review: rebuild a file the mesh once wrote whole, keep links, give back a missing line end, and release a hold only after the write (hq issue 128)

internal/declaration/declaration.go

@@ -169,6 +169,9 @@ type File struct {
 	// the operator all write into it, and the mesh writing it whole took their lines away at the
 	// next change to the mesh's names, silently. Marked blocks are the shape the other tools in
 	// that file already use, and `#` is the comment character of every file this serves.
+	//
+	// Written into, in either format, the file's mode and owner are the machine's: a declared mode
+	// and owner apply only to a file the host creates, and a file that was there keeps its own.
 	Into string `json:"into,omitempty"`
 
 	// At is where a file written into a block has its region added when the file does not hold
@@ -272,10 +275,15 @@ func (f *File) validate(where string, _ bool) []string {
 				break
 			}
 		}
-		// The id is written into the markers, so it has to stay on one line.
+		// The id is written into the markers, so it has to stay on one line — and whitespace at
+		// either end of it is whitespace the host would have to match exactly in a line some
+		// editor may trim.
 		if strings.ContainsAny(f.ID, "\r\n") {
 			problems = append(problems, where+
 				": a file written into a block names its region by its id, and this id spans lines")
+		} else if strings.TrimSpace(f.ID) != f.ID {
+			problems = append(problems, where+
+				": a file written into a block names its region by its id, and this id begins or ends in whitespace")
 		}
 	default:
 		problems = append(problems, fmt.Sprintf(