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From "Jimmy Jarvis (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CB-1318) Cordova Callback Logic Not Implemented Correctly - Native Asynchronous Completion Can Call Wrong Logic in Javascript - Requires Non-recurring Unique ID as Outlined
Date Fri, 31 Aug 2012 01:36:07 GMT

    [ https://issues.apache.org/jira/browse/CB-1318?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13445564#comment-13445564
] 

Jimmy Jarvis commented on CB-1318:
----------------------------------

This seems like a pretty critical bug as it's difficult to detect and resolve for anyone without
a deep understanding asynchronous logic design. I'd proposed we try and get this fix in and
tested as soon as possible. The community of users could be experiencing instability in their
code without realizing it or an easy way to reproduce it because of callback latency, especially
in network calls.

The problem is that the current callback logic in 2.0.0 enables native code to execute incorrect
logic across a page reload. The proposed fix insures native callbacks cannot survive page
reloads. Here is a scenario that demonstrates the problem which could be quite common:

1. HTML/JS: Calls native code (gets assigned callbackId = Service1)

2. ------ NATIVE CODE: Begins working on the request (e.g. REST call, which will take 2 -
5 seconds)

3. HTML/JS: Page refreshes or navigates to a new page. This causes the callbackId to reset
to zero and all entries in the callbacks array to be cleared (Service1 from Step 1 and pending
in Step 2 is now gone)

4. HTML/JS: Calls a different native function (e.g. REST call, Camera, etc. and is assigned
Service1 again)

5. ------ NATIVE CODE: Completes pending REST call and makes asynchronous callback to JS using
it's callbackId = Service1

6. HTML/JS: Finds the new page's Service1 callbackId in the callbacks array and executes its
completion routine, NOT the original REST call's completion routine.

                
> Cordova Callback Logic Not Implemented Correctly -  Native Asynchronous Completion Can
Call Wrong Logic in Javascript - Requires Non-recurring Unique ID as Outlined
> --------------------------------------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: CB-1318
>                 URL: https://issues.apache.org/jira/browse/CB-1318
>             Project: Apache Cordova
>          Issue Type: Bug
>          Components: iOS
>    Affects Versions: 2.0.0
>            Reporter: Jimmy Jarvis
>            Assignee: Shazron Abdullah
>            Priority: Critical
>             Fix For: 2.2.0
>
>   Original Estimate: 1h
>  Remaining Estimate: 1h
>
> This bug creates very difficult to find execution anomalies for any asynchronous callback
that occurs when the calling context has changed.
> PROBLEM: Cordova callback logic can trigger the callback function of an unrelated call
from a different context. When a call from Javascript is made, Cordova generates a new callbackId,
adds it to the cordova.callbacks array, and passes it along to the native code. Some time
later, when the native code completes the request, it injects javascript to call the callbackSuccess
or callbackError function. The respective callback handler looks up the callback attributes
from the cordova.callbacks array and executes the original caller's completion function. The
problem is each page, or reload of a page, resets the callbackId counter to zero and new calls
end up reusing the same callbackId, even though the native code haS not yet completed the
prior request. This can occur upon a new page load or reload upon an empty href in an anchor
tag. A call to native code from page1.html could trigger the completion routine on page2.html
because the native code has no context of the change on the other side of the bridge.
> SOLUTION: Replace the incrementing callbackId with a unique identifier to insure only
the caller's completion logic will be called. I have done this with a Pseudo-GUID generator.
Fix outlined below:
> // REPLACE THIS LINE in both cordova.2.0.0.js and cordova.ios.js
>         callbackId = service + cordova.callbackId++;    // BUG: incrementing Id's can
call unrelated callback logic
> // WITH THIS LINE
>         callbackId = service + ":" + 'xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx'.replace(/[xy]/g,
function(c) {var r = Math.random()*16|0,v=c=='x'?r:r&0x3|0x8;return v.toString(16);});
// Generate unique callbackID with Pseudo-GUID
> This fix insures no code will execute if the caller's context has changed and is a more
well formatted callbackId.

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