Fix Symbol#to_proc (rb_sym_to_proc) to be ractor safe

In non-main ractors, don't use `sym_proc_cache`. It is not thread-safe
to add to this array without a lock and also it leaks procs from one
ractor to another. Instead, we create a new proc each time. If this
results in poor performance we can come up with a solution later.

Fixes [Bug #21354]

Author: luke-gruber

bootstraptest/test_ractor.rb

@@ -2307,6 +2307,18 @@ def initialize(a)
   'ok'
 }
 
+# Using Symbol#to_proc inside ractors
+# [Bug #21354]
+assert_equal 'ok', %q{
+  :inspect.to_proc
+  Ractor.new do
+    # It should not use this cached proc, it should create a new one. If it used
+    # the cached proc, we would get a ractor_confirm_belonging error here.
+    :inspect.to_proc
+  end.take
+  'ok'
+}
+
 # There are some bugs in Windows with multiple threads in same ractor calling ractor actions
 # Ex: https://github.com/ruby/ruby/actions/runs/14998660285/job/42139383905
 unless /mswin/ =~ RUBY_PLATFORM

common.mk

@@ -13836,6 +13836,8 @@ proc.$(OBJEXT): {$(VPATH)}prism/diagnostic.h
 proc.$(OBJEXT): {$(VPATH)}prism/version.h
 proc.$(OBJEXT): {$(VPATH)}prism_compile.h
 proc.$(OBJEXT): {$(VPATH)}proc.c
+proc.$(OBJEXT): {$(VPATH)}ractor.h
+proc.$(OBJEXT): {$(VPATH)}ractor_core.h
 proc.$(OBJEXT): {$(VPATH)}ruby_assert.h
 proc.$(OBJEXT): {$(VPATH)}ruby_atomic.h
 proc.$(OBJEXT): {$(VPATH)}rubyparser.h

proc.c

@@ -22,6 +22,7 @@
 #include "method.h"
 #include "iseq.h"
 #include "vm_core.h"
+#include "ractor_core.h"
 #include "yjit.h"
 
 const rb_cref_t *rb_vm_cref_in_context(VALUE self, VALUE cbase);
@@ -1538,23 +1539,26 @@ rb_sym_to_proc(VALUE sym)
     long index;
     ID id;
 
-    if (!sym_proc_cache) {
-        sym_proc_cache = rb_ary_hidden_new(SYM_PROC_CACHE_SIZE * 2);
-        rb_vm_register_global_object(sym_proc_cache);
-        rb_ary_store(sym_proc_cache, SYM_PROC_CACHE_SIZE*2 - 1, Qnil);
-    }
-
     id = SYM2ID(sym);
-    index = (id % SYM_PROC_CACHE_SIZE) << 1;
 
-    if (RARRAY_AREF(sym_proc_cache, index) == sym) {
-        return RARRAY_AREF(sym_proc_cache, index + 1);
-    }
-    else {
-        proc = sym_proc_new(rb_cProc, ID2SYM(id));
-        RARRAY_ASET(sym_proc_cache, index, sym);
-        RARRAY_ASET(sym_proc_cache, index + 1, proc);
-        return proc;
+    if (rb_ractor_main_p()) {
+        index = (id % SYM_PROC_CACHE_SIZE) << 1;
+        if (!sym_proc_cache) {
+            sym_proc_cache = rb_ary_hidden_new(SYM_PROC_CACHE_SIZE * 2);
+            rb_vm_register_global_object(sym_proc_cache);
+            rb_ary_store(sym_proc_cache, SYM_PROC_CACHE_SIZE*2 - 1, Qnil);
+        }
+        if (RARRAY_AREF(sym_proc_cache, index) == sym) {
+            return RARRAY_AREF(sym_proc_cache, index + 1);
+        }
+        else {
+            proc = sym_proc_new(rb_cProc, ID2SYM(id));
+            RARRAY_ASET(sym_proc_cache, index, sym);
+            RARRAY_ASET(sym_proc_cache, index + 1, proc);
+            return proc;
+        }
+    } else {
+        return sym_proc_new(rb_cProc, ID2SYM(id));
     }
 }