Revert "Revert "Revert "[ruby/fiddle] Use ffi_closure_free by default. (#20)"""

This reverts commit 87f6154bb4.

It turned out that the change fails to build on macOS

https://rubyci.org/logs/rubyci.s3.amazonaws.com/osx1014/ruby-master/log/20200304T074503Z.fail.html.gz
```
+ make 'TESTS=--hide-skip -v fiddle' RUBYOPT=-w test-all
dyld: lazy symbol binding failed: Symbol not found: _ffi_closure_alloc
  Referenced from: /Users/hsbt/Documents/cb/tmp/build/20200304T074503Z/ruby/.ext/x86_64-darwin18/fiddle.bundle
  Expected in: flat namespace

dyld: Symbol not found: _ffi_closure_alloc
  Referenced from: /Users/hsbt/Documents/cb/tmp/build/20200304T074503Z/ruby/.ext/x86_64-darwin18/fiddle.bundle
  Expected in: flat namespace

make: *** [yes-test-all] Abort trap: 6
```

ext/fiddle/closure.c

@@ -13,11 +13,25 @@ typedef struct {
     ffi_type **argv;
 } fiddle_closure;
 
+#if defined(USE_FFI_CLOSURE_ALLOC)
+#elif defined(__OpenBSD__) || defined(__APPLE__) || defined(__linux__)
+# define USE_FFI_CLOSURE_ALLOC 0
+#elif defined(RUBY_LIBFFI_MODVERSION) && RUBY_LIBFFI_MODVERSION < 3000005 && \
+	(defined(__i386__) || defined(__x86_64__) || defined(_M_IX86) || defined(_M_AMD64))
+# define USE_FFI_CLOSURE_ALLOC 0
+#else
+# define USE_FFI_CLOSURE_ALLOC 1
+#endif
+
 static void
 dealloc(void * ptr)
 {
     fiddle_closure * cls = (fiddle_closure *)ptr;
+#if USE_FFI_CLOSURE_ALLOC
     ffi_closure_free(cls->pcl);
+#else
+    munmap(cls->pcl, sizeof(*cls->pcl));
+#endif
     if (cls->argv) xfree(cls->argv);
     xfree(cls);
 }
@@ -191,7 +205,12 @@ allocate(VALUE klass)
     VALUE i = TypedData_Make_Struct(klass, fiddle_closure,
 	    &closure_data_type, closure);
 
+#if USE_FFI_CLOSURE_ALLOC
     closure->pcl = ffi_closure_alloc(sizeof(ffi_closure), &closure->code);
+#else
+    closure->pcl = mmap(NULL, sizeof(ffi_closure), PROT_READ | PROT_WRITE,
+        MAP_ANON | MAP_PRIVATE, -1, 0);
+#endif
 
     return i;
 }
@@ -238,8 +257,17 @@ initialize(int rbargc, VALUE argv[], VALUE self)
     if (FFI_OK != result)
 	rb_raise(rb_eRuntimeError, "error prepping CIF %d", result);
 
+#if USE_FFI_CLOSURE_ALLOC
     result = ffi_prep_closure_loc(pcl, cif, callback,
 		(void *)self, cl->code);
+#else
+    result = ffi_prep_closure(pcl, cif, callback, (void *)self);
+    cl->code = (void *)pcl;
+    i = mprotect(pcl, sizeof(*pcl), PROT_READ | PROT_EXEC);
+    if (i) {
+	rb_sys_fail("mprotect");
+    }
+#endif
 
     if (FFI_OK != result)
 	rb_raise(rb_eRuntimeError, "error prepping closure %d", result);

ext/fiddle/extconf.rb

@@ -7,7 +7,8 @@ bundle = enable_config('bundled-libffi')
 if ! bundle
   dir_config 'libffi'
 
-  pkg_config("libffi")
+  pkg_config("libffi") and
+    ver = pkg_config("libffi", "modversion")
 
   if have_header(ffi_header = 'ffi.h')
     true
@@ -27,20 +28,20 @@ begin
     Dir.glob("#{$srcdir}/libffi-*/").each{|dir| FileUtils.rm_rf(dir)}
     extlibs.run(["--cache=#{cache_dir}", ext_dir])
   end
-  libffi_dir = bundle != false &&
+  ver = bundle != false &&
         Dir.glob("#{$srcdir}/libffi-*/")
         .map {|n| File.basename(n)}
         .max_by {|n| n.scan(/\d+/).map(&:to_i)}
-  unless libffi_dir
+  unless ver
     raise "missing libffi. Please install libffi."
   end
 
-  srcdir = "#{$srcdir}/#{libffi_dir}"
+  srcdir = "#{$srcdir}/#{ver}"
   ffi_header = 'ffi.h'
   libffi = Struct.new(*%I[dir srcdir builddir include lib a cflags ldflags opt arch]).new
-  libffi.dir = libffi_dir
+  libffi.dir = ver
   if $srcdir == "."
-    libffi.builddir = "#{libffi_dir}/#{RUBY_PLATFORM}"
+    libffi.builddir = "#{ver}/#{RUBY_PLATFORM}"
     libffi.srcdir = "."
   else
     libffi.builddir = libffi.dir
@@ -51,6 +52,7 @@ begin
   libffi.a = "#{libffi.lib}/libffi_convenience.#{$LIBEXT}"
   nowarn = CONFIG.merge("warnflags"=>"")
   libffi.cflags = RbConfig.expand("$(CFLAGS)".dup, nowarn)
+  ver = ver[/libffi-(.*)/, 1]
 
   FileUtils.mkdir_p(libffi.dir)
   libffi.opt = CONFIG['configure_args'][/'(-C)'/, 1]
@@ -110,6 +112,12 @@ begin
   $INCFLAGS << " -I" << libffi.include
 end
 
+if ver
+  ver = ver.gsub(/-rc\d+/, '') # If ver contains rc version, just ignored.
+  ver = (ver.split('.') + [0,0])[0,3]
+  $defs.push(%{-DRUBY_LIBFFI_MODVERSION=#{ '%d%03d%03d' % ver }})
+end
+
 have_header 'sys/mman.h'
 
 if have_header "dlfcn.h"