Guard match from GC in String#gsub

We need to guard match from GC because otherwise it could end up being
reclaimed or moved in compaction.

string.c

@@ -5866,8 +5866,7 @@ rb_str_sub(int argc, VALUE *argv, VALUE str)
 static VALUE
 str_gsub(int argc, VALUE *argv, VALUE str, int bang)
 {
-    VALUE pat, val = Qnil, repl, match, match0 = Qnil, dest, hash = Qnil;
-    struct re_registers *regs;
+    VALUE pat, val = Qnil, repl, match0 = Qnil, dest, hash = Qnil;
     long beg, beg0, end0;
     long offset, blen, slen, len, last;
     enum {STR, ITER, MAP} mode = STR;
@@ -5912,8 +5911,8 @@ str_gsub(int argc, VALUE *argv, VALUE str, int bang)
     ENC_CODERANGE_SET(dest, rb_enc_asciicompat(str_enc) ? ENC_CODERANGE_7BIT : ENC_CODERANGE_VALID);
 
     do {
-        match = rb_backref_get();
-        regs = RMATCH_REGS(match);
+        VALUE match = rb_backref_get();
+        struct re_registers *regs = RMATCH_REGS(match);
         if (RB_TYPE_P(pat, T_STRING)) {
             beg0 = beg;
             end0 = beg0 + RSTRING_LEN(pat);
@@ -5970,6 +5969,8 @@ str_gsub(int argc, VALUE *argv, VALUE str, int bang)
         cp = RSTRING_PTR(str) + offset;
         if (offset > RSTRING_LEN(str)) break;
         beg = rb_pat_search(pat, str, offset, need_backref);
+
+        RB_GC_GUARD(match);
     } while (beg >= 0);
     if (RSTRING_LEN(str) > offset) {
         rb_enc_str_buf_cat(dest, cp, RSTRING_LEN(str) - offset, str_enc);

test/ruby/test_string.rb

@@ -1249,6 +1249,10 @@ def test_gsub
     assert_raise(ArgumentError) { S("foo").gsub }
   end
 
+  def test_gsub_gc_compact_stress
+    EnvUtil.under_gc_compact_stress { assert_equal(S("h<e>ll<o>"), S("hello").gsub(/([aeiou])/, S('<\1>'))) }
+  end
+
   def test_gsub_encoding
     a = S("hello world")
     a.force_encoding Encoding::UTF_8
@@ -1292,6 +1296,14 @@ def test_gsub!
     assert_nil(a.sub!(S('X'), S('Y')))
   end
 
+  def test_gsub_bang_gc_compact_stress
+    EnvUtil.under_gc_compact_stress do
+      a = S("hello")
+      a.gsub!(/([aeiou])/, S('<\1>'))
+      assert_equal(S("h<e>ll<o>"), a)
+    end
+  end
+
   def test_sub_hash
     assert_equal('azc', S('abc').sub(/b/, "b" => "z"))
     assert_equal('ac', S('abc').sub(/b/, {}))