Add DBG() to fcsc_next()

... to make it easier to follow the code in fellow_cache_test

motivated by #35

src/fellow_cache.c

@@ -405,6 +405,10 @@ fcsc_next(struct fcscursor *c)
 	return (&c->fcsl->segs[c->u++]);
 }
 
+#define FCSC_NEXT(c) (							\
+	    DBG("fcsc " #c " %p u=%u nsegs=%u next=%p", c, (c)->u,	\
+	    (c)->fdsl->nsegs, (c)->fcsl->next), fcsc_next(c))
+
 /*
  * fdo attribues we want to keep our own enum and, consequently, bitfield
  * stable.  so we define the enum manually and then generate code from the table
@@ -1460,7 +1464,7 @@ fellow_seglist_regions(const struct fellow_fd *ffd,
 
 	fcsc_init(&c, fcsl);
 
-	while ((fcs = fcsc_next(&c)) != NULL)
+	while ((fcs = FCSC_NEXT(&c)) != NULL)
 		fellow_seg_regions(ffd, fcs, region, &n);
 	return (n);
 }
@@ -3087,7 +3091,7 @@ fellow_cache_obj_slim(const struct fellow_cache *fc,
 	AZ(pthread_mutex_lock(&fco->mtx));
 
 	/* anything to do at all? */
-	while ((fcs = fcsc_next(&c)) != NULL)
+	while ((fcs = FCSC_NEXT(&c)) != NULL)
 		if (fcs->alloc.ptr)
 			break;
 
@@ -3096,7 +3100,7 @@ fellow_cache_obj_slim(const struct fellow_cache *fc,
 
 	fcsc_init(&c, &fco->seglist);
 	AZ(pthread_mutex_lock(&lru->lru_mtx));
-	while ((fcs = fcsc_next(&c)) != NULL) {
+	while ((fcs = FCSC_NEXT(&c)) != NULL) {
 		if (fcs->alloc.ptr == NULL)
 			break;
 		ref = 0;
@@ -3802,11 +3806,11 @@ fellow_cache_obj_iter(struct fellow_cache *fc, struct fellow_cache_obj *fco,
 
 	flags = final ? OBJ_ITER_FLUSH : 0;
 	flush = 0;
-	while ((fcs = fcsc_next(&c)) != NULL) {
+	while ((fcs = FCSC_NEXT(&c)) != NULL) {
 		if (ra[n % mod] == NULL) {
 			assert(n == ran);
 
-			fcsra = fcsc_next(&rac);
+			fcsra = FCSC_NEXT(&rac);
 			assert(fcsra == fcs);
 			ran++;
 
@@ -3817,7 +3821,7 @@ fellow_cache_obj_iter(struct fellow_cache *fc, struct fellow_cache_obj *fco,
 
 		raion = 0;
 		while (ran <= n + readahead) {
-			fcsra = fcsc_next(&rac);
+			fcsra = FCSC_NEXT(&rac);
 			if (fcsra == NULL)
 				break;
 			raio[raion++] = fcsra;
@@ -3867,7 +3871,7 @@ fellow_cache_obj_iter(struct fellow_cache *fc, struct fellow_cache_obj *fco,
 		 */
 		AZ(flags & OBJ_ITER_END);
 		nextc = c;
-		fcsnext = fcsc_next(&nextc);
+		fcsnext = FCSC_NEXT(&nextc);
 		flush = 0;
 		if (fcs->state == FCS_BUSY)
 			flush = OBJ_ITER_FLUSH;