util/flow-rate: fix sum calc on index next to base

When the buffer is wrapped around, for any new index, the calculation
must subtract the previous value stored in the buffer. So far, the code
ended up adding to the existing buffer value on the index unless it was
the first index after wrapping around. This is incorrect and would end
up flagging a flow as elephant a lot before than it should be.

Harden the Test06 by checking for such a case.

Bug 7694

src/util-flow-rate.c

@@ -157,9 +157,8 @@ static inline void FlowRateStoreUpdateCurrentRing(
         frs->dir[direction].buf[idx] += pkt_len;
         /* Update the total sum */
         frs->dir[direction].sum += pkt_len;
-    } else if ((idx == frs->dir[direction].last_idx) || (idx == frs->dir[direction].last_idx + 1)) {
-        /* Index matches the last updated index in the ring buffer or is the next index in the
-         * buffer */
+    } else if (idx == frs->dir[direction].last_idx) {
+        /* Index matches the last updated index in the ring buffer */
         /* Add to the existing open time interval */
         frs->dir[direction].buf[idx] += pkt_len;
         /* Update the total sum */
@@ -172,7 +171,10 @@ static inline void FlowRateStoreUpdateCurrentRing(
         DEBUG_VALIDATE_BUG_ON(frs->dir[direction].sum < prev_byte_count);
         /* Sum should get rid of previous count on the same index */
         frs->dir[direction].sum += pkt_len - prev_byte_count;
-        frs->dir[direction].start_ts = p_ts;
+        if (idx != frs->dir[direction].last_idx + 1) {
+            /* Revisited index but not the next to last, so, reset start_ts */
+            frs->dir[direction].start_ts = p_ts;
+        }
     }
     frs->dir[direction].last_idx = idx;
 }
@@ -373,6 +375,15 @@ static int FlowRateTest03(void)
     FAIL_IF(frs->dir[0].start_ts.secs != p5->ts.secs);
     FAIL_IF(frs->dir[0].buf[0] != 43);
 
+    Packet *p6 = UTHBuildPacket((uint8_t *)"meerkat", 7, IPPROTO_TCP);
+    p6->ts.secs = p1->ts.secs + 5;
+    FlowRateStoreUpdate(frs, p6->ts, GET_PKT_LEN(p6), TOSERVER);
+    /* Total length of packet is 47 */
+    FAIL_IF(frs->dir[0].sum != 183);
+    FAIL_IF(frs->dir[0].last_ts.secs != p6->ts.secs);
+    FAIL_IF(frs->dir[0].start_ts.secs != p5->ts.secs);
+    FAIL_IF(frs->dir[0].buf[1] != 47);
+
     FlowRateStoreFree(frs);
     PASS;
 }