Fix: add Thread.start/stop_world + remove GC.sig_suspend/resume

Thread is now the entrypoint for stopping the world. Delegating to the
GC is an implementation detail.

The `sig_resume` and `sig_suspend` properties are now set on
Crystal::System::Thread and only for UNIX.

src/crystal/system/thread.cr

@@ -196,6 +196,14 @@ class Thread
   def resume : Nil
     system_resume
   end
+
+  def stop_world : Nil
+    GC.stop_world
+  end
+
+  def start_world : Nil
+    GC.start_world
+  end
 end
 
 require "./thread_linked_list"

src/crystal/system/unix/pthread.cr

@@ -172,13 +172,13 @@ module Crystal::System::Thread
       # block all signals but sig_resume
       mask = LibC::SigsetT.new
       LibC.sigfillset(pointerof(mask))
-      LibC.sigdelset(pointerof(mask), GC.sig_resume)
+      LibC.sigdelset(pointerof(mask), sig_resume)
 
       # suspend the thread until it receives the sig_resume signal
       LibC.sigsuspend(pointerof(mask))
     end
     LibC.sigemptyset(pointerof(action.@sa_mask))
-    LibC.sigaction(GC.sig_suspend, pointerof(action), nil)
+    LibC.sigaction(sig_suspend, pointerof(action), nil)
   end
 
   private def self.install_sig_resume_signal_handler
@@ -188,13 +188,13 @@ module Crystal::System::Thread
       # do nothing (a handler is still required to receive the signal)
     end
     LibC.sigemptyset(pointerof(action.@sa_mask))
-    LibC.sigaction(GC.sig_resume, pointerof(action), nil)
+    LibC.sigaction(sig_resume, pointerof(action), nil)
   end
 
   private def system_suspend : Nil
     @suspended.set(false)
 
-    if LibC.pthread_kill(@system_handle, GC.sig_suspend) == -1
+    if LibC.pthread_kill(@system_handle, Thread.sig_suspend) == -1
       System.panic("pthread_kill()")
     end
   end
@@ -206,10 +206,30 @@ module Crystal::System::Thread
   end
 
   private def system_resume : Nil
-    if LibC.pthread_kill(@system_handle, GC.sig_resume) == -1
+    if LibC.pthread_kill(@system_handle, Thread.sig_resume) == -1
       System.panic("pthread_kill()")
     end
   end
+
+  protected class_property(sig_suspend : Int32) do
+    # follows bdwgc
+    {% if flag?(:linux) %}
+      LibC::SIGPWR
+    {% elsif LibC.has_constant?(:SIGRTMIN) %}
+      LibC::SIGRTMIN + 6
+    {% else %}
+      LibC::SIGXFSZ
+    {% end %}
+  end
+
+  protected class_property(sig_resume : Int32) do
+    # follows bdwgc
+    {% if LibC.has_constant?(:SIGRTMIN) %}
+      LibC::SIGRTMIN + 5
+    {% else %}
+      LibC::SIGXCPU
+    {% end %}
+  end
 end
 
 # In musl (alpine) the calls to unwind API segfaults

src/crystal/system/win32/thread.cr

@@ -101,10 +101,7 @@ module Crystal::System::Thread
   private def system_wait_suspended : Nil
     # context must be aligned on 16 bytes but we lack a mean to force the
     # alignment on the struct, so we overallocate then realign the pointer:
-    #
-    # we only need to reserve UInt8[sizeof(LibC::CONTEXT) + 16] but that's
-    # invalid crystal
-    local = uninitialized LibC::CONTEXT[2]
+    local = uninitialized UInt8[sizeof(Tuple(LibC::CONTEXT, UInt8[15]))]
     thread_context = Pointer(LibC::CONTEXT).new(local.to_unsafe.address &+ 15_u64 & ~15_u64)
     thread_context.value.contextFlags = LibC::CONTEXT_FULL
 

src/gc/boehm.cr

@@ -162,11 +162,6 @@ lib LibGC
   fun set_warn_proc = GC_set_warn_proc(WarnProc)
   $warn_proc = GC_current_warn_proc : WarnProc
 
-  fun get_suspend_signal = GC_get_suspend_signal : Int
-  fun get_thr_restart_signal = GC_get_thr_restart_signal : Int
-  fun set_suspend_signal = GC_set_suspend_signal(Int) : Int
-  fun set_thr_restart_signal = GC_set_thr_restart_signal(Int) : Int
-
   fun stop_world_external = GC_stop_world_external
   fun start_world_external = GC_start_world_external
 end
@@ -479,26 +474,6 @@ module GC
     end
   {% end %}
 
-  # :nodoc:
-  def sig_suspend : Int32
-    LibGC.get_suspend_signal
-  end
-
-  # :nodoc:
-  def sig_suspend=(value : Int32) : Int32
-    LibGC.set_suspend_signal(value)
-  end
-
-  # :nodoc:
-  def sig_resume : Int32
-    LibGC.get_thr_restart_signal
-  end
-
-  # :nodoc:
-  def sig_resume=(value : Int32) : Int32
-    LibGC.set_thr_restart_signal(value)
-  end
-
   # :nodoc:
   def self.stop_world : Nil
     LibGC.stop_world_external

src/gc/none.cr

@@ -142,52 +142,24 @@ module GC
   #
   # This isn't a GC-safe stop-the-world implementation (it may allocate objects
   # while stopping the world), but the guarantees are enough for the purpose of
-  # gc_none.
+  # gc_none. It could be GC-safe if Thread::LinkedList(T) became a struct, and
+  # Thread::Mutex either became a struct or provide low level abstraction
+  # methods that directly interact with syscalls (without allocating).
   #
-  # Thread safety is guaranteed by the mutex in Crystal::PointerLinkedList:
-  # either a thread is starting and hasn't added itself to the list (it will
-  # block until it can acquire the lock), or is currently adding itself (the
-  # current thread will block until it can acquire the lock).
+  # Thread safety is guaranteed by the mutex in Thread::LinkedList: either a
+  # thread is starting and hasn't added itself to the list (it will block until
+  # it can acquire the lock), or is currently adding itself (the current thread
+  # will block until it can acquire the lock).
   #
   # In both cases there can't be a deadlock since we won't suspend another
-  # thread until it has successfuly added (or removed) itself to the linked
-  # list and released the lock, and the other thread won't progress until it
-  # can add (or remove) itself from the list.
+  # thread until it has successfuly added (or removed) itself to (from) the
+  # linked list and released the lock, and the other thread won't progress until
+  # it can add (or remove) itself from the list.
   #
-  # Finally, we lock the mutex and keep it locked until we resume the world,
-  # so any thread waiting on the mutex will only be resumed when the world is
+  # Finally, we lock the mutex and keep it locked until we resume the world, so
+  # any thread waiting on the mutex will only be resumed when the world is
   # resumed.
 
-  # :nodoc:
-  class_property(sig_suspend : Int32) do
-    # follows bdwgc
-    {% if flag?(:unix) %}
-      {% if flag?(:linux) %}
-        LibC::SIGPWR
-      {% elsif LibC.has_constant?(:SIGRTMIN) %}
-        LibC::SIGRTMIN + 6
-      {% else %}
-        LibC::SIGXFSZ
-      {% end %}
-    {% else %}
-      -1
-    {% end %}
-  end
-
-  # :nodoc:
-  class_property(sig_resume : Int32) do
-    # follows bdwgc
-    {% if flag?(:unix) %}
-      {% if LibC.has_constant?(:SIGRTMIN) %}
-        LibC::SIGRTMIN + 5
-      {% else %}
-        LibC::SIGXCPU
-      {% end %}
-    {% else %}
-      -1
-    {% end %}
-  end
-
   # :nodoc:
   def self.stop_world : Nil
     current_thread = Thread.current