scmutil: use context managers for file handles
Now that we dropped support for Python 2.4, we are able to use context
managers. Let's replace the try..finally pattern in scmutil.py with
context managers, which close files automatically when the context
manager is exited.
There should be no change in behavior with this patch.
Why convert to context managers if nothing is broken? I'm working on
closing file handles in background threads to improve performance on
Windows. As part of this, I realized there could be some future issues
if the background file closing code isn't designed with context
managers in mind. So, I'd like to switch some code to context managers
so I can design an API that works with context managers.
statichttprepo: implement __enter__ and __exit__ on httprangeheader
httprangeheader behaves like a file object. Implement __enter__ and
__exit__ so it can be used as a context manager, just like file objects.
osutil: implement __enter__ and __exit__ on posixfile
So they can be used as context managers.
util: introduce ctxmanager, to avoid nested try/finally blocks
This is similar in spirit to contextlib.nested in Python <= 2.6,
but uses an extra level of indirection to avoid its inability to
clean up if an __enter__ method raises an exception.
Why add this mechanism? It greatly simplifies scoped resource
management, and lets us eliminate several hundred lines of try/finally
blocks. In many of these cases the "finally" is separated from the
"try" by hundreds of lines of code, which makes the connection
between resource acquisition and disposal difficult to follow.
(The preferred mechanism would be the "multi-with" syntax of 2.7+,
but Mercurial can't move to 2.7 for a while.)
Intended use:
>>> with ctxmanager(lambda: file('foo'), lambda: file('bar')) as c:
>>> f1, f2 = c()
This will open both foo and bar when c() is invoked, and will close
both upon exit from the block. If the attempt to open bar raises
an exception, the block will not be entered - but foo will still
be closed.
hghave: support HGMODULEPOLICY for pure
HGMODULEPOLICY was introduced in
4374d819ccd5