Use of signal handlers needs to be improved

[FDelporte]

As reported by Jim Darby:
If you're running as root (never a good idea) and using pigpio directly then pigpio steals all the signal handlers. The JVM doesn't like this as it uses them in running the JVM so it ends up with the program crashing. I'm working on this by getting pi4j-v2 to put back the signal handlers that the JVM uses.

[hackerjimbo]

As a first step I printed out the signal handled at the very start of Java_com_pi4j_library_pigpio_internal_PIGPIO_gpioInitialise to see what it was life before gpioInitialise is called. It looks like quite a lot.

Then I looked at the code in pigpio and found something very interesting. There is actually a compile-time #define, EMBEDDED_IN_VM, which explicitly turns off pigpio's use of signals!

Furthermore, there is a run-time flag as well. The code looks like:

#ifndef EMBEDDED_IN_VM
if (!(gpioCfg.internals & PI_CFG_NOSIGHANDLER))
sigSetHandler();
#endif

So, we can avoid the entire problem and use the OS-supplied pigpio by setting that flag!

This looks like the footprints of someone being here before. I'm going to guess that it's Python. Now all we have to do is to figure out how to set that flag.

[hackerjimbo]

Brace yourselves. Here's the fix (I hope). In the native C bindings we have the function Java_com_pi4j_library_pigpio_internal_PIGPIO_gpioInitialise. This contains the single line return gpioInitialise();.

Immediately before that insert gpioCfgSetInternals (gpioCfgGetInternals () | PI_CFG_NOSIGHANDLER);

I can't actually test this as I can't build an armv6 native library right now, but I strongly believe it should work.

[savageautomate]

@hackerjimbo ,

Here is the compiled JAR with the changes you suggested if you would like to test it out:

• https://www.dropbox.com/s/8cm176mbknk2s0m/pi4j-library-pigpio-2.0-SNAPSHOT.jar

pi4j-v2/libraries/pi4j-library-pigpio/src/main/native/com_pi4j_library_pigpio_internal_PIGPIO.c

Lines 155 to 166 in b3e40b7

	JNIEXPORT jint JNICALL Java_com_pi4j_library_pigpio_internal_PIGPIO_gpioInitialise
	(JNIEnv *env, jclass class)
	{
	// SEE: https://github.com/Pi4J/pi4j-v2/issues/15
	// By default, PIGPIO steals all the signal handlers. The JVM doesn't like this
	// as it uses them in running the JVM so it ends up with the program crashing.
	// The following should disable the signal handlers inside the PIGPIO library.
	gpioCfgSetInternals (gpioCfgGetInternals () | PI_CFG_NOSIGHANDLER);
	// perform initialization of the PIGPIO library
	return gpioInitialise();
	}

[savageautomate]

@hackerjimbo ,

While applying the fix you suggested, I also came across these lines of code:

pi4j-v2/libraries/pi4j-library-pigpio/src/main/java/com/pi4j/library/pigpio/impl/PiGpioNativeImpl.java

Lines 90 to 128 in 5eb404d

	public int gpioInitialise() throws IOException {
	int result = 0;
	logger.trace("[INITIALIZE] -> STARTED");
	if(!this.initialized) {
	// disable socket and pipes interfaces
	int rslt = PIGPIO.gpioCfgInterfaces(3);
	// initialize the PiGpio native library
	result = PIGPIO.gpioInitialise();
	validateResult(result);
	// creat a terminate handler for received signals
	PiGpioSignalCallback terminateSignalHandler = signum -> {
	logger.trace("------------ HERE");
	PIGPIO.gpioTerminate();
	System.exit(signum);
	};
	// apply our terminate handler to these default system signals
	PIGPIO.gpioSetSignalFunc(PiGpioConst.SIGHUP, terminateSignalHandler);
	PIGPIO.gpioSetSignalFunc(PiGpioConst.SIGINT, terminateSignalHandler);
	PIGPIO.gpioSetSignalFunc(PiGpioConst.SIGQUIT, terminateSignalHandler);
	PIGPIO.gpioSetSignalFunc(PiGpioConst.SIGKILL, terminateSignalHandler);
	PIGPIO.gpioSetSignalFunc(PiGpioConst.SIGTERM, terminateSignalHandler);
	PIGPIO.gpioSetSignalFunc(PiGpioConst.SIGSTOP, terminateSignalHandler);
	PIGPIO.gpioSetSignalFunc(PiGpioConst.SIGTSTP, terminateSignalHandler);
	// initialization successful
	this.initialized = true;
	logger.debug("[INITIALIZE] -- INITIALIZED SUCCESSFULLY");
	}
	else{
	logger.warn("[INITIALIZE] -- ALREADY INITIALIZED");
	}
	logger.trace("[INITIALIZE] <- FINISHED");
	return result;
	}

It looks like I was originally registering signal handlers with PiGPIO to try and capture exit/terminate signals and then terminate the PiGPIO library as well as terminate the JVM application with System.exit(signum);. This must be here because I was failing to get the Java application to exit cleanly when PiGPIO was stealing/handling the signals. I suspect this code could now be removed. What do you think?

[savageautomate]

REOPENING only to address the question above (#15 (comment)) about getting rid of Java-side signal handlers registered with PiGPIO. I suspect these can be removed now? Especially if the change @hackerjimbo suggested completely disables signals in PiGPIO.

[hackerjimbo]

@savageautomate Those signal handlers can now never be called as we no longer intercept the signals with PiGPIO. Hopefully :-)

Note that the signals aren't disabled, they're just handled by the JVM. Hopefully the JVM shutdown code should call the handler you registered.

Actually, I just tried a control-c on a running program. I got the following series of errors:

^C[main] WARN com.pi4j.library.pigpio.impl.PiGpioNativeImpl - PIGPIO ERROR: PI_BAD_HANDLE; unknown handle
[main] WARN com.pi4j.library.pigpio.impl.PiGpioNativeImpl - PIGPIO ERROR: PI_BAD_HANDLE; unknown handle
[main] WARN com.pi4j.library.pigpio.impl.PiGpioNativeImpl - PIGPIO ERROR: PI_BAD_HANDLE; unknown handle
[main] WARN com.pi4j.library.pigpio.impl.PiGpioNativeImpl - PIGPIO ERROR: PI_BAD_HANDLE; unknown handle
[main] WARN com.pi4j.library.pigpio.impl.PiGpioNativeImpl - PIGPIO ERROR: PI_BAD_HANDLE; unknown handle
[main] WARN com.pi4j.library.pigpio.impl.PiGpioNativeImpl - PIGPIO ERROR: PI_BAD_HANDLE; unknown handle
[main] WARN com.pi4j.library.pigpio.impl.PiGpioNativeImpl - PIGPIO ERROR: PI_BAD_HANDLE; unknown handle
[main] WARN com.pi4j.library.pigpio.impl.PiGpioNativeImpl - PIGPIO ERROR: PI_BAD_HANDLE; unknown handle
[main] WARN com.pi4j.library.pigpio.impl.PiGpioNativeImpl - PIGPIO ERROR: PI_BAD_HANDLE; unknown handle

[savageautomate]

Removed these signal callbacks and deployed update SNAPSHOT JARs.
CLOSED.