Disable compaction on platforms that can't support it

Manual compaction also requires a read barrier, so we need to disable
even manual compaction on platforms that don't support mprotect.

[Bug #17871]

gc.c

@@ -9998,6 +9998,24 @@ heap_check_moved_i(void *vstart, void *vend, size_t stride, void *data)
 static VALUE
 gc_compact(rb_execution_context_t *ec, VALUE self)
 {
+#if defined(HAVE_SYSCONF) && defined(_SC_PAGE_SIZE)
+    /* If Ruby's heap pages are not a multiple of the system page size, we
+     * cannot use mprotect for the read barrier, so we must disable compaction. */
+    int pagesize;
+    pagesize = (int)sysconf(_SC_PAGE_SIZE);
+    if ((HEAP_PAGE_SIZE % pagesize) != 0) {
+        rb_raise(rb_eNotImpError, "Compaction isn't available on this platform");
+    }
+#endif
+
+    /* If not MinGW, Windows, or does not have mmap, we cannot use mprotect for
+     * the read barrier, so we must disable compaction. */
+#if !defined(__MINGW32__) && !defined(_WIN32)
+    if (!USE_MMAP_ALIGNED_ALLOC) {
+        rb_raise(rb_eNotImpError, "Compaction isn't available on this platform");
+    }
+#endif
+
     /* Run GC with compaction enabled */
     gc_start_internal(ec, self, Qtrue, Qtrue, Qtrue, Qtrue);
 

test/ruby/test_gc_compact.rb

@@ -4,12 +4,32 @@ require 'fiddle'
 require 'etc'
 
 class TestGCCompact < Test::Unit::TestCase
-  class AutoCompact < Test::Unit::TestCase
+  module SupportsCompact
     def setup
       skip "autocompact not supported on this platform" unless supports_auto_compact?
       super
     end
 
+    private
+
+    def supports_auto_compact?
+      return true unless defined?(Etc::SC_PAGE_SIZE)
+
+      begin
+        return GC::INTERNAL_CONSTANTS[:HEAP_PAGE_SIZE] % Etc.sysconf(Etc::SC_PAGE_SIZE) == 0
+      rescue NotImplementedError
+      rescue ArgumentError
+      end
+
+      true
+    end
+  end
+
+  include SupportsCompact
+
+  class AutoCompact < Test::Unit::TestCase
+    include SupportsCompact
+
     def test_enable_autocompact
       before = GC.auto_compact
       GC.auto_compact = true
@@ -59,26 +79,17 @@ class TestGCCompact < Test::Unit::TestCase
     ensure
       GC.auto_compact = before
     end
-
-    private
-
-    def supports_auto_compact?
-      return true unless defined?(Etc::SC_PAGE_SIZE)
-
-      begin
-        return GC::INTERNAL_CONSTANTS[:HEAP_PAGE_SIZE] % Etc.sysconf(Etc::SC_PAGE_SIZE) == 0
-      rescue NotImplementedError
-      rescue ArgumentError
-      end
-
-      true
-    end
   end
 
   def os_page_size
     return true unless defined?(Etc::SC_PAGE_SIZE)
   end
 
+  def setup
+    skip "autocompact not supported on this platform" unless supports_auto_compact?
+    super
+  end
+
   def test_gc_compact_stats
     list = []