nobu	2017-11-23 16:10:56 +0900 (Thu, 23 Nov 2017)

  New Revision: 60884

  https://svn.ruby-lang.org/cgi-bin/viewvc.cgi?view=revision&revision=60884

  Log:
    io.c: read BOM only for reading
    
    * io.c (io_strip_bom): just abandon detecting UTF encoding by BOM
      unless opened for reading.

  Modified files:
    trunk/io.c
    trunk/test/ruby/test_io_m17n.rb
Index: io.c
===================================================================
--- io.c	(revision 60883)
+++ io.c	(revision 60884)
@@ -5970,7 +5970,10 @@ static int https://github.com/ruby/ruby/blob/trunk/io.c#L5970
 io_strip_bom(VALUE io)
 {
     VALUE b1, b2, b3, b4;
+    rb_io_t *fptr;
 
+    GetOpenFile(io, fptr);
+    if (!(fptr->mode & FMODE_READABLE)) return 0;
     if (NIL_P(b1 = rb_io_getbyte(io))) return 0;
     switch (b1) {
       case INT2FIX(0xEF):
Index: test/ruby/test_io_m17n.rb
===================================================================
--- test/ruby/test_io_m17n.rb	(revision 60883)
+++ test/ruby/test_io_m17n.rb	(revision 60884)
@@ -2168,6 +2168,20 @@ EOT https://github.com/ruby/ruby/blob/trunk/test/ruby/test_io_m17n.rb#L2168
     assert_nil(enc)
   end
 
+  def test_bom_non_reading
+    with_tmpdir {
+      enc = nil
+      assert_nothing_raised(IOError) {
+        open("test", "w:bom|utf-8") {|f|
+          enc = f.external_encoding
+          f.print("abc")
+        }
+      }
+      assert_equal(Encoding::UTF_8, enc)
+      assert_equal("abc", File.binread("test"))
+    }
+  end
+
   def test_cbuf
     with_tmpdir {
       fn = "tst"

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