Increase the minimum value for the Fast flag to 4096.

Fix for 8145.

changes/bug8146_etc

@@ -1,5 +1,7 @@
   o Major bugfixes (security, directory authority):
     - When computing directory thresholds, ignore any rejected-as-sybil
       nodes during the computation so that they can't influence Fast,
-      Guard, etc.  Fixes bug 8146.
+      Guard, etc. Fixes bug 8146.
 
+    - When computing thresholds for flags, never let the threshold for
+      the Fast flag to 4096 bytes. Fixes bug 8145.

src/or/dirserv.c

@@ -1986,9 +1986,12 @@ dirserv_compute_performance_thresholds(routerlist_t *rl,
 
   {
     /* We can vote on a parameter for the minimum and maximum. */
+#define ABSOLUTE_MIN_VALUE_FOR_FAST_FLAG 4096
     int32_t min_fast, max_fast;
     min_fast = networkstatus_get_param(NULL, "FastFlagMinThreshold",
-                                       0, 0, INT32_MAX);
+      ABSOLUTE_MIN_VALUE_FOR_FAST_FLAG,
+      ABSOLUTE_MIN_VALUE_FOR_FAST_FLAG,
+      INT32_MAX);
     max_fast = networkstatus_get_param(NULL, "FastFlagMaxThreshold",
                                        INT32_MAX, min_fast, INT32_MAX);
     if (fast_bandwidth < (uint32_t)min_fast)