fsm: add osmo_fsm_inst_state_chg_keep_or_start_timer()

During FSM design for osmo-msc, I noticed that the current behavior that
keep_timer=true doesn't guarantee a running timer can make FSM design a bit
complex, especially when using osmo_tdef for timeout definitions.

A desirable keep_timer=true behavior is one that keeps the previous timer
running, but starts a timer if no timer is running yet.

The simplest example is: a given state repeatedly transitions back to itself,
but wants to set a timeout only on first entering, avoiding to restart the
timeout on re-entering.

Another example is a repeated transition between two or more states, where the
first time we enter this group a timeout should start, but it should not
restart from scratch on every transition.

When using osmo_tdef timeout definitions for this, so far separate meaningless
states have to be introduced that merely set a fixed timeout.

To simplify, add osmo_fsm_inst_state_chg_keep_or_start_timer(), and use this in
osmo_tdef_fsm_inst_state_chg() when both keep_timer == true *and* T != 0.

In tdef_test.ok, the changes show that on first entering state L, the previous
T=1 is now kept with a large remaining timeout. When entering state L from O,
where no timer was running, this time L's T123 is started.

Change-Id: Id647511a4b18e0c4de0e66fb1f35dc9adb9177db

1 files changed, 15 insertions, 0 deletions

diff --git a/include/osmocom/core/fsm.h b/include/osmocom/core/fsm.h
index 13bfb331..c40d7f3c 100644
--- a/include/osmocom/core/fsm.h
+++ b/include/osmocom/core/fsm.h
@@ -254,6 +254,21 @@ int _osmo_fsm_inst_state_chg(struct osmo_fsm_inst *fi, uint32_t new_state,
 int _osmo_fsm_inst_state_chg_keep_timer(struct osmo_fsm_inst *fi, uint32_t new_state,
 					const char *file, int line);
 
+/*! perform a state change while keeping the current timer if running, or starting a timer otherwise.
+ *
+ *  This is useful to keep a timeout across several states, but to make sure that some timeout is actually running.
+ *
+ *  This is a macro that calls _osmo_fsm_inst_state_chg_keep_or_start_timer() with the given
+ *  parameters as well as the caller's source file and line number for logging
+ *  purposes. See there for documentation.
+ */
+#define osmo_fsm_inst_state_chg_keep_or_start_timer(fi, new_state, timeout_secs, T) \
+	_osmo_fsm_inst_state_chg_keep_or_start_timer(fi, new_state, timeout_secs, T, \
+						     __FILE__, __LINE__)
+int _osmo_fsm_inst_state_chg_keep_or_start_timer(struct osmo_fsm_inst *fi, uint32_t new_state,
+						 unsigned long timeout_secs, int T,
+						 const char *file, int line);
+
 /*! dispatch an event to an osmocom finite state machine instance
  *
  *  This is a macro that calls _osmo_fsm_inst_dispatch() with the given