thread_sync.c: avoid reaching across stacks of dead threads

	rb_ensure is insufficient cleanup for fork and we must
	reinitialize all waitqueues in the child process.

	Unfortunately this increases the footprint of ConditionVariable,
	Queue and SizedQueue by 8 bytes on 32-bit (16 bytes on 64-bit).

	[ruby-core:86316] [Bug #14634]

	variable.c: fix thread + fork errors in autoload

	This is fairly non-intrusive bugfix to prevent children
	from trying to reach into thread stacks of the parent.
	I will probably reuse this idea and redo r62934, too
	(same bug).

	* vm_core.h (typedef struct rb_vm_struct): add fork_gen counter
	* thread.c (rb_thread_atfork_internal): increment fork_gen
	* variable.c (struct autoload_data_i): store fork_gen
	* variable.c (check_autoload_data): remove (replaced with get_...)
	* variable.c (get_autoload_data): check fork_gen when retrieving
	* variable.c (check_autoload_required): use get_autoload_data
	* variable.c (rb_autoloading_value): ditto
	* variable.c (rb_autoload_p): ditto
	* variable.c (current_autoload_data): ditto
	* variable.c (autoload_reset): reset fork_gen, adjust indent
	* variable.c (rb_autoload_load): set fork_gen when setting state
	* test/ruby/test_autoload.rb (test_autoload_fork): new test
	  [ruby-core:86410] [Bug #14634]

	thread_sync: redo r62934 to use fork_gen

	Instead of maintaining linked-lists to store all
	rb_queue/rb_szqueue/rb_condvar structs; store only a fork_gen
	serial number to simplify management of these items.

	This reduces initialization costs and avoids the up-front cost
	of resetting all Queue/SizedQueue/ConditionVariable objects at
	fork while saving 8 bytes per-structure on 64-bit.  There are no
	savings on 32-bit.

	* thread.c (rb_thread_atfork_internal): remove rb_thread_sync_reset_all call
	* thread_sync.c (rb_thread_sync_reset_all): remove
	* thread_sync.c (queue_live): remove
	* thread_sync.c (queue_free): remove
	* thread_sync.c (struct rb_queue): s/live/fork_gen/
	* thread_sync.c (queue_data_type): use default free
	* thread_sync.c (queue_alloc): remove list_add
	* thread_sync.c (queue_fork_check): new function
	* thread_sync.c (queue_ptr): call queue_fork_check
	* thread_sync.c (szqueue_free): remove
	* thread_sync.c (szqueue_data_type): use default free
	* thread_sync.c (szqueue_alloc): remove list_add
	* thread_sync.c (szqueue_ptr):  check fork_gen via queue_fork_check
	* thread_sync.c (struct rb_condvar): s/live/fork_gen/
	* thread_sync.c (condvar_free): remove
	* thread_sync.c (cv_data_type): use default free
	* thread_sync.c (condvar_ptr): check fork_gen
	* thread_sync.c (condvar_alloc): remove list_add
	  [ruby-core:86316] [Bug #14634]

	thread_sync.c (condvar_ptr): reset fork_gen after forking

	Otherwise the condition variable waiter list will always
	be empty, which is wrong :x

	[Bug #14725] [Bug #14634]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@66912 b2dd03c8-39d4-4d8f-98ff-823fe69b080e