Monthly Archives: July 2009

The curious case of WindowsFormsParkingWindow

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I was debugging a problem the other day involving WebKit.NET, a webkit wrapper control for winforms. When the webkit control was hosted inside winforms, it had a strange problem of always thinking that it is out of focus. This had the effect of drawing all selections as grayed out.

Everything I threw at it (WM_SETFOCUS, WM_ACTIVATE) seemed to be going into a void, so it was time to break into the source and figure out what exactly was wrong. After a few breakpoints and step overs, I found that the WebKit control searches for the parent window and listens to its messages by subclassing the window. It listens for WM_NCACTIVATE event and then determines whether it has focus or not.

This was working exactly as expected on a normal non-winforms window, so what was different? Debugging the code, I found that it got the root parent window as WindowsFormsParkingWindow. Why is it returning that as the parent window and not the actual parent window I’m not entirely sure. Maybe it’s some special super root window like the Application window in Delphi? It wasn’t shown in Spy++ either. Then again what’s a WindowsFormsParkingWindow? It’s something that I ought to google. Unfortunately google wasn’t very forth coming. Puzzled, I devised a hack by simply temporarily unassigning the immediate parent window of the webkit control WS_CHILD, simply because the webkit find ancestry detection algorithm stops when it encounters a non-child. After the setHostWindow() call, I make the parent window WS_CHILD again. That seemed to work but something lingered in the back of my mind telling that there is a better solution.

The next day as I was about to post a solution on the mailing list describing my solution, just when I was about to talk about this weird WindowsFormsParkingWindow, something in my mind clicked. Suddenly it all seemed to make sense. There was a reason WindowsFormsParkingWindow wasn’t in the tree ancestry when I looked at Spy++. There’s also a reason for the name. It turns out that on creation, all controls get put into WindowsFormsParkingWindow. It just so happens the setHostWindow call was made in the constructor when the parent control was still parked. The solution seemed obvious now. The setHostWindow should be called after the parent is correctly set, ie on the Load event. So, another case closed.

Compiling WebKit/Cairo on Windows with Visual C++ Express

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Just for interest, I decided to build webkit on Windows. This supposed to be a painless task but unfortunately, anything that involves build scripts creates drama. The instructions on compiling are a bit scant, giving the impression that it’s super easy. Maybe it is and I’m just a bit dumb, but anyways after a few hours battling with it I finally got it working. Don’t bother trying with Visual Studio 2008 as you might as well jump off a cliff from compilation errors – Get the Visual C++ Express 2005 edition and SP1 patch. Here are the steps from scratch:

Go to http://nightly.webkit.org/ and get the source tar.bz2.

Extract it somewhere (WinRAR is a good tool for this). This somewhere will now be referred to as {EXTRACTED}.

For compiling on windows, you need windows Download the WebKitSupportLibrary.zip then copy this zip file on the root of your extracted webkit folder. Don’t extract it. This zip gives you unicode/uchar.h or pthread.h which would otherwise be reported missing.

So what’s the difference between webkit/cairo and regular webkit? The regular webkit depends on proprietary Apple libraries such as CoreGraphics. The webkit/cairo build substitutes that dependency with the free Cairo 2D graphics library. The library has a clean C API and contains very compehensive drawing routines. It also sports multiple output backends allowing you to create PDFs or SVGs.

Webkit/cairo has more dependencies on open source libraries such as curl. You either build them or get the pre-built ones some nice folks have put online – Get it here and extract it to a place like {EXTRACTED}\requirements.

Now you have to install Cygwin as noted here on point number 3. The important point I want to highlight is due to some assumptions made in the VS project files, the directory HAS to be: at c:\cygwin (or $(SYSTEMROOT)\cygwin). For Vista, you need to perform additional minor gymnastics which I’m not going to outline here. Read it on the previous link.

You also need to get the Quick Time SDK. Annoyingly, to download quicktime SDK you need to register a Apple Developer Connection (ADC) account. After entering details of your daily life, you’re grant access to the file. Good thing was that you don’t need quick time or itunes installed. You should probably keep the default install directory as well.

Next is setting two environment variables. This involves going into the System Properties in your Control Panel. You can just put add it under the User variables:

  • WEBKITLIBRARIESDIR – point it to your {EXTRACTED}\WebKit\WebKitLibraries\win
  • WEBKITOUTPUTDIR – point it somewhere like c:\webkitdist (create this directory as well)

You will now need to add some VC++ paths. Open C++ Express and go to Tools > Options > Projects > VC++ Directories

On the top drop down, select show directories for include files and add

  • {EXTRACTED}\requirements\include
  • {EXTRACTED}\requirements\include\cairo
  • {EXTRACTED}\requirements\include\curl
  • {EXTRACTED}\requirements\include\libpng13

then select show directories for library files and add

  • {EXTRACTED}\requirements\lib

Lastly in the Executables files section add

  • C:\cygwin\bin – Put this just above $(PATH) but after all the other directories. This is important as if you put it on top, cygwin’s link.exe will conflict with MSVC’s, and you need to put it above $(PATH) or the PATH directories that have similarly named binaries to cygwin’s will conflict.

You will also need to get the Platform SDK as noted in the build instructions if you don’t have it already. Any recent version of the SDK should be alright. Add the executable, include and lib directories to the VC++ directories like the above, described here. The instructions about vcprops editing aren’t really important.

For the include path, you also need to add the mfc includes or the compiler will complain about missing winres.h

Due to some bootstrapping scripts, the first time you compile you can’t do it through the Visual C++ IDE. You will need to compile it via the command line in cygwin. Open the cygwin shell and change directory to {EXTRACTED} ie cd c:/webkit/

Then run:

WebkitTools/Scripts/update-webkit

After it completes, you can start building

WebkitTools/Scripts/build-webkit --wincairo --debug

Remove the –debug flag if you want to compile for Release. Note: Previously the wincairo flag was cairo-win32.

After the first successful build, you can now open {EXTRACTED}\WebKit\win\WebKit.vcproj in Visual Studio. Make sure the build profile is set to Debug_Cairo, set it. You can choose Release_Cairo as well if you so wish. Finally, winlauncher is the test browser you use to try out your new build, so set that as your active project. Don’t forget to copy all the requirements dlls (ie cairo.dll) into the dist\bin folder where winlauncher.exe is or else it’ll complain about missing DLLs.

Detecting the back (or refresh) button click

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While developing a web app, I came across an interesting problem: I had a page which had a button to perform an action. If the button is clicked, the action request is sent to the server side script and redirected back to the same page but with a message displayed on the top of the page (ie Your post has been submitted).

If you then navigate to another page but click back, you would see the same page with the same message popping up. I want to detect that we’re clicking back so we will hide the message. There are plenty of solutions in google, but a lot of them involved setting a cookie (what if cookies are disabled), or a server side script detecting referer (what if page is still cached?), or using time by detecting if the server page load time and the current time differs by a large amount (what if client time is wrong?). Without an ideal solution, I set about finding a new solution. Surely it can’t be hard to detect that we’ve already been in that same page. If only there was a way to save a flag just for that page and for the duration of the page session. I tried modifying the DOM, but that gets reverted when you click back. The onload event also get called again, so you can’t use that to differentiate.

I then remembered that at least on recent browsers, there exists a functionality in forms that retained form field information if you clicked back – very handy if you’re submitting a post and the connection died, you can just click back and your long winded post would be intact.

Solution – Use a hidden form field to detect that we’ve been on this page before

Building on this idea, it’s possible to temporarily store a flag on a hidden form field that says, yep I’ve been on this page before. Here is a code snippet:

[html]<html>
<body>
Try
<a href="http://www.google.com/">jumping to another page</a>
</body>

<script>

document.write("<form style=’display: none’><input name=’__detectback’ id=’__detectback’ value=”></form>");

function checkPageBackOrRefresh(load_id) {
if (document.getElementById(‘__detectback’).value == load_id) {
return true;
} else {
document.getElementById(‘__detectback’).value = load_id;
return false;
}
}

window.onload = function() {
if (checkPageBackOrRefresh(‘tt’))
alert(‘You clicked back or refreshed the page’);
}

</script>

</html>[/html]

Unfortunately, this solution does not work in some browsers where “fast back” (ie, fbcache in firefox) is enabled, as the fast back stores the scripting state so a onload does not trigger again.

The script should work fine with IE7 and IE8. With Firefox, it only works on certain pages. These pages seems to be pages that link to heavy javascripts (ie jquery?).

With fbcache enabled browsers, a possible solution would be to hide the message at the event onbeforeunload so it will not appear even when clicking back.

Pondering per user accounting in Linux

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I’ve been researching for the better part of the day on what the best method to account for bandwidth (and cpu/memory) used by a particular user is. This is useful if you run a hosting business and give out shell access. At first I was looking for a way to meter SSH. There seems to be an old patch for it, but as I continued reading, a old mailing from a mailing list pointed out that there are heaps of ways to generate traffic when you have a shell account (ie wget). In fact you don’t even need shell access – any scripting language that could download will consume bandwidth that may not be accounted for.

So this began my quest to find the best solution to per user accounting in linux. The basic concept is that since the bandwidth consumption is triggered by a process, and owned by a specific user, we should be able to trace traffic to a user and record as such. The advantage is even greater if you run peruser mpm apache or suexec’d php.

I began looking at netfilter/iptables, which had a match -m owner uid. This works only on the OUTPUT chain and will tell you who sent the packet, but unfortunately doesn’t tell you who a packet was destined for.

iptables has a connection tracking feature, that tracks active connections, allowing for stateful packet inspection. If you have the kernel feature enabled, it will also count the traffic numbers, which you can then view in /proc/net/ip_conntrack (or /proc/net/nf_conntrack for newer installations). Using that, and cross referencing it with the netstat -anp and process table will give you an idea of which user owns the connection. This is assuming of course that the process doesn’t setuid to change users.

But then, how are we going to collect all the data? Polling would be extremely slow and tedious and you might miss short lived connections. It seems that using libnetfilter_conntrack, you can subscribe to an event that notifies when connection states have changed (CONFIG_IP_NF_CONNTRACK_EVENTS). Using that, you can record when connections are opened and when they are closed as they happen.

What about processes? Processing accounting can be easily taken care of by the unix acct tools, which monitors processes as they are created and destroyed, provided you have the correct kernel options enabled. But what if you don’t have this option, ie on a VPS – Is there an alternative? The answer is yes, but ugly. You might remember that process information can be access via /proc. What if I set inotify, the file system change mechanism to tell me when /proc has changed? Somebody already thought of this and found it didn’t work quite as expected. The reason for this was mentioned in the linked thread, but the responders did give a good alternative – using ptrace ().

The ptrace command is a powerful unix system call that can manipulate processes it has attached to. It is what the debugger gdb uses to debug running applications. Using the ptrace function, you can set an option to notify the controlling process via SIGTRAP that the ptrace’d process has terminated, or forked/execed. Using this, you can potentially hook into every process and closely monitor their lifecycle. The downside is that you cannot have two ptrace active on the same process, which means application like gdb will fail if your monitoring system is active. Since ptrace is primarily used for debugging, it may also degrade performance of application it has been attached to. So the bottom line is that it looks like it is too extravagant and thus the wrong way to go for implementing a process accounting/monitoring system.

Looks like my quest to find a viable way of accounting per user accounting has so far eluded me. Perhaps the old ways of individual accounting in every application service – apache/ftp/imap/smtp is here to stay.