skytools.fileutil: new module, contains write_atomic()

1 files changed, 37 insertions, 0 deletions

diff --git a/python/skytools/fileutil.py b/python/skytools/fileutil.py
new file mode 100644
index 00000000..fd657fd4
--- /dev/null
+++ b/python/skytools/fileutil.py
@@ -0,0 +1,37 @@
+"""File utilities"""
+
+import os
+
+__all__ = ['write_atomic']
+
+def write_atomic(fn, data, bakext=None, mode='b'):
+    """Write file with rename."""
+
+    if mode not in ['', 'b', 't']:
+        raise ValueError("unsupported fopen mode")
+
+    # write new data to tmp file
+    fn2 = fn + '.new'
+    f = open(fn2, 'w' + mode)
+    f.write(data)
+    f.close()
+
+    # link old data to bak file
+    if bakext:
+        if bakext.find('/') >= 0:
+            raise ValueError("invalid bakext")
+        fnb = fn + bakext
+        try:
+            os.unlink(fnb)
+        except OSError, e:
+            if e.errno != errno.ENOENT:
+                raise
+        try:
+            os.link(fn, fnb)
+        except OSError, e:
+            if e.errno != errno.ENOENT:
+                raise
+
+    # atomically replace file
+    os.rename(fn2, fn)
+