ti-longjohn

Long stack traces for Titanium with configurable call trace length.

This module is a port and partially rewrite of the Node.js longjohn for the Titanium platform.

In addition it has special support to patch async.js's queue.

Requirements

• Titanium SDK 3.4+

Supports

• iOS simulator and device
• Android is untested, probably will not work because Titanium's Android support is based on V8, not JavaScriptCore.

Installation

Download the latest release via git from github:

git clone https://github.com/sttts/ti-longjohn.git

or use gittio:

gittio install ti-longjohn

or use npm:

npm install ti-longjohn

Usage

To use ti-longjohn, require it in your code (probably in some initialization code). That's all!

if (Ti.App.deployType !== 'production') {
  var longjohn = require('longjohn')(global);
}

// ... your code

where global is the global namespace. If you require ti-longjohn from the app.js file, pass this as the global parameter.

Output

When an exception is thrown, ti-longjohn will try to construct a long stack trace which can consist of multiple usual stack traces which are seperated by asyncronous callbacks, e.g. by setTimeout.

The output looks like this:

"TypeError at node_modules/nedb/lib/model.js:31",
"#0 () at node_modules/nedb/lib/datastore.js:296",
"#1 () at node_modules/nedb/lib/datastore.js:528",
"#2 () at node_modules/nedb/lib/cursor.js:174",
"#3 () at node_modules/nedb/lib/datastore.js:530",
"#4 () at node_modules/nedb/node_modules/async/lib/async.js:582",
"#5 () at node_modules/nedb/node_modules/async/lib/async.js:498",
"#8 () at node_modules/process/index.js:14",
"-------- process.nextTick ---------",
"#2 () at node_modules/nedb/node_modules/async/lib/async.js:499",
"#3 () at node_modules/nedb/node_modules/async/lib/async.js:503",
"#4 () at node_modules/nedb/lib/datastore.js:564",
"#5 () at node_modules/nedb/lib/executor.js:40",
"#8 () at node_modules/nedb/node_modules/async/lib/async.js:731",
"#9 () at node_modules/nedb/node_modules/async/lib/async.js:728",
"#10 () at node_modules/nedb/node_modules/async/lib/async.js:24",
"#13 () at node_modules/process/index.js:14",
"-------- async.queue.push ---------",
"#1 () at node_modules/nedb/lib/executor.js:56",
"#2 () at node_modules/nedb/lib/datastore.js:568",
"#3 () at models/thread.js:1687",
"#4 () at lib/async.js:122"

To get a stack trace, ti-longjohn uses the err.backtrace property of an Error object thrown during an exception. It seems that JavaScriptCore in Titanium is not very verbose about function names (hence, the empty () in the stack traces).

Moreover, it cannot be guaranteed by ti-longjohn that the stack traces are always complete.

Use in production

During execution of JavaScript code, ti-longjohn will keep a linked list of error objects in memory, each keeping a stack trace up to an async callback.

Moreover, in order to get a stack trace, a try/catch block is used which is known to be bad for performance.

For these reason it is not recommended to have ti-longjohn activated in production.

Limit traced async calls

longjohn.async_trace_limit = 5;   // defaults to 10
longjohn.async_trace_limit = -1;  // unlimited

Patching async.js

The queue mechanism in async.js breaks the stack trace chain that ti-longjohn tries to build and which it prints on error.

There is special support to patch the async.js queue in such a way that proper long stack traces are created:

var longjohn = require('ti-longjohn')(global);
longjohn.patch_async(require('async'));

Then the code

var q = async.queue(function (x) {
    i.do.not.exist = x;
});
q.push(42, function done() {
    Ti.API.info('done');
});

will lead to a long stack trace:

"TypeError at app.js:19",
"#2 () at node_modules/async/lib/async.js:809",
"#5 () at node_modules/process/index.js:14",
"-------- async.queue.push ---------",
"#1 () at app.js:23"