# HG changeset patch
# User brett
# Date 1200861122 18000
# Node ID 16506464d57ba312e5cbacbbc0c2c85afeec9ddf
# Parent  5f6afe2cf9e19280d9d85997adf91133dede1c7f
[svn] Steel FilenameChecker against race conditions.

Since FilenameChecker was just using os.path.exists() to see whether or not
a filename was free, it had a time of check vs. time of use vulnerability.
Matthew was prodding me about the inconsistency with DirectoryChecker, so
that inspired me to fix this.  This also entailed some refactoring.  Thanks
Matthew.

diff -r 5f6afe2cf9e1 -r 16506464d57b scripts/dtrx
--- a/scripts/dtrx	Sun Jan 20 13:42:48 2008 -0500
+++ b/scripts/dtrx	Sun Jan 20 15:32:02 2008 -0500
@@ -71,11 +71,23 @@
 logger = logging.getLogger('dtrx-log')
 
 class FilenameChecker(object):
+    free_func = os.open
+    free_args = (os.O_CREAT | os.O_EXCL,)
+    free_close = os.close
+
     def __init__(self, original_name):
         self.original_name = original_name
 
     def is_free(self, filename):
-        return not os.path.exists(filename)
+        try:
+            result = self.free_func(filename, *self.free_args)
+        except OSError, error:
+            if error.errno == errno.EEXIST:
+                return False
+            raise
+        if self.free_close:
+            self.free_close(result)
+        return True
 
     def create(self):
         fd, filename = tempfile.mkstemp(prefix=self.original_name + '.',
@@ -84,8 +96,6 @@
         return filename
 
     def check(self):
-        if self.is_free(self.original_name):
-            return self.original_name
         for suffix in [''] + ['.%s' % (x,) for x in range(1, 10)]:
             filename = '%s%s' % (self.original_name, suffix)
             if self.is_free(filename):
@@ -94,14 +104,9 @@
 
 
 class DirectoryChecker(FilenameChecker):
-    def is_free(self, filename):
-        try:
-            os.mkdir(filename)
-        except OSError, error:
-            if error.errno == errno.EEXIST:
-                return False
-            raise
-        return True
+    free_func = os.mkdir
+    free_args = ()
+    free_close = None
 
     def create(self):
         return tempfile.mkdtemp(prefix=self.original_name + '.', dir='.')