Fix a race condition with initial secret_key.py creation

Currently, it's possible (even if unlikely) for multiple backend processes
to create and use different Django `SECRET_KEY` values, because the
following scenario is possible:

* process 1: fail to import secret_key.py
* process 2: fail to import secret_key.py
* process 1: generate a new secret_key.py
* process 1: import secret_key.py
* process 2: generate a new secret_key.py
* process 2: import secret_key.py

Fix this by making it so that `secret_key.py` is created atomically, and
never overwritten if it already exists.

In addition, only generate the secret key if the import fails due to the
module not being found, since other failure reasons suggest incorrect
configuration or data corruption, and so require administrator attention.

cvat/settings/base.py

@@ -21,6 +21,7 @@
 import shutil
 import subprocess
 import sys
+import tempfile
 from datetime import timedelta
 from distutils.util import strtobool
 from enum import Enum
@@ -40,18 +41,43 @@
 ALLOWED_HOSTS = os.environ.get('ALLOWED_HOSTS', 'localhost,127.0.0.1').split(',')
 INTERNAL_IPS = ['127.0.0.1']
 
-try:
-    sys.path.append(BASE_DIR)
-    from keys.secret_key import SECRET_KEY # pylint: disable=unused-import
-except ImportError:
+def generate_secret_key():
+    """
+    Creates secret_key.py in such a way that multiple processes calling
+    this will all end up with the same key (assuming that they share the
+    same "keys" directory).
+    """
 
     from django.utils.crypto import get_random_string
     keys_dir = os.path.join(BASE_DIR, 'keys')
     if not os.path.isdir(keys_dir):
         os.mkdir(keys_dir)
-    with open(os.path.join(keys_dir, 'secret_key.py'), 'w') as f:
-        chars = 'abcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*(-_=+)'
-        f.write("SECRET_KEY = '{}'\n".format(get_random_string(50, chars)))
+
+    chars = 'abcdefghijklmnopqrstuvwxyz0123456789!@#$%^&*(-_=+)'
+    secret_key_fname = 'secret_key.py'
+
+    f = None
+    try:
+        with tempfile.NamedTemporaryFile(
+            mode='wt', dir=keys_dir, prefix=secret_key_fname + ".", delete=False,
+        ) as f:
+            f.write("SECRET_KEY = '{}'\n".format(get_random_string(50, chars)))
+
+        try:
+            os.link(f.name, os.path.join(keys_dir, secret_key_fname))
+        except FileExistsError:
+            # Somebody else created the secret key first.
+            # Discard ours and use theirs.
+            pass
+    finally:
+        if f: # f can be None if the NamedTemporaryFile constructor fails.
+            os.unlink(f.name)
+
+try:
+    sys.path.append(BASE_DIR)
+    from keys.secret_key import SECRET_KEY # pylint: disable=unused-import
+except ModuleNotFoundError:
+    generate_secret_key()
     from keys.secret_key import SECRET_KEY