Add fileutil.replace_file and rename_no_overwrite.

[tahoe-lafs/tahoe-lafs.git] / src / allmydata / util / fileutil.py

diff --git a/src/allmydata/util/fileutil.py b/src/allmydata/util/fileutil.py
index ac82fee9e3cef7d148dc139f258d49d6a1a9b1e5..d77c40671950bde8953f81a0cb17a5e43c4d82f2 100644 (file)
--- a/src/allmydata/util/fileutil.py
+++ b/src/allmydata/util/fileutil.py
@@ -6,6 +6,11 @@ import sys, exceptions, os, stat, tempfile, time, binascii
 from collections import namedtuple
 from errno import ENOENT
 
+if sys.platform == "win32":
+    from ctypes import WINFUNCTYPE, WinError, windll, POINTER, byref, c_ulonglong, \
+        create_unicode_buffer, get_last_error
+    from ctypes.wintypes import BOOL, HANDLE, DWORD, LPCWSTR, LPWSTR, LPVOID
+
 from twisted.python import log
 
 from pycryptopp.cipher.aes import AES
@@ -338,10 +343,6 @@ def to_windows_long_path(path):
 
 have_GetDiskFreeSpaceExW = False
 if sys.platform == "win32":
-    from ctypes import WINFUNCTYPE, windll, POINTER, byref, c_ulonglong, create_unicode_buffer, \
-        get_last_error
-    from ctypes.wintypes import BOOL, DWORD, LPCWSTR, LPWSTR
-
     # <http://msdn.microsoft.com/en-us/library/windows/desktop/ms683188%28v=vs.85%29.aspx>
     GetEnvironmentVariableW = WINFUNCTYPE(
         DWORD,  LPCWSTR, LPWSTR, DWORD,
@@ -518,6 +519,77 @@ def get_available_space(whichdir, reserved_space):
         return 0
 
 
+class ConflictError(Exception):
+    pass
+
+class UnableToUnlinkReplacementError(Exception):
+    pass
+
+def reraise(wrapper):
+    _, exc, tb = sys.exc_info()
+    wrapper_exc = wrapper("%s: %s" % (exc.__class__.__name__, exc))
+    raise wrapper_exc.__class__, wrapper_exc, tb
+
+if sys.platform == "win32":
+    # <https://msdn.microsoft.com/en-us/library/windows/desktop/aa365512%28v=vs.85%29.aspx>
+    ReplaceFileW = WINFUNCTYPE(
+        BOOL,  LPCWSTR, LPCWSTR, LPCWSTR, DWORD, LPVOID, LPVOID,
+        use_last_error=True
+    )(("ReplaceFileW", windll.kernel32))
+
+    REPLACEFILE_IGNORE_MERGE_ERRORS = 0x00000002
+
+    # <https://msdn.microsoft.com/en-us/library/windows/desktop/ms681382%28v=vs.85%29.aspx>
+    ERROR_FILE_NOT_FOUND = 2
+
+    def rename_no_overwrite(source_path, dest_path):
+        os.rename(source_path, dest_path)
+
+    def replace_file(replaced_path, replacement_path, backup_path):
+        precondition_abspath(replaced_path)
+        precondition_abspath(replacement_path)
+        precondition_abspath(backup_path)
+
+        r = ReplaceFileW(replaced_path, replacement_path, backup_path,
+                         REPLACEFILE_IGNORE_MERGE_ERRORS, None, None)
+        if r == 0:
+            # The UnableToUnlinkReplacementError case does not happen on Windows;
+            # all errors should be treated as signalling a conflict.
+            err = get_last_error()
+            if err != ERROR_FILE_NOT_FOUND:
+                raise ConflictError("WinError: %s" % (WinError(err),))
+
+            try:
+                rename_no_overwrite(replacement_path, replaced_path)
+            except EnvironmentError:
+                reraise(ConflictError)
+else:
+    def rename_no_overwrite(source_path, dest_path):
+        # link will fail with EEXIST if there is already something at dest_path.
+        os.link(source_path, dest_path)
+        try:
+            os.unlink(source_path)
+        except EnvironmentError:
+            reraise(UnableToUnlinkReplacementError)
+
+    def replace_file(replaced_path, replacement_path, backup_path):
+        precondition_abspath(replaced_path)
+        precondition_abspath(replacement_path)
+        precondition_abspath(backup_path)
+
+        if not os.path.exists(replacement_path):
+            raise ConflictError("Replacement file not found: %r" % (replacement_path,))
+
+        try:
+            os.rename(replaced_path, backup_path)
+        except OSError as e:
+            if e.errno != ENOENT:
+                raise
+        try:
+            rename_no_overwrite(replacement_path, replaced_path)
+        except EnvironmentError:
+            reraise(ConflictError)
+
 PathInfo = namedtuple('PathInfo', 'isdir isfile islink exists size mtime ctime')
 
 def get_pathinfo(path_u, now=None):