Add the updating_file_with_tmpfile() "tool".

python · ericsnowcurrently · Aug 30, 2021 · Jun 10, 2021 · Aug 10, 2021 · Jun 8, 2021
commit ba9417cf169e15facca67578f01c0e0a618b47ba
diff --git a/Tools/scripts/update_file.py b/Tools/scripts/update_file.py
@@ -6,23 +6,51 @@
 actually change the in-tree generated code.
 """
 
+import contextlib
 import os
+import os.path
 import sys
 
 
-def main(old_path, new_path):
-    with open(old_path, 'rb') as f:
+@contextlib.contextmanager
+def updating_file_with_tmpfile(filename, tmpfile=None):
+    """A context manager for updating a file via a temp file.
+
+    The context manager provides two open files: the source file open
+    for reading, and the temp file, open for writing.
+
+    Upon exiting: both files are closed, and the source file is replaced
+    with the temp file.
+    """
+    # XXX Optionally use tempfile.TemporaryFile?
+    if not tmpfile:
+        tmpfile = filename + '.tmp'
+    elif os.path.isdir(tmpfile):
+        tmpfile = os.path.join(tmpfile, filename + '.tmp')
+
+    try:
+        with open(tmpfile, 'w') as outfile:
+            with open(filename) as infile:
+                yield infile, outfile
+        os.replace(tmpfile, filename)
+    finally:
+        if os.path.exists(tmpfile):
+            os.remove(tmpfile)
+
+
+def update_file_with_tmpfile(filename, tmpfile):
+    with open(filename, 'rb') as f:
         old_contents = f.read()
-    with open(new_path, 'rb') as f:
+    with open(tmpfile, 'rb') as f:
         new_contents = f.read()
     if old_contents != new_contents:
-        os.replace(new_path, old_path)
+        os.replace(tmpfile, filename)
     else:
-        os.unlink(new_path)
+        os.unlink(tmpfile)
 
 
 if __name__ == '__main__':
     if len(sys.argv) != 3:
         print("Usage: %s <path to be updated> <path with new contents>" % (sys.argv[0],))
         sys.exit(1)
-    main(sys.argv[1], sys.argv[2])
+    update_file_with_tmpfile(sys.argv[1], sys.argv[2])