Fix Valgrind memory leak warning by freeing alternate signal stack

[kojix2]

Hello,

This is a proof-of-concept pull request.

Recently, it was reported on the forum that when running Crystal programs with Valgrind, the alternate signal stack allocated by LibC.malloc is not freed before the program ends. Instead, the operating system frees it after the program finishes.

https://forum.crystal-lang.org/t/valgrind-memory-leak-in-helloworld-app-crystal-lang/7583

emrum

==387674== HEAP SUMMARY:
==387674== in use at exit: 8,192 bytes in 1 blocks
==387674== total heap usage: 15 allocs, 14 frees, 12,336 bytes allocated

HertzDevil

For the record, the 8192 bytes leaked here are the alternate stack for the signal handler, which is allocated using LibC.malloc directly

To fix this, I added an at_exit signal handler that switches back to the default stack and then frees the alternate stack. This stops Valgrind from showing a "memory leak" warning.

This pull request also adds a new system-level at_exit handler in AtExitHandlers. I created this because it is not good practice to use the regular at_exit for system tasks. The new handler can be used to free memory allocated by LibC.malloc and handle other system cleanup.

Thank you for reviewing this.

[kojix2]

hello.cr

puts "hello world"

Before

git checkout master
bin/crystal build ./hello.cr
valgrind ./hello

==122981== HEAP SUMMARY:
==122981==     in use at exit: 8,192 bytes in 1 blocks
==122981==   total heap usage: 15 allocs, 14 frees, 12,336 bytes allocated
==122981== 
==122981== LEAK SUMMARY:
==122981==    definitely lost: 8,192 bytes in 1 blocks
==122981==    indirectly lost: 0 bytes in 0 blocks
==122981==      possibly lost: 0 bytes in 0 blocks
==122981==    still reachable: 0 bytes in 0 blocks
==122981==         suppressed: 0 bytes in 0 blocks
==122981== Rerun with --leak-check=full to see details of leaked memory

After

git checkout altstack
bin/crystal build ./hello.cr
valgrind ./hello

==123551== HEAP SUMMARY:
==123551==     in use at exit: 0 bytes in 0 blocks
==123551==   total heap usage: 15 allocs, 15 frees, 12,336 bytes allocated
==123551== 
==123551== All heap blocks were freed -- no leaks are possible

[crysbot]

This pull request has been mentioned on Crystal Forum. There might be relevant details there:

https://forum.crystal-lang.org/t/valgrind-memory-leak-in-helloworld-app-crystal-lang/7583/13