ARROW-4199: [GLib] Add garrow_seekable_input_stream_peek()

Author: Yosuke Shiro <yosuke.shiro615@gmail.com>
Author: Kouhei Sutou <kou@clear-code.com>

Closes #3351 from shiro615/glib-support-peek and squashes the following commits:

1f44576 <Kouhei Sutou> Improve document
a5f0fdf <Yosuke Shiro> Add GARROW_AVAILABLE_IN_0_12
b27c0a0 <Yosuke Shiro> Use g_bytes_new_static to avoid copying the data
f9d9f23 <Yosuke Shiro>  Add support for Peek to InputStream

Author: shiro615

c_glib/arrow-glib/input-stream.cpp

@@ -325,6 +325,30 @@ garrow_seekable_input_stream_read_at(GArrowSeekableInputStream *input_stream,
 }
 
 
+/**
+ * garrow_seekable_input_stream_peek:
+ * @input_stream: A #GArrowSeekableInputStream.
+ * @n_bytes: The number of bytes to be peeked.
+ *
+ * Returns: (transfer full): The data of the buffer, up to the
+ *   indicated number. The data becomes invalid after any operation on
+ *   the stream. If the stream is unbuffered, the data is empty.
+ *
+ *   It should be freed with g_bytes_unref() when no longer needed.
+ *
+ * Since: 0.12.0
+ */
+GBytes *
+garrow_seekable_input_stream_peek(GArrowSeekableInputStream *input_stream,
+                                  gint64 n_bytes)
+{
+  auto arrow_random_access_file =
+    garrow_seekable_input_stream_get_raw(input_stream);
+  auto string_view = arrow_random_access_file->Peek(n_bytes);
+  return g_bytes_new_static(string_view.data(), string_view.size());
+}
+
+
 typedef struct GArrowBufferInputStreamPrivate_ {
   GArrowBuffer *buffer;
 } GArrowBufferInputStreamPrivate;

c_glib/arrow-glib/input-stream.h

@@ -66,6 +66,9 @@ GArrowBuffer *garrow_seekable_input_stream_read_at(GArrowSeekableInputStream *in
                                                    gint64 position,
                                                    gint64 n_bytes,
                                                    GError **error);
+GARROW_AVAILABLE_IN_0_12
+GBytes *garrow_seekable_input_stream_peek(GArrowSeekableInputStream *input_stream,
+                                          gint64 n_bytes);
 
 
 #define GARROW_TYPE_BUFFER_INPUT_STREAM         \

c_glib/test/test-buffer-input-stream.rb

@@ -39,4 +39,12 @@ def test_align
     read_buffer = buffer_input_stream.read(3)
     assert_equal("rld", read_buffer.data.to_s)
   end
+
+  def test_peek
+    buffer = Arrow::Buffer.new("Hello World")
+    buffer_input_stream = Arrow::BufferInputStream.new(buffer)
+    peeked_data = buffer_input_stream.peek(5)
+    assert_equal(buffer_input_stream.read(5).data.to_s,
+                 peeked_data.to_s)
+  end
 end