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/gfileutils.c

@@ -1139,6 +1139,36 @@ fd_should_be_fsynced (int                    fd,
 #endif  /* !HAVE_FSYNC */
 }
 
+static gboolean
+truncate_file (int          fd,
+               off_t        length,
+               const char  *dest_file,
+               GError     **error)
+{
+  while (
+#ifdef G_OS_WIN32
+    g_win32_ftruncate (fd, length) < 0
+#else
+    ftruncate (fd, length) < 0
+#endif
+    )
+    {
+      int saved_errno = errno;
+
+      if (saved_errno == EINTR)
+        continue;
+
+      if (error != NULL)
+        set_file_error (error,
+                        dest_file,
+                        _("Failed to write file “%s”: ftruncate() failed: %s"),
+                        saved_errno);
+      return FALSE;
+    }
+
+  return TRUE;
+}
+
 /* closes @fd once it’s finished (on success or error) */
 static gboolean
 write_to_file (const gchar  *contents,
@@ -1475,6 +1505,8 @@ consistent_out:
         }
 
       do_fsync = fd_should_be_fsynced (direct_fd, filename, flags);
+      if (!truncate_file (direct_fd, 0, filename, error))
+        return FALSE;
       if (!write_to_file (contents, length, g_steal_fd (&direct_fd), filename,
                           do_fsync, error))
         return FALSE;

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);
                 }