Fix GC compaction crash when using local variables in eval

If we have local variables outside of the eval, the local variables names
are IDs. We convert these IDs to char * using rb_id2name. However, these
char * are actually Ruby strings, which may be embedded. This means that
it is not safe to rb_id2name and call any potential GC entrypoints because
if a GC compaction runs, the embedded string may move and the pointer will
change.

For example, if you compile with `-DRGENGC_CHECK_MODE=1`, then the following
script will crash:

    GC.auto_compact = :empty
    GC.stress = true

    o = Object.new

    eval("def o.m(k: 0) k end")

The crash message is:

    test.rb:6: [BUG] Local with constant_id 1 does not exist
    ruby 3.4.0dev (2024-12-17T18:34:57Z prism-local-compac.. 4343467) +PRISM [arm64-darwin24]

    -- C level backtrace information -------------------------------------------
    miniruby(rb_print_backtrace+0x24) [0x10312fec4] vm_dump.c:823
    miniruby(rb_print_backtrace) (null):0
    miniruby(rb_vm_bugreport+0x2d4) [0x1031301b8] vm_dump.c:1155
    miniruby(rb_bug_without_die_internal+0xa8) [0x102dd6a94] error.c:1097
    miniruby(rb_bug+0x28) [0x102dd6b00] error.c:1115
    miniruby(pm_lookup_local_index+0xec) [0x102d61e4c] prism_compile.c:1237
    miniruby(pm_compile_node+0x45d0) [0x102d252f4] prism_compile.c:9334
    miniruby(pm_compile_node+0x1864) [0x102d22588] prism_compile.c:8650
    miniruby(pm_compile_node+0x65ec) [0x102d27310] prism_compile.c:9897
    miniruby(pm_compile_scope_node+0x3008) [0x102d77bcc] prism_compile.c:6584
    miniruby(pm_compile_node+0x5ec4) [0x102d26be8] prism_compile.c:9768
    miniruby(pm_iseq_compile_node+0xac) [0x102d20bf0] prism_compile.c:10069
    miniruby(pm_iseq_new_with_opt_try+0x2c) [0x102e7d088] iseq.c:1029
    miniruby(rb_protect+0x108) [0x102dea9bc] eval.c:1033
    miniruby(pm_iseq_new_with_opt+0x264) [0x102e7c444] iseq.c:1082
    miniruby(pm_iseq_new_eval+0xec) [0x102e7c8e0] iseq.c:961
    miniruby(pm_eval_make_iseq+0x594) [0x1031209cc] vm_eval.c:1770
    miniruby(eval_make_iseq+0x54) [0x103120068] vm_eval.c:1799

Author: peterzhu2118

test/ruby/test_eval.rb

@@ -636,4 +636,19 @@ def test_syntax_error_no_memory_leak
       end
     end;
   end
+
+  def test_outer_local_variable_under_gc_compact_stress
+    omit "compaction is not supported on this platform" unless GC.respond_to?(:compact)
+
+    assert_separately([], <<~RUBY)
+      o = Object.new
+      def o.m = 1
+
+      GC.verify_compaction_references(expand_heap: true, toward: :empty)
+
+      EnvUtil.under_gc_compact_stress do
+        assert_equal(1, eval("o.m"))
+      end
+    RUBY
+  end
 end

vm_eval.c

@@ -1700,7 +1700,8 @@ pm_eval_make_iseq(VALUE src, VALUE fname, int line,
             ID local = ISEQ_BODY(iseq)->local_table[local_index];
 
             if (rb_is_local_id(local)) {
-                const char *name = rb_id2name(local);
+                VALUE name_obj = rb_id2str(local);
+                const char *name = RSTRING_PTR(name_obj);
                 size_t length = strlen(name);
 
                 // Explicitly skip numbered parameters. These should not be sent
@@ -1709,7 +1710,15 @@ pm_eval_make_iseq(VALUE src, VALUE fname, int line,
                     continue;
                 }
 
-                pm_string_constant_init(scope_local, name, length);
+                /* We need to duplicate the string because the Ruby string may
+                 * be embedded so compaction could move the string and the pointer
+                 * will change. */
+                char *name_dup = xmalloc(length + 1);
+                strlcpy(name_dup, name, length + 1);
+
+                RB_GC_GUARD(name_obj);
+
+                pm_string_owned_init(scope_local, (uint8_t *)name_dup, length);
             }
         }