harden USB telemetry transport

main/trikke_transport.c

@@ -0,0 +1,43 @@
+#include "trikke_transport.h"
+
+static bool status_is_valid(trikke_transport_status_t status)
+{
+    return status >= TRIKKE_TRANSPORT_COMPLETE &&
+           status <= TRIKKE_TRANSPORT_FATAL;
+}
+
+void trikke_transport_sender_init(trikke_transport_sender_t *sender)
+{
+    if (sender != NULL) {
+        sender->pending = false;
+    }
+}
+
+trikke_transport_status_t trikke_transport_sender_step(
+    trikke_transport_sender_t *sender,
+    const trikke_transport_t *transport,
+    const uint8_t *packet,
+    size_t packet_size)
+{
+    if (sender == NULL || transport == NULL || transport->begin == NULL ||
+        transport->poll == NULL || packet == NULL || packet_size == 0) {
+        return TRIKKE_TRANSPORT_FATAL;
+    }
+
+    const trikke_transport_status_t status = sender->pending
+        ? transport->poll(transport->context)
+        : transport->begin(transport->context, packet, packet_size);
+    if (!status_is_valid(status)) {
+        sender->pending = false;
+        return TRIKKE_TRANSPORT_FATAL;
+    }
+
+    if (status == TRIKKE_TRANSPORT_PENDING) {
+        sender->pending = true;
+    } else {
+        // RETRY from poll is allowed only when the backend has discarded or
+        // otherwise resolved the old transfer and knows resubmission is safe.
+        sender->pending = false;
+    }
+    return status;
+}