SIGRTMIN does not induce migration

[acarno]

TL; DR: Deliver SIGRTMIN+1 instead of SIGRTMIN to induce migration when using signal triggering.

When the migration library is compiled with type=signal_trigger, delivering SIGRTMIN to a migrateable process should cause a migration proposal. However, the process instead terminates -- the default action for SIGRTMIN.

On investigation, this stems from a discrepancy between the specified signal -- SIGRTMIN according to lib/migration/include/config.h -- and the actual signal that we set:

[anthony@fox4 11:28 NPB-make-popcorn-quiet-signal]$ gdb ./EP_B
...
(gdb) break sigaction
Breakpoint 1 at 0x51bf28: file src/signal/sigaction.c, line 55.
(gdb) run
Starting program: /home/anthony/NPB-make-popcorn-quiet-signal/EP_B

Breakpoint 1, __sigaction (sig=35, sa=0xfffffffff038, old=0x0) at src/signal/sigaction.c:55
55      src/signal/sigaction.c: No such file or directory.

On the second-to-last-line, we see sig=35 -- this is (on my machines at least) SIGRTMIN+1. The reason likely stems from how glibc implements POSIX threads. From man 7 signal:

The Linux kernel supports a range of 33 different real-time signals, numbered 32 to 64. However, the glibc POSIX threads implementation internally uses two (for NPTL) or three (for LinuxThreads) real-time signals (see pthreads(7)), and adjusts the value of SIGRTMIN suitably (to 34 or 35).

The simple solution then is to deliver SIGRTMIN+1 instead of SIGRTMIN.