Introduce signal intercepting example

[GBuella]

It is a nice first attempt to demonstrate the idea, far from complete.

To give it a try, start the dummy program which installs a signal handler:

LD_LIBRARY_PATH=./examples LD_PRELOAD=libsignal_interceptor.so ./examples/signaller

Send it a USR1 signal, and see what happens.

Ref: #4

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This change is 

[GBuella]

BTW, this can be used to handle nasty pointers pass to pmemfile via syscall interfaces, e.g.:

thread_local sigjmp_buf user_pointer_jmb_buf;
thread_local bool jump_back_to_pmemfile;

pmemfile_write(pfp, file, buf, size)
{

  ssize_t result;
  jump_back_to_pmemfile = true;

  if (sigsetjmp(user_pointer_jmb_buf, 0) == 0) {
     memcpy(buf, pmemfile_block, size);
     result = size;
  } else {
    result = -EFAULT;
  }
  jump_back_to_pmemfile = false;
  return result;
}

signal_hook(int sig, siginfo_t *info, void *arg)
{
   if (sig == SIGSEGV && jump_back_to_pmemfile) {
      /* fault due to pointer from user */
      siglongjmp(user_pointer_jmb_buf, 1);
   }
}

[krzycz]

So, this info could be added to the source file and/or to some README.

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Reviewed 3 of 3 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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Comments from Reviewable

[sarahjelinek]

This build fails. Not quite sure why.

[GBuella]

I think there is something really wrong with the docker related scripts, which wasn't manifested until #48 was merged.
It might be a good a idea to copy docker build scripts from the pmemfile repo.

[codecov-io]

Codecov Report

Merging #46 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #46   +/-   ##
=======================================
  Coverage   69.71%   69.71%           
=======================================
  Files          19       19           
  Lines        1433     1433           
  Branches      391      391           
=======================================
  Hits          999      999           
  Misses        235      235           
  Partials      199      199

Flag	Coverage Δ
#pmemfile_tests	65.67% <ø> (ø)	⬆️
#regular_tests	56.38% <ø> (ø)	⬆️

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[marcinslusarz]

Can we use atomic and increase it before filling deferred_queue? That should solve this issue.

[GBuella]

Yes, that would solve this.
But, after I made this PR, I realized it is not easy to prove correctness under all circumstances. A signal can arrive anytime, while serving rt_sigaction, while executing the deferred signals, while executing a signal that arrived while handling a deferred signal... And what if the user installed a signal handler which writes to some log on segfault, or mmap's something into memory on segfault, to be able to continue -- we just defer such a signal, and try to resume instead, which doesn't work.
So, there are a lot of details to work out.