sendme: Add helper functions for DATA cell delivery

When we get a relay DATA cell delivered, we have to decrement the deliver
window on both the circuit and stream level.

This commit adds helper functions to handle the deliver window decrement.

Part of #26840

Signed-off-by: David Goulet <dgoulet@torproject.org>

src/core/or/relay.c

@@ -1548,8 +1548,11 @@ connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
       return connection_exit_begin_conn(cell, circ);
     case RELAY_COMMAND_DATA:
       ++stats_n_data_cells_received;
-      if (( layer_hint && --layer_hint->deliver_window < 0) ||
-          (!layer_hint && --circ->deliver_window < 0)) {
+
+      /* Update our circuit-level deliver window that we received a DATA cell.
+       * If the deliver window goes below 0, we end the connection due to a
+       * protocol failure. */
+      if (sendme_circuit_data_received(circ, layer_hint) < 0) {
         log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
                "(relay data) circ deliver_window below 0. Killing.");
         if (conn) {
@@ -1560,9 +1563,8 @@ connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
         }
         return -END_CIRC_REASON_TORPROTOCOL;
       }
-      log_debug(domain,"circ deliver_window now %d.", layer_hint ?
-                layer_hint->deliver_window : circ->deliver_window);
 
+      /* Consider sending a circuit-level SENDME cell. */
       sendme_circuit_consider_sending(circ, layer_hint);
 
       if (rh.stream_id == 0) {
@@ -1586,7 +1588,11 @@ connection_edge_process_relay_cell(cell_t *cell, circuit_t *circ,
         return 0;
       }
 
-      if (--conn->deliver_window < 0) { /* is it below 0 after decrement? */
+      /* Update our stream-level deliver window that we just received a DATA
+       * cell. Going below 0 means we have a protocol level error so the
+       * circuit is closed. */
+
+      if (sendme_stream_data_received(conn) < 0) {
         log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
                "(relay data) conn deliver_window below 0. Killing.");
         return -END_CIRC_REASON_TORPROTOCOL;

src/core/or/sendme.c

@@ -180,3 +180,36 @@ sendme_process_stream_level(edge_connection_t *conn, circuit_t *circ,
             conn->package_window);
   return 0;
 }
+
+/* Called when a relay DATA cell is received on the given circuit. If
+ * layer_hint is NULL, this means we are the Exit end point else we are the
+ * Client. Update the deliver window and return its new value. */
+int
+sendme_circuit_data_received(circuit_t *circ, crypt_path_t *layer_hint)
+{
+  int deliver_window, domain;
+
+  if (CIRCUIT_IS_ORIGIN(circ)) {
+    tor_assert(layer_hint);
+    --layer_hint->deliver_window;
+    deliver_window = layer_hint->deliver_window;
+    domain = LD_APP;
+  } else {
+    tor_assert(!layer_hint);
+    --circ->deliver_window;
+    deliver_window = circ->deliver_window;
+    domain = LD_EXIT;
+  }
+
+  log_debug(domain, "Circuit deliver_window now %d.", deliver_window);
+  return deliver_window;
+}
+
+/* Called when a relay DATA cell is received for the given edge connection
+ * conn. Update the deliver window and return its new value. */
+int
+sendme_stream_data_received(edge_connection_t *conn)
+{
+  tor_assert(conn);
+  return --conn->deliver_window;
+}

src/core/or/sendme.h

@@ -13,13 +13,19 @@
 #include "core/or/crypt_path_st.h"
 #include "core/or/circuit_st.h"
 
+/* Sending SENDME cell. */
 void sendme_connection_edge_consider_sending(edge_connection_t *edge_conn);
 void sendme_circuit_consider_sending(circuit_t *circ,
                                      crypt_path_t *layer_hint);
 
+/* Processing SENDME cell. */
 int sendme_process_circuit_level(crypt_path_t *layer_hint,
                                  circuit_t *circ, uint16_t cell_body_len);
 int sendme_process_stream_level(edge_connection_t *conn, circuit_t *circ,
                                 uint16_t cell_body_len);
 
+/* Update deliver window functions. */
+int sendme_stream_data_received(edge_connection_t *conn);
+int sendme_circuit_data_received(circuit_t *circ, crypt_path_t *layer_hint);
+
 #endif /* !defined(TOR_SENDME_H) */