scmutil: use context managers for file handles

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.

--- a/mercurial/scmutil.py	Sat Jan 02 15:33:01 2016 -0800
+++ b/mercurial/scmutil.py	Sat Jan 02 15:19:47 2016 -0800
@@ -265,39 +265,24 @@
         return self.__call__(path, mode, text, atomictemp, notindexed)
 
     def read(self, path):
-        fp = self(path, 'rb')
-        try:
+        with self(path, 'rb') as fp:
             return fp.read()
-        finally:
-            fp.close()
 
     def readlines(self, path, mode='rb'):
-        fp = self(path, mode=mode)
-        try:
+        with self(path, mode=mode) as fp:
             return fp.readlines()
-        finally:
-            fp.close()
 
     def write(self, path, data):
-        fp = self(path, 'wb')
-        try:
+        with self(path, 'wb') as fp:
             return fp.write(data)
-        finally:
-            fp.close()
 
     def writelines(self, path, data, mode='wb', notindexed=False):
-        fp = self(path, mode=mode, notindexed=notindexed)
-        try:
+        with self(path, mode=mode, notindexed=notindexed) as fp:
             return fp.writelines(data)
-        finally:
-            fp.close()
 
     def append(self, path, data):
-        fp = self(path, 'ab')
-        try:
+        with self(path, 'ab') as fp:
             return fp.write(data)
-        finally:
-            fp.close()
 
     def basename(self, path):
         """return base element of a path (as os.path.basename would do)
@@ -526,11 +511,10 @@
                     else:
                         # nlinks() may behave differently for files on Windows
                         # shares if the file is open.
-                        fd = util.posixfile(f)
-                        nlink = util.nlinks(f)
-                        if nlink < 1:
-                            nlink = 2 # force mktempcopy (issue1922)
-                        fd.close()
+                        with util.posixfile(f):
+                            nlink = util.nlinks(f)
+                            if nlink < 1:
+                                nlink = 2 # force mktempcopy (issue1922)
                 except (OSError, IOError) as e:
                     if e.errno != errno.ENOENT:
                         raise
@@ -1047,10 +1031,9 @@
     return requirements
 
 def writerequires(opener, requirements):
-    reqfile = opener("requires", "w")
-    for r in sorted(requirements):
-        reqfile.write("%s\n" % r)
-    reqfile.close()
+    with opener('requires', 'w') as fp:
+        for r in sorted(requirements):
+            fp.write("%s\n" % r)
 
 class filecachesubentry(object):
     def __init__(self, path, stat):