prop289: Add two consensus parameters

In order to be able to deploy the authenticated SENDMEs, these two consensus
parameters are needed to control the minimum version that we can emit and
accept.

See section 4 in prop289 for more details.

Note that at this commit, the functions that return the values aren't used so
compilation fails if warnings are set to errors.

Closes #26842

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

src/core/or/sendme.c

@@ -15,6 +15,39 @@
 #include "core/or/circuituse.h"
 #include "core/or/relay.h"
 #include "core/or/sendme.h"
+#include "feature/nodelist/networkstatus.h"
+
+/* The cell version constants for when emitting a cell. */
+#define SENDME_EMIT_MIN_VERSION_DEFAULT 0
+#define SENDME_EMIT_MIN_VERSION_MIN 0
+#define SENDME_EMIT_MIN_VERSION_MAX UINT8_MAX
+
+/* The cell version constants for when accepting a cell. */
+#define SENDME_ACCEPT_MIN_VERSION_DEFAULT 0
+#define SENDME_ACCEPT_MIN_VERSION_MIN 0
+#define SENDME_ACCEPT_MIN_VERSION_MAX UINT8_MAX
+
+/* Return the minimum version given by the consensus (if any) that should be
+ * used when emitting a SENDME cell. */
+static int
+get_emit_min_version(void)
+{
+  return networkstatus_get_param(NULL, "sendme_emit_min_version",
+                                 SENDME_EMIT_MIN_VERSION_DEFAULT,
+                                 SENDME_EMIT_MIN_VERSION_MIN,
+                                 SENDME_EMIT_MIN_VERSION_MAX);
+}
+
+/* Return the minimum version given by the consensus (if any) that should be
+ * accepted when receiving a SENDME cell. */
+static int
+get_accept_min_version(void)
+{
+  return networkstatus_get_param(NULL, "sendme_accept_min_version",
+                                 SENDME_ACCEPT_MIN_VERSION_DEFAULT,
+                                 SENDME_ACCEPT_MIN_VERSION_MIN,
+                                 SENDME_ACCEPT_MIN_VERSION_MAX);
+}
 
 /** Called when we've just received a relay data cell, when we've just
  * finished flushing all bytes to stream <b>conn</b>, or when we've flushed