[Bug #20853] Fix Proc#hash to not change after compaction

The hash value of a Proc must remain constant after a compaction, otherwise
it may not work as the key in a hash table.

common.mk

@@ -13032,6 +13032,7 @@ proc.$(OBJEXT): $(top_srcdir)/internal/compilers.h
 proc.$(OBJEXT): $(top_srcdir)/internal/error.h
 proc.$(OBJEXT): $(top_srcdir)/internal/eval.h
 proc.$(OBJEXT): $(top_srcdir)/internal/gc.h
+proc.$(OBJEXT): $(top_srcdir)/internal/hash.h
 proc.$(OBJEXT): $(top_srcdir)/internal/imemo.h
 proc.$(OBJEXT): $(top_srcdir)/internal/object.h
 proc.$(OBJEXT): $(top_srcdir)/internal/proc.h

proc.c

@@ -15,6 +15,7 @@
 #include "internal/error.h"
 #include "internal/eval.h"
 #include "internal/gc.h"
+#include "internal/hash.h"
 #include "internal/object.h"
 #include "internal/proc.h"
 #include "internal/symbol.h"
@@ -1437,8 +1438,24 @@ rb_hash_proc(st_index_t hash, VALUE prc)
 {
     rb_proc_t *proc;
     GetProcPtr(prc, proc);
-    hash = rb_hash_uint(hash, (st_index_t)proc->block.as.captured.code.val);
-    hash = rb_hash_uint(hash, (st_index_t)proc->block.as.captured.self);
+
+    switch (vm_block_type(&proc->block)) {
+      case block_type_iseq:
+        hash = rb_st_hash_uint(hash, (st_index_t)proc->block.as.captured.code.iseq->body);
+        break;
+      case block_type_ifunc:
+        hash = rb_st_hash_uint(hash, (st_index_t)proc->block.as.captured.code.ifunc->func);
+        break;
+      case block_type_symbol:
+        hash = rb_st_hash_uint(hash, rb_any_hash(proc->block.as.symbol));
+        break;
+      case block_type_proc:
+        hash = rb_st_hash_uint(hash, rb_any_hash(proc->block.as.proc));
+        break;
+      default:
+        rb_bug("rb_hash_proc: unknown block type %d", vm_block_type(&proc->block));
+    }
+
     return rb_hash_uint(hash, (st_index_t)proc->block.as.captured.ep);
 }
 

test/ruby/test_proc.rb

@@ -168,6 +168,24 @@ class TestProc < Test::Unit::TestCase
     assert_operator(procs.map(&:hash).uniq.size, :>=, 500)
   end
 
+  def test_hash_does_not_change_after_compaction
+    # [Bug #20853]
+    [
+      "proc {}", # iseq backed proc
+      "{}.to_proc", # ifunc backed proc
+      ":hello.to_proc", # symbol backed proc
+    ].each do |proc|
+      assert_separately([], <<~RUBY)
+        p1 = #{proc}
+        hash = p1.hash
+
+        GC.verify_compaction_references(expand_heap: true, toward: :empty)
+
+        assert_equal(hash, p1.hash, "proc is `#{proc}`")
+      RUBY
+    end
+  end
+
   def test_block_par
     assert_equal(10, Proc.new{|&b| b.call(10)}.call {|x| x})
     assert_equal(12, Proc.new{|a,&b| b.call(a)}.call(12) {|x| x})