Use a lock file (sbbslist.json.lock) to protect the database (sbbslist.json)

from corruption during concurrent operations. Although the list file is not
open shareable, I was observing corruption on Vertrauen when multiple sbbslist
tasks (e.g. verification and maintenance) were running concurrently on
different systems. The .lock file method is crude, but is working to protect
the list from concurrent opens of any kind (read or write).

exec/load/sbbslist_lib.js

 // $Id$
+// @format.tab-size 4
 
 // Synchronet BBS List (SBL) v4 Library
 
@@ -402,39 +403,78 @@ function parse(f)
 	return list;
 }
 
+function lock_fname()
+{
+	return list_fname + '.lock';
+}
+
+function lock(op, timeout /* in seconds */, max)
+{
+	var start = time();
+	if(timeout === undefined)
+		timeout = 60;
+	if(max === undefined)
+		max = 24 * 60 * 60;
+
+	while(!file_mutex(lock_fname(), op, max)) {
+		if(time() - start >= timeout) {
+        		log(LOG_ERR, "SBBSLIST: Timeout locking " + list_fname + " for " + op);
+			return false;
+		}
+		sleep(1000);
+	}
+	return true;
+}
+
+function unlock()
+{
+	return file_remove(lock_fname());
+}
+
 function read_list()
 {
+	if(!lock("read"))
+		return false;
     var f = new File(list_fname);
     log(LOG_DEBUG, "Opening list file: " + f.name);
     if(!f.open("r")) {
         log(LOG_ERR, "SBBSLIST: Error " + f.error + " opening " + f.name);
+		unlock();
         return [];
     }
 	var list = parse(f);
     f.close();
+	unlock();
     return list;
 }
 
 function write_list(list)
 {
+	if(!lock("write"))
+		return false;
     var out = new File(list_fname);
     log(LOG_DEBUG, "SBBSLIST: Opening / creating list file: " + list_fname);
     if(!out.open("w+")) {
         log(LOG_ERR, "SBBSLIST: Error " + out.error + " creating " + out.name);
+		unlock();
         return false;
     }
     log(LOG_DEBUG, "SBBSLIST: Writing list file: " + out.name + " (" + list.length + " BBS entries)");
     out.write(JSON.stringify(list, null, 4));
     out.close();
+	unlock();
     return true;
 }
 
 function append(bbs)
 {
+	if(!lock("append"))
+		return false;
     var f = new File(list_fname);
     log(LOG_DEBUG, "SBBSLIST: Opening / creating list file: " + list_fname);
     if(!f.open(f.exists ? 'r+':'w+')) {
         log(LOG_ERR, "SBBSLIST: Error " + f.error + " creating " + f.name);
+		unlock();
         return false;
     }
 	var list = parse(f);
@@ -443,21 +483,26 @@ function append(bbs)
 	f.truncate();
     f.write(JSON.stringify(list, null, 4));
     f.close();
+	unlock();
     return true;
 }
 
 function remove(bbs)
 {
+	if(!lock("remove"))
+		return false;
     var f = new File(list_fname);
     log(LOG_INFO, "SBBSLIST: Opening / creating list file: " + list_fname);
     if(!f.open('r+')) {
         log(LOG_ERR, "SBBSLIST: Error " + f.error + " opening " + f.name);
+		unlock();
         return false;
     }
     var list = parse(f);
 	var index = find_value(list, "name", bbs.name); 
 	if(index < 0 ) {
 		f.close();
+		unlock();
 		return false;
 	}
 	list.splice(index, 1);
@@ -465,21 +510,26 @@ function remove(bbs)
 	f.truncate();
     f.write(JSON.stringify(list, null, 4));
     f.close();
+	unlock();
     return true;
 }
 
 function replace(bbs)
 {
+	if(!lock("replace"))
+		return false;
     var f = new File(list_fname);
     log(LOG_INFO, "SBBSLIST: Opening / creating list file: " + list_fname);
     if(!f.open('r+')) {
         log(LOG_ERR, "SBBSLIST: Error " + f.error + " opening " + f.name);
+		unlock();
         return false;
     }
     var list = parse(f);
 	var index = find_value(list, "name", bbs.name); 
 	if(index < 0 ) {
 		f.close();
+		unlock();
 		return false;
 	}
 	list[index] = bbs;
@@ -487,6 +537,7 @@ function replace(bbs)
 	f.truncate();
     f.write(JSON.stringify(list, null, 4));
     f.close();
+	unlock();
     return true;
 }