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

David Holmes david.holmes at oracle.com
Thu Feb 4 00:59:21 UTC 2016

Adding back hotspot-runtime-dev

On 4/02/2016 2:07 AM, Roger Riggs wrote:
> Hi,
> The current wording is explicit about signal support being
> implementation and os dependent,
> it can say its release dependent too.

So you think an API that is implementation dependent, OS dependent and 
release dependent, is a good candidate for the Write-Once-Run-Anywhere 
Java platform as a core API addition ???

At best this should be in some side add-on (anyone remember javax.*).


> Roger
> On 2/3/2016 11:00 AM, Chris Hegarty wrote:
>> On 03/02/16 15:43, Thomas Stüfe wrote:
>>> On Wed, Feb 3, 2016 at 4:20 PM, Thomas Stüfe <thomas.stuefe at gmail.com>
>>> wrote:
>>>> On Wed, Feb 3, 2016 at 4:05 AM, David Holmes <david.holmes at oracle.com>
>>>> wrote:
>>>>> On 3/02/2016 8:08 AM, Stuart Marks wrote:
>>>>>> Hi Roger,
>>>>>> It will be good to get this into the JDK. Lots of people have been
>>>>>> asking for this.
>>>>> I think this API is a big mistake. The primary usecase seems to be
>>>>> control-C interception for utilities like jshell. Adding a general
>>>>> purpose
>>>>> signal raising and handling mechanism to the JDK does not seem like
>>>>> a good
>>>>> solution to me. While you would need to use signal management under
>>>>> the
>>>>> covers I think it would be much cleaner to expose an API that actually
>>>>> captures what it is you want here: a mechanism to manage
>>>>> "interrupt" and
>>>>> "terminate" events at the VM level, in a clean cross-platform way.
>>>> I agree with David. One problem I see is that it is difficult to write
>>>> portable java applications with this API. Not only are WIndows and
>>>> Posix
>>>> are very different, but also there are also sublte differences between
>>>> Posix platforms. For instance, in the jbs SIGTRAP was mentioned as a
>>>> possible signal someone wanted to raise, but SIGTRAP is used by the
>>>> JIT in
>>>> the powerpc port. So applications using Signal.of("SIGTRAP") would
>>>> run fine
>>>> on x86, but cause a crash on powerpc.
>>>> Kind Regards, Thomas
>>> Just occurred to me that a second, subtle problem may be that once java
>>> developers start using signals for their own application, we are barred
>>> from using the same signals in the future for our own purposes, even
>>> if we
>>> do not use them now. At least not without breaking those java
>>> applications.
>> This is an excellent point. The API should include appropriate wording
>> to indicate that there is no guarantee that an invocation of
>> Signal.of(...) that succeeds in one release, is guaranteed to succeed
>> in another, future, release.
>> -Chris.
>>> ..Thomas
>>>> Aside: If you want to see some prior art in this area look at
>>>>> PosixSignalHandler API in the Real-Time Specification for Java.
>>>>> Which reminds me - do you propose to support the POSIX real-time
>>>>> signals?
>>>>> David
>>>>> -----
>>>>> I have a few comments on the API.
>>>>>> 1) Is there a way to query the set of signals supported? This
>>>>>> might be a
>>>>>> Set<String> returned by a static method, for example. I agree that
>>>>>> signal strings outside this set shouldn't be supported.
>>>>>> 2) The Signal class spec mentions SIGINT, SIGHUP, and SIGTERM
>>>>>> explicitly. Are these required to be implemented on all platforms, or
>>>>>> just on "unix-like" platforms, are they just examples? What
>>>>>> signals are
>>>>>> available on Windows?
>>>>>> 3) raise() is spec'd to throw an exception if there's no handler
>>>>>> registered. But wouldn't it make sense to allow it if the default
>>>>>> handler is registered?
>>>>>> 4) In an earlier message you said that the Signal object is a
>>>>>> capability, so the security check is on getting a reference. It
>>>>>> seems to
>>>>>> me that setting a handler is in a different category from raising a
>>>>>> signal; this suggests to me that using the same object as a
>>>>>> capability
>>>>>> for both should be rethought.
>>>>>> 5) I don't understand the asymmetry between register() and
>>>>>> unregister().
>>>>>> Your earlier exchanges with Chris and with Gerard touched on this,
>>>>>> specifically, the requirement that the caller pass unregister() a
>>>>>> reference to the old handler in order for unregistration to work. You
>>>>>> had said this was safer, if there are uncoordinated pieces of code
>>>>>> attempting to set/unset signal handlers.
>>>>>> It looks to me like this API is really about maintaining process
>>>>>> global
>>>>>> state consisting of a single handler -- user-specified or default
>>>>>> -- for
>>>>>> each supported signal. (I agree that it shouldn't try to have a
>>>>>> stack or
>>>>>> a chain of handlers.) There are a few other things that are global
>>>>>> like
>>>>>> this, such as the security manager and policy,
>>>>>> System.setIn/Out/Err, and
>>>>>> so forth. As such, uncoordinated access to the signal API is
>>>>>> pretty much
>>>>>> broken no matter what. Thus I don't think it makes sense to have a
>>>>>> CAS-like protocol for unregistering a handler, to protect against the
>>>>>> case where "somebody else" might have registered a handler different
>>>>>> from yours.
>>>>>> Something like this might make sense:
>>>>>>       void register(Consumer<Signal> handler);
>>>>>>       void unregister();
>>>>>> The register() call would be pretty much as currently specified; the
>>>>>> unregister() call would restore the default handler. Alternatively,
>>>>>> register(null) could be used instead of unregister(), but this is
>>>>>> quite
>>>>>> minor.
>>>>>> Thanks,
>>>>>> s'marks
>>>>>> On 2/1/16 8:02 AM, 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

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