From 2aafcd5df547f8d009b6d2e197a517906ad37c8a Mon Sep 17 00:00:00 2001
From: Unrud <unrud@outlook.com>
Date: Fri, 23 Oct 2020 21:28:19 +0200
Subject: [PATCH] Use renameat2 on Linux for atomic exchanging of files

---
 radicale/pathutils.py                         | 58 +++++++++++++++++++
 .../multifilesystem/create_collection.py      | 10 ++--
 2 files changed, 62 insertions(+), 6 deletions(-)

diff --git a/radicale/pathutils.py b/radicale/pathutils.py
index 6a2cb1b..400a604 100644
--- a/radicale/pathutils.py
+++ b/radicale/pathutils.py
@@ -24,7 +24,9 @@ Helper functions for working with the file system.
 import contextlib
 import os
 import posixpath
+import sys
 import threading
+from tempfile import TemporaryDirectory
 
 if os.name == "nt":
     import ctypes
@@ -66,6 +68,23 @@ if os.name == "nt":
 elif os.name == "posix":
     import fcntl
 
+HAVE_RENAMEAT2 = False
+if sys.platform == "linux":
+    import ctypes
+
+    RENAME_EXCHANGE = 2
+    try:
+        renameat2 = ctypes.CDLL(None, use_errno=True).renameat2
+    except AttributeError:
+        pass
+    else:
+        HAVE_RENAMEAT2 = True
+        renameat2.argtypes = [
+            ctypes.c_int, ctypes.c_char_p,
+            ctypes.c_int, ctypes.c_char_p,
+            ctypes.c_uint]
+        renameat2.restype = ctypes.c_int
+
 
 class RwLock:
     """A readers-Writer lock that locks a file."""
@@ -127,6 +146,45 @@ class RwLock:
                     self._writer = False
 
 
+def rename_exchange(src, dst):
+    """Exchange the files or directories `src` and `dst`.
+
+    Both `src` and `dst` must exist but may be of different types.
+
+    On Linux with renameat2 the operation is atomic.
+    On other platforms it's not atomic.
+
+    """
+    src_dir, src_base = os.path.split(src)
+    dst_dir, dst_base = os.path.split(dst)
+    src_dir = src_dir or os.curdir
+    dst_dir = dst_dir or os.curdir
+    if not src_base or not dst_base:
+        raise ValueError("Invalid arguments: %r -> %r" % (src, dst))
+    if HAVE_RENAMEAT2:
+        src_base_bytes = os.fsencode(src_base)
+        dst_base_bytes = os.fsencode(dst_base)
+        src_dir_fd = os.open(src_dir, 0)
+        try:
+            dst_dir_fd = os.open(dst_dir, 0)
+            try:
+                if renameat2(src_dir_fd, src_base_bytes,
+                             dst_dir_fd, dst_base_bytes,
+                             RENAME_EXCHANGE) != 0:
+                    errno = ctypes.get_errno()
+                    raise OSError(errno, os.strerror(errno))
+            finally:
+                os.close(dst_dir_fd)
+        finally:
+            os.close(src_dir_fd)
+    else:
+        with TemporaryDirectory(
+                prefix=".Radicale.tmp-", dir=src_dir) as tmp_dir:
+            os.rename(dst, os.path.join(tmp_dir, "interim"))
+            os.rename(src, dst)
+            os.rename(os.path.join(tmp_dir, "interim"), src)
+
+
 def fsync(fd):
     if os.name == "posix" and hasattr(fcntl, "F_FULLFSYNC"):
         fcntl.fcntl(fd, fcntl.F_FULLFSYNC)
diff --git a/radicale/storage/multifilesystem/create_collection.py b/radicale/storage/multifilesystem/create_collection.py
index 6be6345..fe0290e 100644
--- a/radicale/storage/multifilesystem/create_collection.py
+++ b/radicale/storage/multifilesystem/create_collection.py
@@ -55,12 +55,10 @@ class StorageCreateCollectionMixin:
                 elif props.get("tag") == "VADDRESSBOOK":
                     col._upload_all_nonatomic(items, suffix=".vcf")
 
-            # This operation is not atomic on the filesystem level but it's
-            # very unlikely that one rename operations succeeds while the
-            # other fails or that only one gets written to disk.
-            if os.path.exists(filesystem_path):
-                os.rename(filesystem_path, os.path.join(tmp_dir, "delete"))
-            os.rename(tmp_filesystem_path, filesystem_path)
+            if os.path.lexists(filesystem_path):
+                pathutils.rename_exchange(tmp_filesystem_path, filesystem_path)
+            else:
+                os.rename(tmp_filesystem_path, filesystem_path)
             self._sync_directory(parent_dir)
 
         return self._collection_class(