Reapply 644908c1, "Garbage-collect SMJS objects before flushing caches."

Reapply commit 644908c130 of 2007-05-28,
"Garbage-collect SMJS objects before flushing caches." which was reverted
in commit 6cd6a843d1 on 2007-05-30.
It is now safe to apply because bug 956 has been fixed.

doc/events.txt

@@ -113,6 +113,26 @@ Description:
 
  Open Lua console dialog.
 
+-------------------------------------------------------------------------------
+Name:		flush-caches
+Managed By:	The scripting subsystem/backends
+Triggered When:
+
+ Elinks is going to free its caches.  This happens when the user chooses
+ ACT_MAIN_CACHE_MINIMIZE, but currently also on ACT_MAIN_FORGET_CREDENTIALS
+ and when ELinks is quitting.
+
+Arguments:
+
+ None
+
+Description:
+
+ If scripting backends hold pointers to cache entries, they should try
+ to release those pointers so that ELinks can free the entries.  This
+ may involve a full garbage collection.  Also, if backends have some
+ caches of their own, they should flush them.
+
 -------------------------------------------------------------------------------
 Name:		follow-url
 Managed By:	The scripting subsystem/backends

src/main/main.c

@@ -315,6 +315,13 @@ terminate_all_subsystems(void)
 void
 shrink_memory(int whole)
 {
+#ifdef CONFIG_SCRIPTING
+	/* The SMJS pre-format-html hook constructs an SMJS object for
+	 * each cache entry.  Give all scripting modules a cue to garbage
+	 * collect any such objects so that the entries can be freed.  */
+	if (whole)
+		trigger_event_name("flush-caches");
+#endif
 	shrink_dns_cache(whole);
 	shrink_format_cache(whole);
 	garbage_collection(whole);

src/scripting/smjs/hooks.c

@@ -122,10 +122,26 @@ end:
 	return ret;
 }
 
+static enum evhook_status
+script_hook_flush_caches(va_list ap, void *data)
+{
+	/* script_hook_pre_format_html() calls smjs_get_cache_entry_object()
+	 * for each struct cache_entry.  The resulting SMJS objects hold
+	 * references to the structs, and these references prevent ELinks
+	 * from freeing the cache entries.  (The resource info dialog shows
+	 * that the entries are "in use".)  SMJS does not immediately collect
+	 * these objects as garbage.  If we're really trying to flush the
+	 * caches then ask SMJS to run a check.  */
+	if (smjs_ctx)
+		JS_GC(smjs_ctx);
+	return EVENT_HOOK_STATUS_NEXT;
+}
+
 struct event_hook_info smjs_scripting_hooks[] = {
 	{ "goto-url", 0, script_hook_goto_url, NULL },
 	{ "follow-url", 0, script_hook_follow_url, NULL },
 	{ "pre-format-html", 0, script_hook_pre_format_html, NULL },
+ 	{ "flush-caches", 0, script_hook_flush_caches, NULL },
 
 	NULL_EVENT_HOOK_INFO,
 };