fix: harden high-fanout delivery under load

server/db/mysql/adapter.go

@@ -4836,6 +4836,27 @@ func isDupe(err error) bool {
 	return ok && myerr.Number == 1062
 }
 
+func isInvalidConnection(err error) bool {
+	return errors.Is(err, ms.ErrInvalidConn)
+}
+
+// retryInvalidConnection retries only operations which are independently
+// idempotent. Callers must not use it for unconstrained writes.
+func retryInvalidConnection(operation string, fn func() error) error {
+	err := fn()
+	if !isInvalidConnection(err) {
+		return err
+	}
+	immetrics.IncDBInvalidConnectionRetry(operation, "attempt")
+	err = fn()
+	if err == nil || isDupe(err) {
+		immetrics.IncDBInvalidConnectionRetry(operation, "recovered")
+	} else {
+		immetrics.IncDBInvalidConnectionRetry(operation, "failed")
+	}
+	return err
+}
+
 func isMissingTable(err error) bool {
 	if err == nil {
 		return false