Implement an (in-memory) Avatar cache - reducing the number of disk accesses

when using features that support avatars (e.g. reading msgs, listing files,
listing BBSes in the BBS list).
When used in the terminal server, the cache is located in bbs.mods.avatar_cache
otherwise it's located in the scope of the load()'d library. Applications must
use the lib's read() method to take advantage of the cache. The other lower
level functions (e.g. read_localuser, read_netuser) by-pass the cache on read
but do update the cache with the result. So, generally, avatars should be only
loaded from disk one time during a session/logon.

exec/load/avatar_lib.js

@@ -13,6 +13,31 @@ const FTN_4D_PATTERN = /^(\d+):(\d+)\/(\d+)\.(\d+)$/;
 
 const size = defs.width * defs.height * 2;	// 2 bytes per cell for char and attributes
 
+var cache = {};
+if(js.global.bbs) {
+	if(!bbs.mods.avatar_cache)
+		bbs.mods.avatar_cache = {};
+	cache = bbs.mods.avatar_cache;
+}
+
+function cache_key(username, netaddr)
+{
+	var key = username;
+	if(netaddr)
+		key += ( '@' + netaddr );
+	return key;
+}
+
+function cache_set(username, obj, netaddr)
+{
+	cache[cache_key(username, netaddr)] = obj;
+}
+
+function cache_get(username, netaddr)
+{
+	return cache[cache_key(username, netaddr)];
+}
+
 function local_library()
 {
 	return format("%savatars/", system.text_dir);
@@ -94,6 +119,7 @@ function write_localuser(usernum, obj)
 	}
 	var result = file.iniSetObject("avatar", obj);
 	file.close();
+	cache_set(usernum, obj);
 	return result;
 }
 
@@ -111,7 +137,8 @@ function enable_localuser(usernum, enabled)
 		obj.disabled = !enabled;
 		obj.updated = new Date();
 		result = file.iniSetObject("avatar", obj);
-	}	
+		cache_set(usernum, obj);
+	}
 	file.close();
 	return result;
 }
@@ -124,6 +151,7 @@ function remove_localuser(usernum)
 	}
 	var result = file.iniRemoveSection("avatar");
 	file.close();
+	cache_set(usernum, undefined);
 	return result;
 }
 
@@ -135,6 +163,7 @@ function write_netuser(username, netaddr, obj)
 	}
 	var result = file.iniSetObject(username, obj);
 	file.close();
+	cache_set(username, obj, netaddr);
 	return result;
 }
 
@@ -155,7 +184,7 @@ function read_localuser(usernum)
 function read_netuser(username, netaddr)
 {
 	var filename = this.netuser_fname(netaddr);
-	if(!filename)
+	if(!filename && netaddr.indexOf('@') == -1)
 		filename = this.bbsuser_fname(netaddr);
 	if(!filename)
 		return false;
@@ -170,12 +199,18 @@ function read_netuser(username, netaddr)
 
 function read(usernum, username, netaddr)
 {
-	if(parseInt(usernum, 10) >= 1)
-		return read_localuser(usernum);
+	var usernum = parseInt(usernum, 10);
+	var obj = cache_get(usernum >= 1 ? usernum : username, netaddr);
+	if(obj !== undefined)	// null and false are also valid cached avatar values
+		return obj;
+	if(usernum >= 1)
+		obj = read_localuser(usernum);
 	else if(!netaddr)
-		return read_localuser(system.matchuser(username));
+		obj = read_localuser(system.matchuser(username));
 	else
-		return read_netuser(username, netaddr);
+		obj = read_netuser(username, netaddr);
+	cache_set(usernum >= 1 ? usernum : username, obj, netaddr);
+	return obj;
 }
 
 function update_localuser(usernum, data)