expire: Introduce EXP_Reduce()

This commit introduces EXP_Reduce(), a function to reduce object timers.
The goal is to provide a function better suited to soft-purging objects,
as EXP_Rearm() has some non-obvious disadvantages when used or this
purpose.

When EXP_Rearm() is used to soft-purge an object by setting its TTL to
0, the expiry is effectively reset to the start of the objects grace
period. This happens because the object TTL includes a time delta
between object insertion time (oc->t_origin) and now. The result is that
a soft-purge extends the lifetime of an already stale object, and
repeated soft-purges can keep the object away from expiry indefinitely.

The EXP_Reduce() function is better suited for soft-purging, as it only
updates an objects TTL if it would reduce the objects lifetime. The
function also has facilities for reducing grace and keep.

bin/varnishd/cache/cache_expire.c

@@ -219,6 +219,28 @@ EXP_Insert(struct worker *wrk, struct objcore *oc)
 	}
 }
 
+/*--------------------------------------------------------------------
+ * Reduce object timers
+ */
+
+void
+EXP_Reduce(struct objcore *oc, vtim_real now,
+    vtim_dur ttl, vtim_dur grace, vtim_dur keep)
+{
+
+	CHECK_OBJ_NOTNULL(oc, OBJCORE_MAGIC);
+	assert(oc->refcnt > 0);
+
+	if (!isnan(ttl) && now + ttl - oc->t_origin >= oc->ttl)
+		ttl = NAN;
+	if (!isnan(grace) && grace >= oc->grace)
+		grace = NAN;
+	if (!isnan(keep) && keep >= oc->keep)
+		keep = NAN;
+
+	EXP_Rearm(oc, now, ttl, grace, keep);
+}
+
 /*--------------------------------------------------------------------
  * We have changed one or more of the object timers, tell the exp_thread
  *

bin/varnishd/cache/cache_varnishd.h

@@ -240,6 +240,8 @@ void EXP_Remove(struct objcore *, const struct objcore *);
 /* cache_exp.c */
 void EXP_Rearm(struct objcore *oc, vtim_real now,
     vtim_dur ttl, vtim_dur grace, vtim_dur keep);
+void EXP_Reduce(struct objcore *oc, vtim_real now,
+    vtim_dur ttl, vtim_dur grace, vtim_dur keep);
 
 /* From cache_main.c */
 void BAN_Init(void);