Disable auto compaction on platforms that can't support it

Both explicit compaction routines (gc_compact and the verify references form)
need to clear the heap before executing compaction.  Otherwise some
objects may not be alive, and we'll need the read barrier.  The heap
must only contain *live* objects if we want to disable the read barrier
during explicit compaction.

The previous commit was missing the "clear the heap" phase from the
"verify references" explicit compaction function.

Fixes [Bug #17306]

gc.c

@@ -682,7 +682,7 @@ typedef struct rb_objspace {
 	unsigned int dont_gc : 1;
 	unsigned int dont_incremental : 1;
 	unsigned int during_gc : 1;
-        unsigned int during_compacting : 1;
+        unsigned int during_compacting : 2;
 	unsigned int gc_stressful: 1;
 	unsigned int has_hook: 1;
 	unsigned int during_minor_gc : 1;
@@ -3090,6 +3090,17 @@ Init_heap(void)
 {
     rb_objspace_t *objspace = &rb_objspace;
 
+#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 automatic
+     * compaction. */
+    int pagesize;
+    pagesize = (int)sysconf(_SC_PAGE_SIZE);
+    if ((HEAP_PAGE_SIZE % pagesize) != 0) {
+        ruby_enable_autocompact = 0;
+    }
+#endif
+
     objspace->next_object_id = INT2FIX(OBJ_ID_INITIAL);
     objspace->id_to_obj_tbl = st_init_table(&object_id_hash_type);
     objspace->obj_to_id_tbl = st_init_numtable();
@@ -4389,6 +4400,11 @@ static VALUE gc_move(rb_objspace_t *objspace, VALUE scan, VALUE free);
 static void
 lock_page_body(rb_objspace_t *objspace, struct heap_page_body *body)
 {
+    /* If this is an explicit compaction (GC.compact), we don't need a read
+     * barrier, so just return early. */
+    if (objspace->flags.during_compacting >> 1) {
+        return;
+    }
 #if defined(_WIN32)
     DWORD old_protect;
 
@@ -4405,6 +4421,11 @@ lock_page_body(rb_objspace_t *objspace, struct heap_page_body *body)
 static void
 unlock_page_body(rb_objspace_t *objspace, struct heap_page_body *body)
 {
+    /* If this is an explicit compaction (GC.compact), we don't need a read
+     * barrier, so just return early. */
+    if (objspace->flags.during_compacting >> 1) {
+        return;
+    }
 #if defined(_WIN32)
     DWORD old_protect;
 
@@ -4624,8 +4645,12 @@ gc_compact_finish(rb_objspace_t *objspace, rb_heap_t *heap)
 {
     GC_ASSERT(heap->sweeping_page == heap->compact_cursor);
 
+    /* If this is an explicit compaction (GC.compact), no read barrier was set
+     * so we don't need to unprotect pages or uninstall the SEGV handler */
+    if (!(objspace->flags.during_compacting >> 1)) {
         gc_unprotect_pages(objspace, heap);
         uninstall_handlers();
+    }
 
     /* The mutator is allowed to run during incremental sweeping. T_MOVED
      * objects can get pushed on the stack and when the compaction process
@@ -5134,6 +5159,12 @@ gc_compact_start(rb_objspace_t *objspace, rb_heap_t *heap)
     memset(objspace->rcompactor.considered_count_table, 0, T_MASK * sizeof(size_t));
     memset(objspace->rcompactor.moved_count_table, 0, T_MASK * sizeof(size_t));
 
+    /* If this is an explicit compaction (GC.compact), we don't need a read
+     * barrier, so just return early. */
+    if (objspace->flags.during_compacting >> 1) {
+        return;
+    }
+
     /* Set up read barrier for pages containing MOVED objects */
     install_handlers();
 }
@@ -7030,7 +7061,7 @@ gc_marks_start(rb_objspace_t *objspace, int full_mark)
 #endif
 	objspace->flags.during_minor_gc = FALSE;
         if (ruby_enable_autocompact) {
-            objspace->flags.during_compacting = TRUE;
+            objspace->flags.during_compacting |= TRUE;
         }
 	objspace->profile.major_gc_count++;
 	objspace->rgengc.uncollectible_wb_unprotected_objects = 0;
@@ -8057,7 +8088,9 @@ gc_start(rb_objspace_t *objspace, int reason)
 
     /* reason may be clobbered, later, so keep set immediate_sweep here */
     objspace->flags.immediate_sweep = !!((unsigned)reason & GPR_FLAG_IMMEDIATE_SWEEP);
-    objspace->flags.during_compacting = !!((unsigned)reason & GPR_FLAG_COMPACT);
+
+    /* Explicitly enable compaction (GC.compact) */
+    objspace->flags.during_compacting = (!!((unsigned)reason & GPR_FLAG_COMPACT) << 1);
 
     if (!heap_allocated_pages) return FALSE; /* heap is not ready */
     if (!(reason & GPR_FLAG_METHOD) && !ready_to_gc(objspace)) return TRUE; /* GC is not allowed */
@@ -9247,11 +9280,27 @@ heap_check_moved_i(void *vstart, void *vend, size_t stride, void *data)
     return 0;
 }
 
+static VALUE
+gc_compact(rb_execution_context_t *ec, VALUE self)
+{
+    /* Clear the heap. */
+    gc_start_internal(ec, self, Qtrue, Qtrue, Qtrue, Qfalse);
+
+    /* At this point, all references are live and the mutator is not allowed
+     * to run, so we don't need a read barrier. */
+    gc_start_internal(ec, self, Qtrue, Qtrue, Qtrue, Qtrue);
+
+    return gc_compact_stats(ec, self);
+}
+
 static VALUE
 gc_verify_compaction_references(rb_execution_context_t *ec, VALUE self, VALUE double_heap, VALUE toward_empty)
 {
     rb_objspace_t *objspace = &rb_objspace;
 
+    /* Clear the heap. */
+    gc_start_internal(ec, self, Qtrue, Qtrue, Qtrue, Qfalse);
+
     RB_VM_LOCK_ENTER();
     {
         gc_rest(objspace);
@@ -9865,6 +9914,16 @@ gc_disable(rb_execution_context_t *ec, VALUE _)
 static VALUE
 gc_set_auto_compact(rb_execution_context_t *ec, VALUE _, VALUE v)
 {
+#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 automatic
+     * compaction. */
+    int pagesize;
+    pagesize = (int)sysconf(_SC_PAGE_SIZE);
+    if ((HEAP_PAGE_SIZE % pagesize) != 0) {
+        rb_raise(rb_eNotImpError, "Automatic compaction isn't available on this platform");
+    }
+#endif
     ruby_enable_autocompact = RTEST(v);
     return v;
 }

gc.rb

@@ -199,8 +199,7 @@ module GC
   end
 
   def self.compact
-    Primitive.gc_start_internal true, true, true, true
-    Primitive.gc_compact_stats
+    Primitive.gc_compact
   end
 
   # call-seq:

test/ruby/test_gc_compact.rb

@@ -1,8 +1,15 @@
 # frozen_string_literal: true
 require 'test/unit'
 require 'fiddle'
+require 'etc'
 
 class TestGCCompact < Test::Unit::TestCase
+  class AutoCompact < Test::Unit::TestCase
+    def setup
+      skip "autocompact not supported on this platform" unless supports_auto_compact?
+      super
+    end
+
     def test_enable_autocompact
       before = GC.auto_compact
       GC.auto_compact = true
@@ -53,6 +60,25 @@ class TestGCCompact < Test::Unit::TestCase
       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 test_gc_compact_stats
     list = []