Do not use `fcopyfile` if appending to non-empty file [Bug #18388]

`fcopyfile` appends `src` to `to` and then truncates `to` to it's
original size.

io.c

@@ -11483,6 +11483,13 @@ nogvl_fcopyfile(struct copy_stream_struct *stp)
         return 0;
     if (lseek(stp->dst_fptr->fd, 0, SEEK_CUR) > (off_t)0) /* if dst IO was already written */
         return 0;
+    if (fcntl(stp->dst_fptr->fd, F_GETFL) & O_APPEND) {
+        /* fcopyfile(3) appends src IO to dst IO and then truncates
+         * dst IO to src IO's original size. */
+        off_t end = lseek(stp->dst_fptr->fd, 0, SEEK_END);
+        lseek(stp->dst_fptr->fd, 0, SEEK_SET);
+        if (end > (off_t)0) return 0;
+    }
 
     if (src_offset > (off_t)0) {
         off_t r;

test/ruby/test_io.rb

@@ -476,6 +476,18 @@ class TestIO < Test::Unit::TestCase
     }
   end
 
+  def test_copy_stream_append_to_nonempty
+    with_srccontent("foobar") {|src, content|
+      preface = 'preface'
+      File.write('dst', preface)
+      File.open('dst', 'ab') do |dst|
+        ret = IO.copy_stream(src, dst)
+        assert_equal(content.bytesize, ret)
+        assert_equal(preface + content, File.read("dst"))
+      end
+    }
+  end
+
   def test_copy_stream_smaller
     with_srccontent {|src, content|