sfemem_iterator: fix readahead for when segments are not yet available

src/fellow_cache.c

@@ -3062,6 +3062,18 @@ fcs_state_read_needwait(enum fcos_state s)
 	}
 }
 
+/*
+ * ra[] is a ring of pointers to fcses which we (potentially)
+ * read ahead.
+ *
+ * ra[n % mod] is the current fcs, anything after that are read
+ * aheads.
+ * ran the next element to be read ahead
+ *
+ * c is a cursor tracking the current fcs
+ * rac is a cursor tracking the read ahead
+ *
+ */
 struct fellow_cache_res
 fellow_cache_obj_iter(struct fellow_cache *fc, struct fellow_cache_obj *fco,
     void *priv, objiterate_f func, int final)
@@ -3090,26 +3102,30 @@ fellow_cache_obj_iter(struct fellow_cache *fc, struct fellow_cache_obj *fco,
 
 	fcsc_init(&c, &fco->seglist);
 	rac = c;
-	(void) fcsc_next(&rac);
-	nextc = rac;
 
 	flags = final ? OBJ_ITER_FLUSH : 0;
 	flush = 0;
 	while ((fcs = fcsc_next(&c)) != NULL) {
 		if (ra[n % mod] == NULL) {
+			assert(n == ran);
+
+			fcsra = fcsc_next(&rac);
+			assert(fcsra == fcs);
+			ran++;
+
 			fellow_cache_seg_ref_in(fc, FCIO_SYNC, &fcs, 1);
 			ra[n % mod] = fcs;
 		}
 		assert(ra[n % mod] == fcs);
 
 		raion = 0;
-		while (ran < n + readahead) {
+		while (ran <= n + readahead) {
 			fcsra = fcsc_next(&rac);
 			if (fcsra == NULL)
 				break;
-			ran++;
 			raio[raion++] = fcsra;
 			TAKEZN(ra[ran % mod], fcsra);
+			ran++;
 		}
 		if (raion)
 			fellow_cache_seg_ref_in(fc, FCIO_ASYNC,
@@ -3153,6 +3169,7 @@ fellow_cache_obj_iter(struct fellow_cache *fc, struct fellow_cache_obj *fco,
 		 * derefn == mod - 1 == 0
 		 */
 		AZ(flags & OBJ_ITER_END);
+		nextc = c;
 		fcsnext = fcsc_next(&nextc);
 		flush = 0;
 		if (fcsnext == NULL || fcsnext->state == FCS_USABLE) {