In Zlib::GzipReader#eof? check if we're actually at eof

Only consider it eof if we read ahead and something fills the buf.
If not, we may only have empty blocks and the footer.

Fixes #56

ext/zlib/zlib.c

@@ -3500,6 +3500,9 @@ static VALUE
 rb_gzfile_eof_p(VALUE obj)
 {
     struct gzfile *gz = get_gzfile(obj);
+    while (!ZSTREAM_IS_FINISHED(&gz->z) && ZSTREAM_BUF_FILLED(&gz->z) == 0) {
+	gzfile_read_more(gz, Qnil);
+    }
     return GZFILE_IS_FINISHED(gz) ? Qtrue : Qfalse;
 }
 

test/zlib/test_zlib.rb

@@ -1205,6 +1205,38 @@ def test_double_close
       }
     end
 
+    # Various methods of Zlib::GzipReader failed when to reading files
+    # just a few bytes larger than GZFILE_READ_SIZE.
+    def test_gzfile_read_size_boundary
+      Tempfile.create("test_zlib_gzip_read_size_boundary") {|t|
+        t.close
+        # NO_COMPRESSION helps with recreating the error condition.
+        # The error happens on compressed files too, but it's harder to reproduce.
+        # For example, ~12750 bytes are needed to trigger the error using __FILE__.
+        # We avoid this because the test file will change over time.
+        Zlib::GzipWriter.open(t.path, Zlib::NO_COMPRESSION) do |gz|
+          gz.print("\n" * 2024) # range from 2024 to 2033 triggers the error
+          gz.flush
+        end
+
+        Zlib::GzipReader.open(t.path) do |f|
+          f.readpartial(1024) until f.eof?
+          assert_raise(EOFError) { f.readpartial(1) }
+        end
+
+        Zlib::GzipReader.open(t.path) do |f|
+          f.readline until f.eof?
+          assert_raise(EOFError) { f.readline }
+        end
+
+        Zlib::GzipReader.open(t.path) do |f|
+          b = f.readbyte until f.eof?
+          f.ungetbyte(b)
+          f.readbyte
+          assert_raise(EOFError) { f.readbyte }
+        end
+      }
+    end
   end
 
   class TestZlibGzipWriter < Test::Unit::TestCase