Harden the libfyaml backend after code review

Mark the reconstructed SyntaxError message UTF-8 instead of US-ASCII, so
a diagnostic that embeds a multibyte snippet of the input does not raise
Encoding::CompatibilityError when concatenated with UTF-8. Add
RB_GC_GUARD for the anchor and tag strings in the emitter (matching the
existing guard on the scalar value) so their C pointers cannot dangle if
a GC runs inside fy_emit_event_create.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: hsbt

ext/psych/psych_emitter_fy.c

@@ -313,6 +313,8 @@ static VALUE scalar(VALUE self, VALUE value, VALUE anchor, VALUE tag,
 
     do_emit(e, event);
     RB_GC_GUARD(value);
+    RB_GC_GUARD(anchor);
+    RB_GC_GUARD(tag);
     return self;
 }
 
@@ -333,6 +335,8 @@ static VALUE start_sequence(VALUE self, VALUE anchor, VALUE tag,
             NIL_P(tag) ? NULL : StringValueCStr(tag));
 
     do_emit(e, event);
+    RB_GC_GUARD(anchor);
+    RB_GC_GUARD(tag);
     return self;
 }
 
@@ -362,6 +366,8 @@ static VALUE start_mapping(VALUE self, VALUE anchor, VALUE tag,
             NIL_P(tag) ? NULL : StringValueCStr(tag));
 
     do_emit(e, event);
+    RB_GC_GUARD(anchor);
+    RB_GC_GUARD(tag);
     return self;
 }
 
@@ -383,6 +389,7 @@ static VALUE alias(VALUE self, VALUE anchor)
 
     do_emit(e, fy_emit_event_create(e->emit, FYET_ALIAS,
             NIL_P(anchor) ? NULL : StringValueCStr(anchor)));
+    RB_GC_GUARD(anchor);
     return self;
 }
 

ext/psych/psych_parser_fy.c

@@ -112,7 +112,11 @@ static VALUE make_exception(psych_fy_parser_t *parser, VALUE path)
         void *iter = NULL;
         struct fy_diag_error *err = fy_diag_errors_iterate(diag, &iter);
         if (err) {
-            if (err->msg) problem = rb_usascii_str_new2(err->msg);
+            /* The message may embed a snippet of the (possibly multibyte)
+             * input, so mark it UTF-8 rather than US-ASCII. */
+            if (err->msg) {
+                problem = rb_enc_str_new_cstr(err->msg, rb_utf8_encoding());
+            }
             if (err->line >= 0) line = (size_t)err->line;
             if (err->column >= 0) column = (size_t)err->column;
         }