gfileutils: Add a missing ftruncate() call when writing files

When calling `g_file_set_contents_full()` without
`G_FILE_SET_CONTENTS_CONSISTENT`, the file is written by opening it,
`write()`ing to it, then closing it.

This is fine as long as the file is not longer than the new content you
want to set its contents to. If it is, the last bit of the old content
remains, because `g_file_set_contents_full()` was missing an
`ftruncate()` call.

Fix that, and change the tests to catch truncation failures in future.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #3144

glib/tests/fileutils.c

@@ -1603,6 +1603,8 @@ test_set_contents_full (void)
           gsize len;
           gboolean ret;
           GStatBuf statbuf;
+          const gchar *original_contents = "a string which is longer than what will be overwritten on it";
+          size_t original_contents_len = strlen (original_contents);
 
           g_test_message ("Flags %d and test %" G_GSIZE_FORMAT, flags, i);
 
@@ -1617,7 +1619,7 @@ test_set_contents_full (void)
 
                 fd = g_file_open_tmp (NULL, &file_name, &error);
                 g_assert_no_error (error);
-                g_assert_cmpint (write (fd, "a", 1), ==, 1);
+                g_assert_cmpint (write (fd, original_contents, original_contents_len), ==, original_contents_len);
                 g_assert_no_errno (g_fsync (fd));
                 close (fd);
 
@@ -1713,7 +1715,7 @@ test_set_contents_full (void)
 
                   g_file_get_contents (file_name, &target_contents, NULL, &error);
                   g_assert_no_error (error);
-                  g_assert_cmpstr (target_contents, ==, "a");
+                  g_assert_cmpstr (target_contents, ==, original_contents);
 
                   g_free (target_contents);
                 }