Awasu » 2017 » January
Tuesday 31st January 2017 9:27 PM [Awasu News]

Awasu 3.0.4.alpha3 has been released here, mostly minor bug fixes and enhancements. Only a few things left to do, and that'll close off this release cycle - woo hoo! :clap:

Wednesday 25th January 2017 6:05 PM [General]

Awasu has had many optimizations made to it over the past few releases, and one of them was a change to how it captures output from plugins. The old code used to set up temp files to capture stdout and stderr, configured the plugin process to send its output to them, then read the captured content when the plugin finished. This was changed in 3.0.4.alpha2 to use in-memory buffers, thus removing the need to use slow disk files.

This worked pretty well, except for one rare, bizarre problem: if Awasu crashed was forcibly terminated via Task Manager while plugins were running, when it was restarted, it was unable to open the server socket it normally listens on for API requests, because another process had it open. The plugins would also hang indefinitely. Awasu was recently changed to automatically find an alternative port, which is kinda cool, but (1) it also kinda sucks if you have scripts that have this port hard-coded, and (2) it's indicative of a serious problem, a zombie kinda problem 🙁

When Awasu is started, it checks to see if another copy is already running, and transfers control to that if it is, so the old process was dead enough that the new copy of Awasu wasn't seeing it, but it was alive enough that it was keeping the old socket (and presumably other stuff) open.

I hate zombies.

I finally figured out what the problem was, and thought it might make a good blog post, since it was an interesting situation, and as a PSA to correct an omission in the MSDN example. However, when I went looking for the MSDN example, it did the necessary magic to avoid this problem :wall: but since it doesn't explain why it performs these incantations, it's maybe worth jotting down these notes for anyone else running into the same problem...

The problem

The old code[1]Error handling has been removed for clarity. created the pipe [2]Awasu captures both stdout and stderr, but stderr is not important for this example, and so is omitted. and launched the plugin process something like this:

// create a pipe to capture stdout
SECURITY_ATTRIBUTES secAttrs ; 
memset( &secAttrs , 0 , sizeof(secAttrs) ) ; 
secAttrs.nLength = sizeof(secAttrs) ; 
secAttrs.bInheritHandle = TRUE ; // the handles returned will be inheritable
HANDLE hStdoutRead , hStdoutWrite ; 
BOOL rc = CreatePipe( &hStdoutRead , &hStdoutWrite , &secAttrs , 0 ) ;

// start the child process
STARTUPINFO startupInfo ; 
memset( &startupInfo , 0 , sizeof(startupInfo) ) ; 
startupInfo.cb = sizeof(startupInfo) ; 
startupInfo.dwFlags = STARTF_USESHOWWINDOW | STARTF_USESTDHANDLES ;
startupInfo.wShowWindow = SW_HIDE ;
startupInfo.hStdOutput = hStdoutWrite ;
PROCESS_INFORMATION processInfo ;
BOOL rc = CreateProcess(
    NULL , pCmdLine ,
    NULL , NULL ,
    TRUE , // the child process will get our inheritable handles
    CREATE_NO_WINDOW ,
    NULL , pWorkingDir ,
    &startupInfo , &processInfo
) ;

The idea is that we create a pipe to capture the output from the plugin process, with 2 handles to access it[3]There are some things you need to be careful about when using this technique, but they are (sorta) well-known. One (hStdoutWrite) is given to the plugin process so that it can write to the pipe[4]It's actually connected to the process' stdout, and it doesn't even know anything funky is going on., while the other (hStdoutRead) is used by Awasu to read from it.

Inheriting handles

But what does it mean to inherit handles, anyway?

When you open something like a file, or socket, or registry key, the Windows kernel creates an in-memory object to manage it, then gives you a handle to it, which is an alias for the underlying kernel object. The thing about these aliases is:

  • they're only valid within one process (typically the one that opened the underlying object)
  • there can be more than one of them, even across multiple processes, referring to the same underlying object

When the code above runs, Windows creates the pipe buffer and 2 objects to manage reading/writing it, and gives us handles (i.e. aliases) to those underlying kernel objects:

  • handle #1 (an alias for kernel object #12[5]These numbers are internal to the kernel, and have no meaning to user programs.
  • handle #2 (an alias for kernel object #47)


When the child process is created, it inherits these handles[6]Because CreateProcess() was told to do this, and these handles were created as inheritable. i.e. it gets its own copy of them, that refer to the same underlying kernel objects:

The child process needs to inherit these, since it needs the pipe write handle. When it outputs something, it goes to hStdoutWrite[7]Because we passed it in to CreateProcess() as the process's stdout., and ends up in the pipe buffer. From there, Awasu can read it via its hStdoutRead handle.


The solution

You would think that if the reading end of a pipe disappears, anyone trying to write to it would get a "broken pipe" error, but as described above, the plugins would hang indefinitely and the parent awasu.exe got stuck in zombie limbo 🙁

If we update our diagram to reflect Awasu disappearing, we can see what's happening:

The child process also inherited a copy of the pipe's read handle (even though it doesn't actually use it), and since the process is still running, Windows thinks the underlying kernel object is still in use and so can't close it. So, since something is still connected to the reading end of the pipe, it's still alive, the plugin merrily writes its output to the pipe buffer, until it fills up, and then waits for someone to read the output, to free up some space so that it can write out more output. Except, of course, the only thing with a read handle to the pipe is the child process itself[8]Which it never reads from, since it doesn't need to, that's Awasu's job., so it waits forever, and all this is also apparently enough to stop Windows from fully cleaning up the dead-but-not-quite Awasu process. Sigh...


Once we understand what's going on, the fix is easy: don't inherit the hStdoutRead handle. We set things up like this:


Now, if Awasu disappears, there are no more handles to the pipe's read handle, so Windows will close it, realize the pipe is broken, then returns an error to the plugin the next time it tries to write to it, and in the grand Shakespearian tradition, everyone dies as they should.



   [ + ]

1. Error handling has been removed for clarity.
2. Awasu captures both stdout and stderr, but stderr is not important for this example, and so is omitted.
3. There are some things you need to be careful about when using this technique, but they are (sorta) well-known
4. It's actually connected to the process' stdout, and it doesn't even know anything funky is going on.
5. These numbers are internal to the kernel, and have no meaning to user programs.
6. Because CreateProcess() was told to do this, and these handles were created as inheritable.
7. Because we passed it in to CreateProcess() as the process's stdout.
8. Which it never reads from, since it doesn't need to, that's Awasu's job.
Tuesday 3rd January 2017 9:07 PM [General]

I'm not a big one for browser plugins and add-ons, but one thing I absolutely love is this simple, but very cool, bookmarklet. In the author's own words:

I hate sticky headers. I want to kill sticky headers.

So I made this bookmarklet.

It removes those unbelievably annoying full-page overlays, header bars, chat boxes, anything that stays in a fixed position, from a web page[1]Technobabble: it looks for anything with a CSS position of "fixed" and removes it from the DOM.. I so want this in Awasu, so I was mulling over how best to implement it, when it occurred to me that you could already do it :wall:

Simply create a new user tool[2]Choose "Organize user tools..." from the Tools menu. called, say, "Kill stickies", with the following Javascript:
javascript:(function(){(function(){var i,elements=document.querySelectorAll('body *');for(i=0;i<elements.length;i++){if(getComputedStyle(elements[i]).position==='fixed'){elements[i].parentNode.removeChild(elements[i]);}}})()})()

Then, when you browse to a page that has these pesky things on it, just run the user tool and they'll be removed from the page! 😎

Wishing y'all a great and sticky-free 2017 ::-):

   [ + ]

1. Technobabble: it looks for anything with a CSS position of "fixed" and removes it from the DOM.
2. Choose "Organize user tools..." from the Tools menu.