fsm: support graceful osmo_fsm_inst_term() cascades

Add global flag osmo_fsm_term_safely() -- if set to true, enable the following
behavior:

Detect osmo_fsm_inst_term() occuring within osmo_fsm_inst_term():
- collect deallocations until the outermost osmo_fsm_inst_term() is done.
- call osmo_fsm_inst_free() *after* dispatching the parent event.

If a struct osmo_fsm_inst enters osmo_fsm_inst_term() while another is already
within osmo_fsm_inst_term(), do not directly deallocate it, but talloc-reparent
it to a separate talloc context, to be deallocated with the outermost FSM inst.

The effect is that all osmo_fsm_inst freed within an osmo_fsm_inst_term()
cascade will stay allocated until all osmo_fsm_inst_term() are complete and all
of them will be deallocated at the same time.

Mark the deferred deallocation state as __thread in an attempt to make cascaded
deallocation handling threadsafe.  Keep the enable/disable flag separate, so
that it is global and not per-thread.

The feature is showcased by fsm_dealloc_test.c: with this feature, all of those
wild deallocation scenarios succeed.

Make fsm_dealloc_test a normal regression test in testsuite.at.

Rationale:

It is difficult to gracefully handle deallocations of groups of FSM instances
that reference each other. As soon as one child dispatching a cleanup event
causes its parent to deallocate before fsm.c was ready for it, deallocation
will hit a use-after-free. Before this patch, by using parent_term events and
distinct "terminating" FSM states, parent/child FSMs can be taught to wait for
all children to deallocate before deallocating the parent. But as soon as a
non-child / non-parent FSM instance is involved, or actually any other
cleanup() action that triggers parent FSMs or parent talloc contexts to become
unused, it is near impossible to think of all possible deallocation events
ricocheting, and to avoid running into freeing FSM instances that were still in
the middle of osmo_fsm_inst_term(), or FSM instances to enter
osmo_fsm_inst_term() more than once. This patch makes deallocation of "all
possible" setups of complex cross referencing FSM instances easy to handle
correctly, without running into use-after-free or double free situations, and,
notably, without changing calling code.

Change-Id: I8eda67540a1cd444491beb7856b9fcd0a3143b18

1 files changed, 1 insertions, 0 deletions

diff --git a/include/osmocom/core/fsm.h b/include/osmocom/core/fsm.h
index 07bcd126..c9e1e0cf 100644
--- a/include/osmocom/core/fsm.h
+++ b/include/osmocom/core/fsm.h
@@ -121,6 +121,7 @@ struct osmo_fsm_inst {
 
 void osmo_fsm_log_addr(bool log_addr);
 void osmo_fsm_log_timeouts(bool log_timeouts);
+void osmo_fsm_term_safely(bool term_safely);
 
 /*! Log using FSM instance's context, on explicit logging subsystem and level.
  * \param fi  An osmo_fsm_inst.