From 166be7d7684b4bd309661603a35710e0ed752680 Mon Sep 17 00:00:00 2001
From: Ulf Magnusson <ulfalizer@gmail.com>
Date: Fri, 7 Dec 2018 23:24:39 +0100
Subject: Save existing configuration to .<filename>.old in write_config()

Add a default-True 'save_old' flag to write_config(). If 'save_old' is
True and an existing configuration file is being overwritten, a copy of
the old configuration file is saved to .<filename>.old (e.g.
.config.old) in the same directory.

Errors are ignored, as the old configuration would usually just be a
nice-to-have, and not essential.

The same functionality could be added for minimal configuration files
and headers, but it's probably most useful for configuration files.
---
 kconfiglib.py | 40 +++++++++++++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/kconfiglib.py b/kconfiglib.py
index 6ffac90..b854186 100644
--- a/kconfiglib.py
+++ b/kconfiglib.py
@@ -1244,7 +1244,8 @@ class Kconfig(object):
                                 .format(self.config_prefix, sym.name, val))
 
     def write_config(self, filename,
-                     header="# Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)\n"):
+                     header="# Generated by Kconfiglib (https://github.com/ulfalizer/Kconfiglib)\n",
+                     save_old=True):
         r"""
         Writes out symbol values in the .config format. The format matches the
         C implementation, including ordering.
@@ -1264,7 +1265,19 @@ class Kconfig(object):
           Text that will be inserted verbatim at the beginning of the file. You
           would usually want each line to start with '#' to make it a comment,
           and include a final terminating newline.
+
+        save_old (default: True):
+          If True and <filename> already exists, a copy of it will be saved to
+          .<filename>.old in the same directory before the new configuration is
+          written. The leading dot is added only if the filename doesn't
+          already start with a dot.
+
+          Errors are silently ignored if .<filename>.old cannot be written
+          (e.g. due to being a directory).
         """
+        if save_old:
+            _save_old(filename)
+
         with self._open(filename, "w") as f:
             f.write(header)
 
@@ -5776,6 +5789,31 @@ def _touch_dep_file(sym_name):
     os.close(os.open(
         sym_path, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o644))
 
+def _save_old(path):
+    # See write_config()
+
+    dirname, basename = os.path.split(path)
+    if basename.startswith("."):
+        basename += ".old"
+    else:
+        basename = "." + basename + ".old"
+    backup = os.path.join(dirname, basename)
+
+    try:
+        # os.replace() would be nice here, but it's Python 3 (3.3+) only
+        if os.name == "posix":
+            # Will remove .<filename>.old if it already exists on POSIX
+            # systems
+            os.rename(path, backup)
+        else:
+            import shutil
+            shutil.copyfile(path, backup)
+    except:
+        # Ignore errors from 'filename' missing as well as other errors. The
+        # backup file is more of a nice-to-have, and not worth erroring out
+        # over e.g. if .<filename>.old happens to be a directory.
+        pass
+
 def _decoding_error(e, filename, macro_linenr=None):
     # Gives the filename and context for UnicodeDecodeError's, which are a pain
     # to debug otherwise. 'e' is the UnicodeDecodeError object.
-- 
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