diff tests/test-atomictempfile.py @ 14007:d764463b433e

atomictempfile: avoid infinite recursion in __del__().

The problem is that a programmer using atomictempfile directly can
make an innocent everyday mistake -- not enough args to the
constructor -- which escalates badly.  You would expect a simple
TypeError crash in that case, but you actually get an infinite
recursion that is surprisingly difficult to kill: it happens between
__del__() and __getattr__(), and Python does not handle infinite
recursion from __del__() well.

The fix is to not implement __getattr__(), but instead assign instance
attributes for the methods we wish to delegate to the builtin file
type: write() and fileno(). I've audited mercurial.* and hgext.* and
found no users of atomictempfile using methods other than write() and
rename(). I audited third-party extensions and found one (snap)
passing an atomictempfile to util.fstat(), so I also threw in
fileno().

The last time I submitted a similar patch, Matt proposed that we make
atomictempfile a subclass of file instead of wrapping it. Rejected on
grounds of unnecessary complexity: for one thing, it would make the
Windows implementation of posixfile quite a bit more complex. It would
have to become a subclass of file rather than a simple function -- but
since it's written in C, this is non-obvious and non-trivial.
Furthermore, there's nothing wrong with wrapping objects and
delegating methods: it's a well-established pattern that works just
fine in many cases. Subclassing is not the answer to all of life's
problems.

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tests/test-atomictempfile.py	Sun Apr 24 19:25:10 2011 -0400
@@ -0,0 +1,49 @@
+import os
+import glob
+from mercurial.util import atomictempfile
+
+# basic usage
+def test1_simple():
+    if os.path.exists('foo'):
+        os.remove('foo')
+    file = atomictempfile('foo')
+    (dir, basename) = os.path.split(file._tempname)
+    assert not os.path.isfile('foo')
+    assert basename in glob.glob('.foo-*')
+
+    file.write('argh\n')
+    file.rename()
+
+    assert os.path.isfile('foo')
+    assert basename not in glob.glob('.foo-*')
+    print 'OK'
+
+# close() removes the temp file but does not make the write
+# permanent -- essentially discards your work (WTF?!)
+def test2_close():
+    if os.path.exists('foo'):
+        os.remove('foo')
+    file = atomictempfile('foo')
+    (dir, basename) = os.path.split(file._tempname)
+
+    file.write('yo\n')
+    file.close()
+
+    assert not os.path.isfile('foo')
+    assert basename not in os.listdir('.')
+    print 'OK'
+
+# if a programmer screws up and passes bad args to atomictempfile, they
+# get a plain ordinary TypeError, not infinite recursion
+def test3_oops():
+    try:
+        file = atomictempfile()
+    except TypeError:
+        print "OK"
+    else:
+        print "expected TypeError"
+
+if __name__ == '__main__':
+    test1_simple()
+    test2_close()
+    test3_oops()