dir2config: introduce #!gen

cmd/dkl-dir2config/assemble.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"log"
 	"os"
+	"os/exec"
 	"strings"
 
 	"gopkg.in/yaml.v2"
@@ -116,9 +117,26 @@ func eachFragment(path string, searchList []FS, walk func(io.Reader) error) (err
 			continue
 		}
 
+		genCmd, found := strings.CutPrefix(line, "#!gen ")
+		if found {
+			cmdArgs := strings.Fields(genCmd)
+			if Debug {
+				log.Print("#!gen ", cmdArgs)
+			}
+
+			cmd := "gen/" + cmdArgs[0]
+			args := cmdArgs[1:]
+			genOutput, err := exec.Command(cmd, args...).Output()
+			if err != nil {
+				return fmt.Errorf("gen %v: %w", cmdArgs, err)
+			}
+			walk(bytes.NewBuffer(genOutput))
+			continue
+		}
+
 		includePath, found := strings.CutPrefix(line, "#!include ")
 		if !found {
-			continue // or break?
+			continue
 		}
 
 		includePath = strings.TrimSpace(includePath)
@@ -127,8 +145,7 @@ func eachFragment(path string, searchList []FS, walk func(io.Reader) error) (err
 		}
 		err = eachFragment(includePath, searchList, walk)
 		if err != nil {
-			err = fmt.Errorf("include %q: %w", includePath, err)
+			return fmt.Errorf("include %q: %w", includePath, err)
-			return
 		}
 	}