Fix `IO#readchar` to support UTF-8 char reading; fix #4712

This fix only effective when `MRB_UTF8_STRING` is set.

mrbgems/mruby-io/mrblib/io.rb

@@ -283,9 +283,10 @@ class IO
 
   def readchar
     _read_buf
-    c = @buf[0]
-    @buf[0] = ""
-    c
+    _readchar(@buf)
+#    c = @buf[0]
+#    @buf[0] = ""
+#    c
   end
 
   def getc

mrbgems/mruby-io/src/io.c

@@ -1454,14 +1454,13 @@ mrb_io_pwrite(mrb_state *mrb, mrb_value io)
 #endif /* MRB_WITH_IO_PREAD_PWRITE */
 
 static mrb_value
-io_bufread(mrb_state *mrb, mrb_value self)
+io_bufread(mrb_state *mrb, mrb_value str, mrb_int len)
 {
-  mrb_value str, str2;
-  mrb_int len, newlen;
+  mrb_value str2;
+  mrb_int newlen;
   struct RString *s;
   char *p;
 
-  mrb_get_args(mrb, "Si", &str, &len);
   s = RSTRING(str);
   mrb_str_modify(mrb, s);
   p = RSTR_PTR(s);
@@ -1474,6 +1473,40 @@ io_bufread(mrb_state *mrb, mrb_value self)
   return str2;
 }
 
+static mrb_value
+mrb_io_bufread(mrb_state *mrb, mrb_value self)
+{
+  mrb_value str;
+  mrb_int len;
+
+  mrb_get_args(mrb, "Si", &str, &len);
+  return io_bufread(mrb, str, len);
+}
+
+static mrb_value
+mrb_io_readchar(mrb_state *mrb, mrb_value self)
+{
+  mrb_value buf;
+  unsigned char c;
+  mrb_int len = 1;
+
+  mrb_get_args(mrb, "S", &buf);
+  mrb_assert(RSTRING_PTR(buf) > 0);
+#ifdef MRB_UTF8_STRING
+  c = RSTRING_PTR(buf)[0];
+  if (c & 0x80) {
+    len = mrb_utf8len(RSTRING_PTR(buf), RSTRING_END(buf));
+    if (len == 1 && RSTRING_LEN(buf) < 5) { /* partial UTF-8 */
+      /* refill the buffer */
+      mrb_value b = mrb_io_sysread_common(mrb, mrb_sysread_dummy, self, mrb_nil_value(), 4096, 0);
+      mrb_str_concat(mrb, buf, b);
+    }
+    len = mrb_utf8len(RSTRING_PTR(buf), RSTRING_END(buf));
+  }
+#endif
+  return io_bufread(mrb, buf, len);
+}
+
 void
 mrb_init_io(mrb_state *mrb)
 {
@@ -1511,5 +1544,6 @@ mrb_init_io(mrb_state *mrb)
   mrb_define_method(mrb, io, "pread",      mrb_io_pread,      MRB_ARGS_ANY());    /* ruby 2.5 feature */
   mrb_define_method(mrb, io, "pwrite",     mrb_io_pwrite,     MRB_ARGS_ANY());    /* ruby 2.5 feature */
 
-  mrb_define_class_method(mrb, io, "_bufread",   io_bufread,        MRB_ARGS_REQ(2));
+  mrb_define_method(mrb, io, "_readchar",  mrb_io_readchar,   MRB_ARGS_REQ(1));
+  mrb_define_class_method(mrb, io, "_bufread",   mrb_io_bufread,    MRB_ARGS_REQ(2));
 }