Hi Stuart,
On 2/3/2016 5:32 PM, Stuart Marks wrote:
On 2/2/16 7:05 PM, David Holmes wrote:
On 3/02/2016 8:08 AM, Stuart Marks wrote:
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.
OK, I've looked some at the implementation, and there's more going on
here than meets the eye.
I was under the impression (or at least I was hoping) that the API
would expose a carefully curated set of signals that are (a) known to
be useful to applications, and (b) are safe for the JVM to allow
applications to handle. Examples of this would include SIGWINCH and
SIGTSTP, which are common for Unix applications to want to handle, as
well as the Control-C (SIGINT) case that jshell among others want to
handle.
The initial task was to provide a replacement for sun.misc.Signal.
Sun.misc.Signal provides to Java the same set of
signals the VM is able to handle and exposes via the JVM_findSignal,
JVM_RegisterSignal, and JVM_RaiseSignal.
The VM carefully handles the native state and delivers a Java safe
notification that the signal occurred.
But I tried out the patch and looked through the Hotspot signal
handling code, and the set of signals exposed is much broader than I
would have expected. On Mac OS X, the signals for which a handler can
be registered include the following:
SIGABRT
SIGALRM
SIGBUS
SIGCHLD
SIGCONT
SIGEMT
SIGHUP
SIGINT
SIGIO
SIGPIPE
SIGPROF
SIGSYS
SIGTERM
SIGTRAP
SIGTSTP
SIGTTIN
SIGTTOU
SIGURG
SIGUSR1
SIGUSR2
SIGVTALRM
SIGWINCH
SIGXCPU
SIGXFSZ
I'm quite surprised by this. It seems quite unwise to expose all of
these. Perhaps this is what David is concerned about. If so, I'm
starting to share his concern.
Java is used as an application programming language and environment and
as a system programming
environment. Java applications are intended to be competitive with
other programming environments, of which
most support handling and raising of signals. The native command
shells, python, etc all support signals
to control and manage applications. Each supports use of signals within
and between applications.
We have encouraged the use of Java for similar programming, process and
control functions.
Signals are a well known and used mechanism to communicate between the
OS and applications
and between applications. Some signal use is restricted because the
Java runtime
uses them (SIGSEGV, SIGFPE, SIGILL, etc.)
of them the application is restricted from using. The others are
handled uniformly to notify the client
and to be able to raise them.
Signals are a valuable communication mechanism and I don't think the
Java runtime should
arbitrarily restrict signal use based on an unquantified risk.
In addition, the signals for which a Signal instance can be gotten via
Signal.of(), but which cannot be handled (throws UOE), include:
SIGFPE
SIGILL
SIGKILL
SIGQUIT
SIGSEGV
SIGSTOP
The first 5, I'd understand can't be handled because the VM treats them
specially.
I'll need to dig into the VM differences to see why it is the way it is.
It's very strange to expose Signal instances representing these
signals when they can't be handled. They can't be raised either (at
least in Roger's first patch) since raising a signal is prohibited if
there's no handler installed.
Thanks, Roger
I still think a signal-handling API, even a system-specific one, can
be useful for a well curated set of signals. But this implementation
seems to bring an internal interface directly out to the API. That
doesn't seem like the right approach.
s'marks
