ func (s *Server) UpdateConfigurationFiles() {
 		f := cf
 
 		pool.Submit(func() {
-			file, err := s.Filesystem().UnixFS().Touch(f.FileName, ufs.O_RDWR|ufs.O_CREATE, 0o644)
+			filename := replaceParserConfigPathVariables(f.FileName, s.Config().EnvVars)
+			file, err := s.Filesystem().UnixFS().Touch(filename, ufs.O_RDWR|ufs.O_CREATE, 0o644)
 			if err != nil {
 				s.Log().WithField("file_name", f.FileName).WithField("error", err).Error("failed to open file for configuration")
 				return