Improve dbuf_read() error reporting

Previous code reported non-ZIO errors only via return value, but
not via parent ZIO.  It could cause NULL-dereference panics due
to dmu_buf_hold_array_by_dnode() ignoring the return value,
relying solely on parent ZIO status.

Reported by:	Ameer Hamza <ahamza@ixsystems.com>
Signed-off-by:	Alexander Motin <mav@FreeBSD.org>
Sponsored by:	iXsystems, Inc.

module/zfs/dbuf.c

@@ -1669,10 +1669,10 @@ dbuf_read_impl(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags,
 	 * parent's rwlock, which would be a lock ordering violation.
 	 */
 	dmu_buf_unlock_parent(db, dblt, tag);
-	(void) arc_read(zio, db->db_objset->os_spa, bpp,
+	return (arc_read(zio, db->db_objset->os_spa, bpp,
 	    dbuf_read_done, db, ZIO_PRIORITY_SYNC_READ, zio_flags,
-	    &aflags, &zb);
-	return (err);
+	    &aflags, &zb));
+
 early_unlock:
 	DB_DNODE_EXIT(db);
 	mutex_exit(&db->db_mtx);
@@ -1759,7 +1759,7 @@ dbuf_fix_old_data(dmu_buf_impl_t *db, uint64_t txg)
 }
 
 int
-dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
+dbuf_read(dmu_buf_impl_t *db, zio_t *pio, uint32_t flags)
 {
 	int err = 0;
 	boolean_t prefetch;
@@ -1822,13 +1822,13 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
 
 		db_lock_type_t dblt = dmu_buf_lock_parent(db, RW_READER, FTAG);
 
-		if (zio == NULL && (db->db_state == DB_NOFILL ||
+		if (pio == NULL && (db->db_state == DB_NOFILL ||
 		    (db->db_blkptr != NULL && !BP_IS_HOLE(db->db_blkptr)))) {
 			spa_t *spa = dn->dn_objset->os_spa;
-			zio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);
+			pio = zio_root(spa, NULL, NULL, ZIO_FLAG_CANFAIL);
 			need_wait = B_TRUE;
 		}
-		err = dbuf_read_impl(db, zio, flags, dblt, FTAG);
+		err = dbuf_read_impl(db, pio, flags, dblt, FTAG);
 		/*
 		 * dbuf_read_impl has dropped db_mtx and our parent's rwlock
 		 * for us
@@ -1849,9 +1849,10 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
 		 */
 		if (need_wait) {
 			if (err == 0)
-				err = zio_wait(zio);
+				err = zio_wait(pio);
 			else
-				VERIFY0(zio_wait(zio));
+				(void) zio_wait(pio);
+			pio = NULL;
 		}
 	} else {
 		/*
@@ -1878,7 +1879,7 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
 				ASSERT(db->db_state == DB_READ ||
 				    (flags & DB_RF_HAVESTRUCT) == 0);
 				DTRACE_PROBE2(blocked__read, dmu_buf_impl_t *,
-				    db, zio_t *, zio);
+				    db, zio_t *, pio);
 				cv_wait(&db->db_changed, &db->db_mtx);
 			}
 			if (db->db_state == DB_UNCACHED)
@@ -1887,6 +1888,13 @@ dbuf_read(dmu_buf_impl_t *db, zio_t *zio, uint32_t flags)
 		}
 	}
 
+	if (pio && err != 0) {
+		zio_t *zio = zio_null(pio, pio->io_spa, NULL, NULL, NULL,
+		    ZIO_FLAG_CANFAIL);
+		zio->io_error = err;
+		zio_nowait(zio);
+	}
+
 	return (err);
 }