[Bug #22461] Fix dangling pointers after associating encoding

`rb_enc_associate` may re-allocate the string buffer because of the
terminator length change.

sprintf.c

@@ -234,6 +234,18 @@ rb_f_sprintf(int argc, const VALUE *argv)
     return rb_str_format(argc - 1, argv + 1, GETNTHARG(0));
 }
 
+static char *
+reassoc_enc(VALUE result, rb_encoding *enc)
+{
+    return RSTRING_PTR(rb_enc_associate(result, enc));
+}
+
+static char *
+reassoc_enc_index(VALUE result, int encidx)
+{
+    return RSTRING_PTR(rb_enc_associate_index(result, encidx));
+}
+
 VALUE
 rb_str_format_ary(int argc, const VALUE *argv, VALUE fmt, VALUE ary)
 {
@@ -295,8 +307,7 @@ rb_str_format_ary(int argc, const VALUE *argv, VALUE fmt, VALUE ary)
     blen = 0;
     bsiz = 120;
     result = rb_str_buf_new(bsiz);
-    rb_enc_associate(result, enc);
-    buf = RSTRING_PTR(result);
+    buf = reassoc_enc(result, enc);
     memset(buf, 0, bsiz);
     ENC_CODERANGE_SET(result, coderange);
 
@@ -487,7 +498,7 @@ rb_str_format_ary(int argc, const VALUE *argv, VALUE fmt, VALUE ary)
                 }
                 if (encidx >= 0 && encidx != rb_enc_to_index(enc)) {
                     /* special case */
-                    rb_enc_associate_index(result, encidx);
+                    buf = reassoc_enc_index(result, encidx);
                     enc = rb_enc_from_index(encidx);
                     coderange = ENC_CODERANGE_VALID;
                 }
@@ -550,13 +561,13 @@ rb_str_format_ary(int argc, const VALUE *argv, VALUE fmt, VALUE ary)
                         if (flags&FMINUS) {
                             FILL_(' ', width);
                         }
-                        rb_enc_associate(result, enc);
+                        buf = reassoc_enc(result, enc);
                         break;
                     }
                 }
                 PUSH(RSTRING_PTR(str), len);
                 RB_GC_GUARD(str);
-                rb_enc_associate(result, enc);
+                buf = reassoc_enc(result, enc);
             }
             break;
 

test/ruby/test_sprintf.rb

@@ -574,4 +574,16 @@ class TestSprintf < Test::Unit::TestCase
       assert_equal i, str.bytesize
     end
   end
+
+  def test_dangling_pointer_after_encoding_set
+    str = "\u0141" * 61
+    wide_encodings = [
+      Encoding::UTF_16BE, Encoding::UTF_16LE,
+      Encoding::UTF_32BE, Encoding::UTF_32LE,
+    ]
+    all_assertions_foreach(nil, *wide_encodings) do |enc|
+      s = str.encode(enc)
+      assert_equal(s, ("%s" % s), enc.name)
+    end
+  end
 end