Fix gang write late_arrival bug

When a write comes in via dmu_sync_late_arrival, its txg is equal to the open TXG. If that write gangs, and we have not yet activated the new gang header feature, and the gang header we pick can store a larger gang header, we will try to schedule the upgrade for the open TXG + 1. In debug mode, this causes an assertion to trip. This PR sets the TXG for activating the feature to be at most the current open TXG.

Signed-off-by: Paul Dagnelie <[email protected]>
Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.

Author: Paul Dagnelie

include/sys/txg.h

@@ -137,6 +137,7 @@ extern boolean_t txg_stalled(struct dsl_pool *dp);
 
 /* returns TRUE if someone is waiting for the next txg to sync */
 extern boolean_t txg_sync_waiting(struct dsl_pool *dp);
+extern uint64_t txg_cur_open(struct dsl_pool *dp);
 
 extern void txg_verify(spa_t *spa, uint64_t txg);
 

module/zfs/txg.c

@@ -867,6 +867,12 @@ txg_verify(spa_t *spa, uint64_t txg)
 }
 #endif
 
+uint64_t
+txg_cur_open(dsl_pool_t *dp)
+{
+	return (dp->dp_tx.tx_open_txg);
+}
+
 /*
  * Per-txg object lists.
  */

module/zfs/zio.c

@@ -3319,7 +3319,8 @@ zio_write_gang_block(zio_t *pio, metaslab_class_t *mc)
 	    spa_feature_is_enabled(spa, SPA_FEATURE_DYNAMIC_GANG_HEADER) &&
 	    !spa_feature_is_active(spa, SPA_FEATURE_DYNAMIC_GANG_HEADER)) {
 		dmu_tx_t *tx =
-		    dmu_tx_create_assigned(spa->spa_dsl_pool, txg + 1);
+		    dmu_tx_create_assigned(spa->spa_dsl_pool, MIN(txg + 1,
+		    txg_cur_open(spa->spa_dsl_pool)));
 		dsl_sync_task_nowait(spa->spa_dsl_pool,
 		    zio_update_feature,
 		    (void *)SPA_FEATURE_DYNAMIC_GANG_HEADER, tx);