Prevent zdb(8) from occasionally hanging on I/O

The zdb(8) command may not terminate in the case where the pool
gets suspended and there is a caller in zio_wait() blocking on
an outstanding read I/O that will never complete.  This can in
turn cause ztest(1) to block indefinitely despite the deadman.

Resolve the issue by setting the default failure mode for zdb(8)
to panic.  In user space we always want the command to terminate
when forward progress is no longer possible.

Reviewed-by: Tim Chase <[email protected]>
Reviewed by: Thomas Caputi <[email protected]>
Reviewed-by: Giuseppe Di Natale <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes openzfs#6999

cmd/zdb/zdb.c

@@ -4542,13 +4542,23 @@ main(int argc, char **argv)
 			}
 		} else {
 			error = open_objset(target, DMU_OST_ANY, FTAG, &os);
+			if (error == 0)
+				spa = dmu_objset_spa(os);
 		}
 	}
 	nvlist_free(policy);
 
 	if (error)
 		fatal("can't open '%s': %s", target, strerror(error));
 
+	/*
+	 * Set the pool failure mode to panic in order to prevent the pool
+	 * from suspending.  A suspended I/O will have no way to resume and
+	 * can prevent the zdb(8) command from terminating as expected.
+	 */
+	if (spa != NULL)
+		spa->spa_failmode = ZIO_FAILURE_MODE_PANIC;
+
 	argv++;
 	argc--;
 	if (!dump_opt['R']) {