SIgnal chaining, JVM_handle_(linux|bsd|aix)_signal, and backward compatibility

Thomas Stüfe thomas.stuefe at
Mon Nov 2 07:40:25 UTC 2020


While working on some signal handler fixes and cleanups ([1]), I noticed
that we export JVM_handle_(linux|bsd)_signal() on all POSIX platforms.

I wondered why we do this. See also my first question at runtime-dev [2] -
the initial reaction was "probably no reason anymore".

But then, I see a carefully crafted comment in [3]:

// This routine may be used by user applications as a "hook" to catch
// The user-defined signal handler must pass unrecognized signals to this
// routine

and I also found some bug reports [4], [5], from 2001 and 2004:
JDK-4864136 : "JVM_handle_linux_signal is private in 1.4.2-beta"
JDK-4408646 : "JVM_handle_solaris_signal must be a global function"

So, to me this looks like JVM_handle_xxx_signals() was an official, or at
least deliberate, interface for foreign code to interact with our signal
handling. Specifically, this reads like a third party could install its
signal handlers over ours, and as long as it queried our signal handler
first by calling JVM_handle_xxx_signal(), we still could coexist with it.

But now we have signal chaining [6], which achieves the same by preloading
the libjsig library. Which some consider to be ewww [7] :-), but still
seems to me like the official solution to that problem. Using signal
chaining there would be no need to manually call JVM_handle_xxx_signal()
from outside since the interposed sigaction() in libjsig would take care of
all this stuff automatically.

My question is:
- if JVM_handle_xxx_signal() is (had been?) an official interface, should
there not be some official documentation and regression tests? Was there? I
could not find anything.
- if it is not an official interface, would it be okay to lay it to rest?
  - Pro: it is made obsolete by signal chaining, and removing it would
reduce complexity of signal handling somewhat
  - Con: applications may exist out there which use that interface, and I'd
hate to break them. We give customers enough reasons to cling to old JDK
versions as it is.
     Also, arguably, this interface is useful. Not everyone likes
preloading libjsig, and the signal chaining mechanism is also a whole lot
more fragile.

However, if we let it live on, should we not document and test it?

I am really interested in the history of this. Was this just a solution for
a single customer?

Thanks, Thomas


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