ext/json/generator/generator.c: prevent GC of temporary strings

We need to guard temporary strings from being collected while we
append to the JSON buffer (which may allocate memory).  The
RSTRING_PAIR macro is dangerous since it preserves no pointer to
the original string VALUE, allowing GC to reap the object while
we're still using the (C) string pointer.

The included test case shows data corruption with large
Bignums without this fix.

3 files changed, 26 insertions, 5 deletions

diff --git a/ext/json/ext/generator/generator.c b/ext/json/ext/generator/generator.c
index 4e44178..a75118a 100644
--- a/ext/json/ext/generator/generator.c
+++ b/ext/json/ext/generator/generator.c
@@ -349,6 +349,16 @@ static void fbuffer_append(FBuffer *fb, const char *newstr, unsigned long len)
     }
 }
 
+static void fbuffer_append_str(FBuffer *fb, VALUE str)
+{
+    const char *newstr = RSTRING_PTR(str);
+    unsigned long len = RSTRING_LEN(str);
+
+    RB_GC_GUARD(str);
+
+    fbuffer_append(fb, newstr, len);
+}
+
 static void fbuffer_append_char(FBuffer *fb, char newchr)
 {
     fbuffer_inc_capa(fb, 1);
@@ -852,7 +862,7 @@ static void generate_json_fixnum(FBuffer *buffer, VALUE Vstate, JSON_Generator_S
 static void generate_json_bignum(FBuffer *buffer, VALUE Vstate, JSON_Generator_State *state, VALUE obj)
 {
     VALUE tmp = rb_funcall(obj, i_to_s, 0);
-    fbuffer_append(buffer, RSTRING_PAIR(tmp));
+    fbuffer_append_str(buffer, tmp);
 }
 
 static void generate_json_float(FBuffer *buffer, VALUE Vstate, JSON_Generator_State *state, VALUE obj)
@@ -869,7 +879,7 @@ static void generate_json_float(FBuffer *buffer, VALUE Vstate, JSON_Generator_St
             rb_raise(eGeneratorError, "%u: %s not allowed in JSON", __LINE__, StringValueCStr(tmp));
         }
     }
-    fbuffer_append(buffer, RSTRING_PAIR(tmp));
+    fbuffer_append_str(buffer, tmp);
 }
 
 static void generate_json(FBuffer *buffer, VALUE Vstate, JSON_Generator_State *state, VALUE obj)
@@ -897,7 +907,7 @@ static void generate_json(FBuffer *buffer, VALUE Vstate, JSON_Generator_State *s
     } else if (rb_respond_to(obj, i_to_json)) {
         tmp = rb_funcall(obj, i_to_json, 1, Vstate);
         Check_Type(tmp, T_STRING);
-        fbuffer_append(buffer, RSTRING_PAIR(tmp));
+        fbuffer_append_str(buffer, tmp);
     } else {
         tmp = rb_funcall(obj, i_to_s, 0);
         Check_Type(tmp, T_STRING);
diff --git a/ext/json/ext/generator/generator.h b/ext/json/ext/generator/generator.h
index ee496fe..2a83149 100644
--- a/ext/json/ext/generator/generator.h
+++ b/ext/json/ext/generator/generator.h
@@ -45,8 +45,6 @@
 #define RSTRING_LEN(string) RSTRING(string)->len
 #endif
 
-#define RSTRING_PAIR(string) RSTRING_PTR(string), RSTRING_LEN(string)
-
 /* fbuffer implementation */
 
 typedef struct FBufferStruct {
diff --git a/tests/test_json_generate.rb b/tests/test_json_generate.rb
index 9b0cff4..bc4e395 100755
--- a/tests/test_json_generate.rb
+++ b/tests/test_json_generate.rb
@@ -177,4 +177,17 @@ EOT
     assert_raises(JSON::NestingError) { ary.to_json(s) }
     assert_equal 19, s.depth
   end
+
+  def test_gc
+    bignum_too_long_to_embed_as_string = 1234567890123456789012345
+    expect = bignum_too_long_to_embed_as_string.to_s
+    stress, GC.stress = GC.stress, true
+
+    10.times do |i|
+      tmp = bignum_too_long_to_embed_as_string.to_json
+      assert_equal expect, tmp
+    end
+    ensure
+      GC.stress = stress
+  end if GC.respond_to?(:stress=)
 end