fix dsl_scan_ds_clone_swapped logic

The was incorrect with respect to swapping dataset IDs both in the
on-disk ZAP object and the in-memory queue.

In both cases, if ds1 was already present, then it would be first
replaced with ds2 and then ds would be replaced back with ds1.
Also, both cases did not properly handle a situation where both ds1 and
ds2 are already queued.  A duplicate insertion would be attempted and
its failure would result in a panic.

This change should fix openzfs#9140.

Signed-off-by: Andriy Gapon <avg@FreeBSD.org>

module/zfs/dsl_scan.c

@@ -2184,52 +2184,57 @@ dsl_scan_ds_clone_swapped(dsl_dataset_t *ds1, dsl_dataset_t *ds2, dmu_tx_t *tx)
 {
 	dsl_pool_t *dp = ds1->ds_dir->dd_pool;
 	dsl_scan_t *scn = dp->dp_scan;
-	uint64_t mintxg;
+	uint64_t mintxg1, mintxg2;
+	boolean_t ds1_queued, ds2_queued;
 
 	if (!dsl_scan_is_running(scn))
 		return;
 
 	ds_clone_swapped_bookmark(ds1, ds2, &scn->scn_phys.scn_bookmark);
 	ds_clone_swapped_bookmark(ds1, ds2, &scn->scn_phys_cached.scn_bookmark);
 
-	if (scan_ds_queue_contains(scn, ds1->ds_object, &mintxg)) {
+	ds1_queued = scan_ds_queue_contains(scn, ds1->ds_object, &mintxg1);
+	ds2_queued = scan_ds_queue_contains(scn, ds2->ds_object, &mintxg2);
+	if (ds1_queued)
 		scan_ds_queue_remove(scn, ds1->ds_object);
-		scan_ds_queue_insert(scn, ds2->ds_object, mintxg);
-	}
-	if (scan_ds_queue_contains(scn, ds2->ds_object, &mintxg)) {
+	if (ds2_queued)
 		scan_ds_queue_remove(scn, ds2->ds_object);
-		scan_ds_queue_insert(scn, ds1->ds_object, mintxg);
+	if (ds1_queued)
+		scan_ds_queue_insert(scn, ds2->ds_object, mintxg1);
+	if (ds2_queued)
+		scan_ds_queue_insert(scn, ds1->ds_object, mintxg2);
+
+	ds1_queued = zap_lookup_int_key(dp->dp_meta_objset,
+	    scn->scn_phys.scn_queue_obj, ds1->ds_object, &mintxg1) == 0;
+	ds2_queued = zap_lookup_int_key(dp->dp_meta_objset,
+	    scn->scn_phys.scn_queue_obj, ds2->ds_object, &mintxg2) == 0;
+
+	/* Sanity checking. */
+	if (ds1_queued) {
+		ASSERT3U(mintxg1, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
+		ASSERT3U(mintxg1, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
+	}
+	if (ds2_queued) {
+		ASSERT3U(mintxg2, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
+		ASSERT3U(mintxg2, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
 	}
 
-	if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
-	    ds1->ds_object, &mintxg) == 0) {
-		int err;
-		ASSERT3U(mintxg, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
-		ASSERT3U(mintxg, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
-		VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
+	if (ds1_queued && ds2_queued) {
+		/* Nothing to do if both are queued (or both are not queued). */
+	} else if (ds1_queued) {
+		VERIFY3S(0, ==, zap_remove_int(dp->dp_meta_objset,
 		    scn->scn_phys.scn_queue_obj, ds1->ds_object, tx));
-		err = zap_add_int_key(dp->dp_meta_objset,
-		    scn->scn_phys.scn_queue_obj, ds2->ds_object, mintxg, tx);
-		VERIFY(err == 0 || err == EEXIST);
-		if (err == EEXIST) {
-			/* Both were there to begin with */
-			VERIFY(0 == zap_add_int_key(dp->dp_meta_objset,
-			    scn->scn_phys.scn_queue_obj,
-			    ds1->ds_object, mintxg, tx));
-		}
+		VERIFY3S(0, ==, zap_add_int_key(dp->dp_meta_objset,
+		    scn->scn_phys.scn_queue_obj, ds2->ds_object, mintxg1, tx));
 		zfs_dbgmsg("clone_swap ds %llu; in queue; "
 		    "replacing with %llu",
 		    (u_longlong_t)ds1->ds_object,
 		    (u_longlong_t)ds2->ds_object);
-	}
-	if (zap_lookup_int_key(dp->dp_meta_objset, scn->scn_phys.scn_queue_obj,
-	    ds2->ds_object, &mintxg) == 0) {
-		ASSERT3U(mintxg, ==, dsl_dataset_phys(ds1)->ds_prev_snap_txg);
-		ASSERT3U(mintxg, ==, dsl_dataset_phys(ds2)->ds_prev_snap_txg);
-		VERIFY3U(0, ==, zap_remove_int(dp->dp_meta_objset,
+	} else if (ds2_queued) {
+		VERIFY3S(0, ==, zap_remove_int(dp->dp_meta_objset,
 		    scn->scn_phys.scn_queue_obj, ds2->ds_object, tx));
-		VERIFY(0 == zap_add_int_key(dp->dp_meta_objset,
-		    scn->scn_phys.scn_queue_obj, ds1->ds_object, mintxg, tx));
+		VERIFY3S(0, ==, zap_add_int_key(dp->dp_meta_objset,
+		    scn->scn_phys.scn_queue_obj, ds1->ds_object, mintxg2, tx));
 		zfs_dbgmsg("clone_swap ds %llu; in queue; "
 		    "replacing with %llu",
 		    (u_longlong_t)ds2->ds_object,