posix, windows: introduce cachestat

This class contains a stat result, and possibly other file info to reliably
determine between two points in time whether a file has changed.

Uniquely identifying a file gives us that reliability because we either
atomic rename or append. So one of two will happen: the file 'id' will change,
or the size of the file will change.

posix implements it simply by calling os.stat() and checking if the result
has st_ino.

For now on Windows we always assume the path is uncacheable. This can be
improved on NTFS due to file IDs: http://msdn.microsoft.com/en-us/library/aa363788(v=vs.85).aspx

So we need to find out if a file path is on an NTFS drive, for that we have:

- GetVolumeInformation, which unfortunately only works with a root path (but is available on XP)
- GetVolumeInformationByHandleW, works on a full file path but requires Vista or higher

mercurial/posix.py

@@ -349,5 +349,21 b' def hidewindow():'
     """
     pass
 
+class cachestat(object):
+    def __init__(self, path):
+        self.stat = os.stat(path)
+
+    def cacheable(self):
+        return bool(self.stat.st_ino)
+
+    def __eq__(self, other):
+        try:
+            return self.stat == other.stat
+        except AttributeError:
+            return False
+
+    def __ne__(self, other):
+        return not self == other
+
 def executablepath():
     return None # available on Windows only

mercurial/util.py

@@ -24,6 +24,7 b" if os.name == 'nt':"
 else:
     import posix as platform
 
+cachestat = platform.cachestat
 checkexec = platform.checkexec
 checklink = platform.checklink
 executablepath = platform.executablepath

mercurial/windows.py

@@ -286,4 +286,11 b' def isexec(f):'
 
 from win32 import *
 
+class cachestat(object):
+    def __init__(self, path):
+        pass
+
+    def cacheable(self):
+        return False
+
 expandglobs = True

tests/hghave

@@ -101,6 +101,16 b' def has_inotify():'
 def has_fifo():
     return hasattr(os, "mkfifo")
 
+def has_cacheable_fs():
+    from mercurial import util
+
+    fd, path = tempfile.mkstemp(prefix=tempprefix)
+    os.close(fd)
+    try:
+        return util.cachestat(path).cacheable()
+    finally:
+        os.remove(path)
+
 def has_lsprof():
     try:
         import _lsprof
@@ -200,6 +210,7 b' checks = {'
     "baz": (has_baz, "GNU Arch baz client"),
     "bzr": (has_bzr, "Canonical's Bazaar client"),
     "bzr114": (has_bzr114, "Canonical's Bazaar client >= 1.14"),
+    "cacheable": (has_cacheable_fs, "cacheable filesystem"),
     "cvs": (has_cvs, "cvs client/server"),
     "darcs": (has_darcs, "darcs client"),
     "docutils": (has_docutils, "Docutils text processing library"),