RFR 9: 8087286 Need a way to handle control-C and possibly some other signals

[Roger Riggs]

Hi Peter,

On 2/2/2016 6:44 AM, Peter Levart wrote:
> Hi Roger,
>
> Is this public API supposed to replace sun.misc.Signal? I don't see 
> why not. Comparing the APIs, thought they are different in style, I 
> can't find a feature of sun.misc.Signal that wouldn't be supported by 
> java.util.Signal except for NativeSignalHandler which in my 
> understanding is currently only used to implement the 
> SignalHandler.SIG_DFL and SignalHandler.SIG_IGN special "native" 
> handlers. Is there any use of NativeSignalHandler to dispatch to 
> arbitrary native procedure?
I don't think so and have not included it in the replacement API.
>
> As to the implementation: Registering signal handlers with this API is 
> protected by a runtime permission. Nevertheless, since this is public 
> API and handlers are supplied by user code, should dispatching to the 
> handlers be performed in a specially prepared Thread with no 
> permissions? Current dispatch implementation constructs new Thread 
> with constructor that uses the AccessController.getContext() inherited 
> from the thread that calls the Signal.dispatch() method which is a 
> special "Signal Dispatcher" thread spawned in the VM 
> (os::signal_init). I haven't dug any deeper, but is it possible that 
> "Signal Dispatcher" thread uses system protection domain?
yes, the spawned thread should follow the typical use of InnocuousThread.
The Signal Dispatcher thread has very limited code it executes and does 
not need any privs either.
Will check.
>
> As to the API. Is registerDefault() really needed to be exposed to the 
> public? I can see it is currently used to install default non-native 
> handler(s) for TERM, INT, HUP signals in the boot-up sequence in 
> j.l.System. Do you want users to be able to override these default 
> handlers?
There is not a strong use case and it can be removed from the public API.
> Also the new implementation only sets the defaultConsumer field which 
> is used only when some normal handler is unregister-ed, so this does 
> not work correctly...
WIll fix.
>
> Also if this is to become public API, There's a chance users would 
> want to add a handler to the chain of existing handlers or override 
> them. So what about an API that allows registering/unregistering a 
> default (non-native) handler and other handlers above it in a uniform 
> way, like:
The problem with chaining, as in the current API, is that there is no 
way to know what the next
handler in the chain will do.  If it is the default one for INT, TERM, 
HUP, it will call Shutdown and exit.
So without extra information and cooperation chaining is risky.
If the handler knows something about the other actors in the 
environment, it can coordinate with them directly.
For the use cases that have been raised for existing use of 
sun.misc.Signal, they are simple interactive
environments that want to give the appearance of being able to interrupt 
using control-c.

I've been aiming for the simplest API that would support the current use 
cases.

Thanks, Roger


>
>
> public final class Signal {
>
>     private static final ConcurrentMap<String, Signal>
>         SIGNAL_BY_NAME = new ConcurrentHashMap<>(4);
>     private static final ConcurrentMap<Integer, Signal>
>         SIGNAL_BY_NUMBER = new ConcurrentHashMap<>(4);
>
>     public static Signal of(String name) {
>         Signal signal = SIGNAL_BY_NAME.get(name);
>         if (signal != null) {
>             return signal;
>         }
>
>         int number;
>         if (!name.startsWith("SIG") || name.length() <= 3 ||
>             (number = findSignal0(name.substring(3))) < 0) {
>             throw new UnsupportedOperationException("Unknown signal: " 
> + name);
>         }
>
>         signal = SIGNAL_BY_NUMBER.computeIfAbsent(
>             number,
>             new Function<Integer, Signal>() {
>                 @Override
>                 public Signal apply(Integer number) {
>                     return new Signal(name, number);
>                 }
>             }
>         );
>
>         SIGNAL_BY_NAME.putIfAbsent(name, signal);
>
>         return signal;
>     }
>
>     private final String name;
>     private final int number;
>     private volatile HandlerChain handlerChain;
>     private long savedNativeHandler;
>
>     private Signal(String name, int number) {
>         this.name = name;
>         this.number = number;
>     }
>
>     public String name() { return name; }
>
>     public int number() { return number; }
>
>     public void raise() { raise0(number); }
>
>     public void register(BiConsumer<Signal, Runnable> handler) {
>         synchronized (this) {
>             HandlerChain oldChain = handlerChain;
>             handlerChain = new HandlerChain(handler, oldChain);
>             if (oldChain == null) {
>                 // set native to dispatch to Singnal.dispatch()
>                 savedNativeHandler = handle1(2);
>             }
>         }
>     }
>
>     public boolean unregister(BiConsumer<Signal, Runnable> handler) {
>         synchronized (this) {
>             HandlerChain oldChain = handlerChain;
>             if (oldChain != null && oldChain.handler == handler) {
>                 if (oldChain.next == null) {
>                     // restore saved native handler
>                     long oldNativeHandler = handle1(savedNativeHandler);
>                     assert oldNativeHandler == 2L;
>                 }
>                 handlerChain = oldChain.next;
>                 return true;
>             } else {
>                 return false;
>             }
>         }
>     }
>
>     // following two should probably be hidden from public API
>
>     public void nativeIgnore() {
>         synchronized (this) {
>             if (handlerChain == null) {
>                 handle1(1); // ignore signal
>             } else {
>                 throw new IllegalStateException(
>                     "Can't ignore signal after handlers have already 
> been registered.");
>             }
>         }
>     }
>
>     public void nativeDefault() {
>         synchronized (this) {
>             if (handlerChain == null) {
>                 handle1(0); // default native handler
>             } else {
>                 throw new IllegalStateException(
>                     "Can't restore signal after handlers have already 
> been registered.");
>             }
>         }
>     }
>
>     private long handle1(long nativeHandler) {
>         long oldNativeHandler = handle0(number, nativeHandler);
>         if (oldNativeHandler == -1L) {
>             throw new UnsupportedOperationException(
>                 "Signal already used by VM or OS: " + name);
>         }
>         return oldNativeHandler;
>     }
>
>     /*
>      * Called by the VM to execute Java signal handlers.
>      */
>     private static void dispatch(int number) {
>         Signal signal = SIGNAL_BY_NUMBER.get(number);
>         if (signal != null) {
>             HandlerChain handlerChain = signal.handlerChain;
>             if (handlerChain != null) {
>                 new InnocuousThread(() -> handlerChain.accept(signal))
>                     .start();
>             }
>         }
>     }
>
>     /**
>      * Find the signal number, given a name.
>      *
>      * @param sigName the signal name
>      * @return the signal number or -1 for unknown signals.
>      */
>     private static native int findSignal0(String sigName);
>
>     /* Registers a native signal handler, and returns the old handler.
>      * Handler values:
>      *   0     default handler
>      *   1     ignore the signal
>      *   2     call back to Signal.dispatch
>      *   other arbitrary native signal handlers
>      * @param nativeH the index or address of the new signal handler
>      * @return the previous index or address
>      */
>     private static native long handle0(int sig, long nativeH);
>
>     /*
>      * Raise a given signal number.
>      * @param sig the signal number to raise
>      */
>     private static native void raise0(int sig);
>
>
>     private static class HandlerChain implements Consumer<Signal> {
>         final BiConsumer<Signal, Runnable> handler;
>         final HandlerChain next;
>
>         HandlerChain(BiConsumer<Signal, Runnable> handler, 
> HandlerChain next) {
>             this.handler = handler;
>             this.next = next;
>         }
>
>         @Override
>         public void accept(Signal signal) {
>             handler.accept(signal, () -> {
>                 if (next != null) {
>                     next.accept(signal);
>                 }
>             });
>         }
>     }
> }
>
>
>
>
> Regards, Peter
>
>
> On 02/01/2016 05:02 PM, Roger Riggs wrote:
>> Please review an API addition to handle signals such as SIGINT, 
>> SIGHUP, and SIGTERM.
>> This JEP 260 motivated alternative to sun.misc.Signal supports the 
>> use case for
>> interactive applications that need to handle Control-C and other 
>> signals.
>>
>> The new java.util.Signal class provides a settable primary signal 
>> handler and a default
>> signal handler.  The primary signal handler can be unregistered and 
>> handling is restored
>> to the default signal handler.  System initialization registers 
>> default signal handlers
>> to terminate on SIGINT, SIGHUP, and SIGTERM.  Use of the Signal API 
>> requires
>> a permission if a SecurityManager is set.
>>
>> The sun.misc.Signal implementation is modified to be layered on a common
>> thread and dispatch mechanism. The VM handling of native signals is 
>> not affected.
>> The command option to reduce signal use by the runtime with -Xrs is 
>> unmodified.
>>
>> The changes to hotspot are minimal to rename the hardcoded callback 
>> to the Java
>> Signal dispatcher.
>>
>> Please review and comment on the API and implementation.
>>
>> javadoc:
>> http://cr.openjdk.java.net/~rriggs/signal-doc/
>>
>> Webrev:
>> jdk: http://cr.openjdk.java.net/~rriggs/webrev-signal-8087286/
>> hotspot: http://cr.openjdk.java.net/~rriggs/webrev-hs-signal-8087286/
>>
>> Issue:
>> https://bugs.openjdk.java.net/browse/JDK-8087286
>>
>> JEP 260:
>> https://bugs.openjdk.java.net/browse/JDK-8132928
>>
>> Thanks, Roger
>>
>>
>