cmd/app: common configuration file

Generally useful for shared storage and environment.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/app/main.go

@@ -7,6 +7,7 @@ package main
 import (
 	"context"
 	"errors"
+	"io"
 	"log"
 	"os"
 	"os/exec"
@@ -134,14 +135,32 @@ func main() {
 			}
 
 			var config *hst.Config
+			var r io.Reader
 			f, err := os.Open(base.Append("app", args[0]).String())
 			if err != nil {
 				return err
 			}
-			config, err = parse(args[0], base, f)
+			r = f
+
+			var common *os.File
+			if common, err = os.Open(base.Append("common").String()); err != nil {
+				if !errors.Is(err, os.ErrNotExist) {
+					_ = f.Close()
+					return err
+				}
+			} else {
+				r = io.MultiReader(f, common)
+			}
+
+			config, err = parse(args[0], base, r)
 			if closeErr := f.Close(); err == nil {
 				err = closeErr
 			}
+			if common != nil {
+				if closeErr := common.Close(); err == nil {
+					err = closeErr
+				}
+			}
 			if err != nil {
 				return err
 			}