fileutil.py: add rename_no_overwrite and replace_file.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

diff --git a/src/allmydata/test/test_util.py b/src/allmydata/test/test_util.py
index a2bba112599cecab5d1456d14bcbee08d17fef77..fd399f6f1b16d4516cac7be47d50afb0988585a7 100644 (file)
--- a/src/allmydata/test/test_util.py
+++ b/src/allmydata/test/test_util.py
@@ -453,6 +453,73 @@ class FileUtil(ReallyEqualMixin, unittest.TestCase):
         self.failIf(os.path.exists(fn))
         self.failUnless(os.path.exists(fn2))
 
+    def test_rename_no_overwrite(self):
+        workdir = fileutil.abspath_expanduser_unicode(u"test_rename_no_overwrite")
+        fileutil.make_dirs(workdir)
+
+        source_path = os.path.join(workdir, "source")
+        dest_path   = os.path.join(workdir, "dest")
+
+        # when neither file exists
+        self.failUnlessRaises(OSError, fileutil.rename_no_overwrite, source_path, dest_path)
+
+        # when only dest exists
+        fileutil.write(dest_path,   "dest")
+        self.failUnlessRaises(OSError, fileutil.rename_no_overwrite, source_path, dest_path)
+        self.failUnlessEqual(fileutil.read(dest_path),   "dest")
+
+        # when both exist
+        fileutil.write(source_path, "source")
+        self.failUnlessRaises(OSError, fileutil.rename_no_overwrite, source_path, dest_path)
+        self.failUnlessEqual(fileutil.read(source_path), "source")
+        self.failUnlessEqual(fileutil.read(dest_path),   "dest")
+
+        # when only source exists
+        os.remove(dest_path)
+        fileutil.rename_no_overwrite(source_path, dest_path)
+        self.failUnlessEqual(fileutil.read(dest_path), "source")
+        self.failIf(os.path.exists(source_path))
+
+    def test_replace_file(self):
+        workdir = fileutil.abspath_expanduser_unicode(u"test_replace_file")
+        fileutil.make_dirs(workdir)
+
+        backup_path      = os.path.join(workdir, "backup")
+        replaced_path    = os.path.join(workdir, "replaced")
+        replacement_path = os.path.join(workdir, "replacement")
+
+        # when none of the files exist
+        self.failUnlessRaises(fileutil.ConflictError, fileutil.replace_file, replaced_path, replacement_path, backup_path)
+
+        # when only replaced exists
+        fileutil.write(replaced_path,    "foo")
+        self.failUnlessRaises(fileutil.ConflictError, fileutil.replace_file, replaced_path, replacement_path, backup_path)
+        self.failUnlessEqual(fileutil.read(replaced_path), "foo")
+
+        # when both replaced and replacement exist, but not backup
+        fileutil.write(replacement_path, "bar")
+        fileutil.replace_file(replaced_path, replacement_path, backup_path)
+        self.failUnlessEqual(fileutil.read(backup_path),   "foo")
+        self.failUnlessEqual(fileutil.read(replaced_path), "bar")
+        self.failIf(os.path.exists(replacement_path))
+
+        # when only replacement exists
+        os.remove(backup_path)
+        os.remove(replaced_path)
+        fileutil.write(replacement_path, "bar")
+        self.failUnlessRaises(fileutil.ConflictError, fileutil.replace_file, replaced_path, replacement_path, backup_path)
+        self.failUnlessEqual(fileutil.read(replacement_path), "bar")
+        self.failIf(os.path.exists(replaced_path))
+        self.failIf(os.path.exists(backup_path))
+
+        # when replaced, replacement and backup all exist
+        fileutil.write(replaced_path,    "foo")
+        fileutil.write(backup_path,      "bak")
+        fileutil.replace_file(replaced_path, replacement_path, backup_path)
+        self.failUnlessEqual(fileutil.read(backup_path),   "foo")
+        self.failUnlessEqual(fileutil.read(replaced_path), "bar")
+        self.failIf(os.path.exists(replacement_path))
+
     def test_du(self):
         basedir = "util/FileUtil/test_du"
         fileutil.make_dirs(basedir)
@@ -579,6 +646,7 @@ class FileUtil(ReallyEqualMixin, unittest.TestCase):
         disk = fileutil.get_disk_stats('.', 2**128)
         self.failUnlessEqual(disk['avail'], 0)
 
+
 class PollMixinTests(unittest.TestCase):
     def setUp(self):
         self.pm = pollmixin.PollMixin()

diff --git a/src/allmydata/util/fileutil.py b/src/allmydata/util/fileutil.py
index 6dcf4ee1072da18394602518e85d336d19db5c4c..87fdda64f4c6c1a7949d530b131172409b883ecb 100644 (file)
--- a/src/allmydata/util/fileutil.py
+++ b/src/allmydata/util/fileutil.py
@@ -3,6 +3,7 @@ Futz with files like a pro.
 """
 
 import sys, exceptions, os, stat, tempfile, time, binascii
+from errno import EEXIST
 
 from twisted.python import log
 
@@ -546,8 +547,7 @@ def get_available_space(whichdir, reserved_space):
 
 
 if sys.platform == "win32":
-    from ctypes import WINFUNCTYPE, windll, WinError
-    from ctypes.wintypes import BOOL, HANDLE, DWORD, LPCWSTR, LPVOID
+    from ctypes.wintypes import BOOL, HANDLE, DWORD, LPCWSTR, LPVOID, WinError, get_last_error
 
     # <http://msdn.microsoft.com/en-us/library/aa363858%28v=vs.85%29.aspx>
     CreateFileW = WINFUNCTYPE(
@@ -603,3 +603,63 @@ else:
     def flush_volume(path):
         # use sync()?
         pass
+
+
+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":
+    from ctypes import WINFUNCTYPE, windll, WinError, get_last_error
+    from ctypes.wintypes import BOOL, DWORD, LPCWSTR, LPVOID
+
+    # <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
+
+    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()
+            raise ConflictError("WinError: %s" % (WinError(err)))
+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)
+
+        try:
+            os.rename(replaced_path, backup_path)
+            rename_no_overwrite(replacement_path, replaced_path)
+        except EnvironmentError:
+            reraise(ConflictError)