[fpc-pascal] HookSignal, "halt", and unhandled exceptions

[Seth Grover]

Greetings! An interesting problem I've run into:

Under Linux, I have a small example program written in C (we'll call
it cprog) which uses a FPC-compiled shared object library
(libfpcso1.so) inside which I want to catch access violations in
try/except blocks. I don't care about access violations in the C
program, and by "don't care" I mean "don't care if it halts the
program."

Thanks to HookSignal(RTL_SIGDEFAULT) (see
http://www.freepascal.org/docs-html/rtl/sysutils/hooksignal.html) once
the .so installs its signal handler, catching the exceptions in the
try/except blocks works beautifully.

However, if a segfault is raised in cprog, strange behavior happens.
The FPC signal handler catches the signal, and goes into the "handle
unhandled exception" code, which is what I wou
ld expect. It raises a runtime error 217, which is what I would
expect. However, it does not halt the program. What actually happens
is it gets into an infinite loop of raising runtime error 217. Here's
the output. Note that the first 3 lines of output are me testing that
the exceptions are handled correctly in the try/except block in the
.so, which they are. Then my c program causes a segfault, after which
I get the infinity r.e. 217 output:

===================================================
$ ./cprog

handled inside so 2: Access violation
handled inside so 2: Division by zero
called libc, handled inside so 2: Access violation

An unhandled exception occurred at $08048534 :
EAccessViolation : Access violation
  $08048534 line 50 of cprog.c

Runtime error 217 at $F7761740
  $F7761740
  $F77617B4
  $F7783DA5
  $F77647FE
  $F75D1BD6
  $08048431

Runtime error 217 at $F7761740
  $F7761740
  $F77617B4
  $F7783DA5
  $F77647FE
  $F75D1BD6
  $08048431

An unhandled exception occurred at $F77617EF :
EAccessViolation :
  $F77617EF
  $F7783DA5
  $F77647FE

Runtime error 217 at $F7761740
  $F7761740

Runtime error 217 at $F7761740
  $F7761740

Runtime error 217 at $F7761740
  $F7761740

Runtime error 217 at $F7761740
  $F7761740

An unhandled exception occurred at $F77617EF :
EAccessViolation :
  $F77617EF
  $F7783DA5
  $F77647FE
  $F7783DA5
  $F77647FE

Runtime error 217 at $F7761740
... (at this point it continues like this forever)
===================================================

It seems the cause of this is that halt, which calls InternalExit and
System_exit, which in turn calls haltproc, does not actually halt the
program in the case of a library. Instead, haltproc for the library
just calls InternalExit again and returns.

I took my test case to an even simpler one, and simple removed the
try/except block around the access violation in my test .so. So now,
all my test C program is call libfpcso1.so's "crash" function and I
get the infinite loop of r.e. 217's, never returning to the main
program at all.

I understand that it's only possible to have one signal handler
installed at a time (which, in this example, I'm perfectly happy with;
HookSignal does exactly what I want), but it seems wrong that an
unhandled exception in a library should cause your program to go into
an infinite loop, rather than doing the same thing an unhanded
exception would do in an executable.

Thoughts? Should I log it as a bug?

Thanks,

-SG

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Seth Grover
sethdgrover[at]gmail[dot]com