Avoid creating autorelease pointers when spawning subprocesses

* Instead use stack_alloc in #with_array_of_strings_pointer.

Author: eregon

src/main/ruby/truffleruby/core/posix.rb

@@ -326,12 +326,15 @@ def self.with_array_of_ints(ints)
 
   def self.with_array_of_strings_pointer(strings)
     Truffle::FFI::MemoryPointer.new(:pointer, strings.size + 1) do |ptr|
-      pointers = strings.map do |str|
-        Truffle::FFI::MemoryPointer.from_string(str)
+      pointers = Truffle::FFI::Pool.stack_alloc(*strings.map { |s| s.bytesize + 1 })
+      begin
+        pointers.zip(strings) { |sp, s| sp.put_string(0, s) }
+        pointers << Truffle::FFI::Pointer::NULL
+        ptr.write_array_of_pointer pointers
+        yield(ptr)
+      ensure
+        Truffle::FFI::Pool.stack_free(pointers[0])
       end
-      pointers << Truffle::FFI::Pointer::NULL
-      ptr.write_array_of_pointer pointers
-      yield(ptr)
     end
   end