On 21/06/2010 03:51, Dimitry Golubovsky wrote:
I am experimenting with the new feature in GHC such as custom Handles. I use GHC 6.12.2, base 4.2.0.1
I have pasted the code in question as http://hpaste.org/fastcgi/hpaste.fcgi/view?id=26413#a26413
I am trying to create a Handle using a thread as data producer. In my example is is a union of directories, and the Handle contents is newline-separated list of full paths of all files in the union. This list is produced by a thread which reads contents of all directories in the union, turns them into absolute filenames, and sends to the channel. The method of BufferedIO responsible for filling in the Handle's buffer reads data from the channel and places it in the buffer.
The problem is, after the contents of the Handle (via hGetContents) has been consumed, the program crashes with the following message:
<program name>: thread blocked indefinitely in an MVar operation
that is, the BlockedIndefinitelyOnMVar exception has occurred.
What is done wrong in my code? The dirClose method seems to be called (its output shows up), however with or without the call to killThread, the exception occurs anyway.
So the problem is that fillReadBuffer has to return 0 to indicate EOF, but you are killing the slave thread as soon as it has reached the end of the stream. So you get one call to fillReadBuffer that returns the data up to the end of the stream, and the next call that should return 0 blocks on the Chan indefinitely because the slave thread has already been killed. Cheers, Simon