Cracked Pipe

Pipes can get blocked as an old post of mine illustrates. So, how do we ensure that we can read and write to and from pipes in the effectful world?

Well, my first (rather poor) attempt was to use Java's PipedInputStream and PipedOutputStream and have ZIO handle the multithreading aspects of it. Although the test passes it is far from exhaustive.

One of the problems is that unbeknown to me that Java's piped IO classes are very thread sensitive. Consequently, my ZIO test was failing with:

    Fiber failed.

    A checked error was not handled.

    java.io.IOException: Read end dead

        at java.io.PipedInputStream.checkStateForReceive(PipedInputStream.java:262)

        at java.io.PipedInputStream.awaitSpace(PipedInputStream.java:268)

        at java.io.PipedInputStream.receive(PipedInputStream.java:231)

        at java.io.PipedOutputStream.write(PipedOutputStream.java:149)

        at java.io.OutputStream.write(OutputStream.java:75)

        at uk.co.odinconsultants.fp.zio.streams.LargePipeMain$.writing$1(LargePipeMain.scala:42)

        at uk.co.odinconsultants.fp.zio.streams.LargePipeMain$.$anonfun$piping$3(LargePipeMain.scala:58)

        at zio.internal.FiberContext.evaluateNow(FiberContext.scala:458)

        at zio.internal.FiberContext.$anonfun$evaluateLater$1(FiberContext.scala:687)

        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)

        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)

        at java.lang.Thread.run(Thread.java:748)

This happens because the JDK code has something in it that looks like this:

...

        } else if (readSide != null && !readSide.isAlive()) {

            throw new IOException("Read end dead");

        }

...

and to be fair, the Java API documentation says:

"A pipe is said to be broken if a thread that was providing data bytes to the connected piped output stream is no longer alive."

But if your effect system uses fibres instead of threads then it is free to stop and start threads as it pleases. The bottom line is that wrapping side effecting code in a ZIO or a Cats IO might not be the first step in making your code more FP.