Validate max_retries on both target forms (closes #221)

internal/handlers/target_edit.go

@@ -133,20 +133,28 @@ func (h *Handlers) applyTargetEdit(
 		return
 	}
 
-	target.Name = name
-	target.Config = configJSON
-
 	// Retries are offered only by the forms for target types that
 	// retry, so an absent field means "this form does not edit
 	// retries" rather than "set them to zero". Reading it
 	// unconditionally would silently disable retries on any target
 	// saved from a form that does not render the input.
+	//
+	// A field that IS submitted but does not parse is a 400, through
+	// the same validator the create path uses. It is rejected before
+	// anything is written, so a typo cannot destroy the retry count
+	// the target is already delivering with.
 	if r.PostForm.Has("max_retries") {
-		target.MaxRetries = parseNonNegativeInt(
-			r.PostFormValue("max_retries"),
-		)
+		retries, ok := targetMaxRetries(w, r, target.MaxRetries)
+		if !ok {
+			return
+		}
+
+		target.MaxRetries = retries
 	}
 
+	target.Name = name
+	target.Config = configJSON
+
 	err = h.db.DB().Save(target).Error
 	if err != nil {
 		h.serverError(w, "failed to update target", err)