[utils] Make JSON file writes atomic (Fixes #3549)

youtube_dl/utils.py

@@ -24,6 +24,7 @@
 import struct
 import subprocess
 import sys
+import tempfile
 import traceback
 import xml.etree.ElementTree
 import zlib
@@ -228,18 +229,36 @@ def compat_print(s):
         assert type(s) == type(u'')
         print(s)
 
-# In Python 2.x, json.dump expects a bytestream.
-# In Python 3.x, it writes to a character stream
-if sys.version_info < (3,0):
-    def write_json_file(obj, fn):
-        with open(fn, 'wb') as f:
-            json.dump(obj, f)
-else:
-    def write_json_file(obj, fn):
-        with open(fn, 'w', encoding='utf-8') as f:
-            json.dump(obj, f)
 
-if sys.version_info >= (2,7):
+def write_json_file(obj, fn):
+    """ Encode obj as JSON and write it to fn, atomically """
+
+    # In Python 2.x, json.dump expects a bytestream.
+    # In Python 3.x, it writes to a character stream
+    if sys.version_info < (3, 0):
+        mode = 'wb'
+        encoding = None
+    else:
+        mode = 'w'
+        encoding = 'utf-8'
+    tf = tempfile.NamedTemporaryFile(
+        suffix='.tmp', prefix=os.path.basename(fn) + '.',
+        dir=os.path.dirname(fn),
+        delete=False)
+
+    try:
+        with tf:
+            json.dump(obj, tf)
+        os.rename(tf.name, fn)
+    except:
+        try:
+            os.remove(tf.name)
+        except OSError:
+            pass
+        raise
+
+
+if sys.version_info >= (2, 7):
     def find_xpath_attr(node, xpath, key, val):
         """ Find the xpath xpath[@key=val] """
         assert re.match(r'^[a-zA-Z-]+$', key)