win32: correctly break hardlinks on network drives (issue761)

win32.nlinks() was often returning 1 instead of the correct
hardlinks count when reading from network drives. This made
commit or push to a repository on a network share to fail
breaking the hardlinks in the datastore, possibly causing
integrity errors in repositories linked locally on the remote
side.

Here is what the MSDN says about GetFileInformationByHandle():

  Depending on the underlying network features of the operating
  system and the type of server connected to, the
  GetFileInformationByHandle function may fail, return partial
  information, or full information for the given file.

In practice, we never got the correct hardlinks count when
reading from and to many combinations of Window XP, 2003, Vista
and 7, via network drives or RDP shares. It always returned 1
instead. The only setup returning an accurate links count was a
samba on Debian.

To avoid this, Mercurial now breaks the hardlinks unconditionally
when writing to a network drive.

mercurial/win32.py

@@ -1,204 +1,205 b''
 # win32.py - utility functions that use win32 API
 #
 # Copyright 2005-2009 Matt Mackall <[e-mail]> and others
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 """Utility functions that use win32 API.
 
 Mark Hammond's win32all package allows better functionality on
 Windows. This module overrides definitions in util.py. If not
 available, import of this module will fail, and generic code will be
 used.
 """
 
 import win32api
 
 import errno, os, sys, pywintypes, win32con, win32file, win32process
 import winerror, win32gui, win32console
 import osutil, encoding
 from win32com.shell import shell, shellcon
 
 def os_link(src, dst):
     try:
         win32file.CreateHardLink(dst, src)
-        # CreateHardLink sometimes succeeds on mapped drives but
-        # following nlinks() returns 1. Check it now and bail out.
-        if nlinks(src) < 2:
-            try:
-                win32file.DeleteFile(dst)
-            except:
-                pass
-            # Fake hardlinking error
-            raise OSError(errno.EINVAL, 'Hardlinking not supported')
     except pywintypes.error:
         raise OSError(errno.EINVAL, 'target implements hardlinks improperly')
     except NotImplementedError: # Another fake error win Win98
         raise OSError(errno.EINVAL, 'Hardlinking not supported')
 
 def _getfileinfo(pathname):
     """Return number of hardlinks for the given file."""
     try:
         fh = win32file.CreateFile(pathname,
             win32file.GENERIC_READ, win32file.FILE_SHARE_READ,
             None, win32file.OPEN_EXISTING, 0, None)
         try:
             return win32file.GetFileInformationByHandle(fh)
         finally:
             fh.Close()
     except pywintypes.error:
         return None
 
 def nlinks(pathname):
     """Return number of hardlinks for the given file."""
     res = _getfileinfo(pathname)
     if res is not None:
-        return res[7]
+        links = res[7]
     else:
-        return os.lstat(pathname).st_nlink
+        links = os.lstat(pathname).st_nlink
+    if links < 2:
+        # Known to be wrong for most network drives
+        dirname = os.path.dirname(pathname)
+        if not dirname:
+            dirname = '.'
+        dt = win32file.GetDriveType(dirname + '\\')
+        if dt == 4 or dt == 1:
+            # Fake hardlink to force COW for network drives
+            links = 2
+    return links
 
 def samefile(fpath1, fpath2):
     """Returns whether fpath1 and fpath2 refer to the same file. This is only
     guaranteed to work for files, not directories."""
     res1 = _getfileinfo(fpath1)
     res2 = _getfileinfo(fpath2)
     if res1 is not None and res2 is not None:
         # Index 4 is the volume serial number, and 8 and 9 contain the file ID
         return res1[4] == res2[4] and res1[8] == res2[8] and res1[9] == res2[9]
     else:
         return False
 
 def samedevice(fpath1, fpath2):
     """Returns whether fpath1 and fpath2 are on the same device. This is only
     guaranteed to work for files, not directories."""
     res1 = _getfileinfo(fpath1)
     res2 = _getfileinfo(fpath2)
     if res1 is not None and res2 is not None:
         return res1[4] == res2[4]
     else:
         return False
 
 def testpid(pid):
     '''return True if pid is still running or unable to
     determine, False otherwise'''
     try:
         handle = win32api.OpenProcess(
             win32con.PROCESS_QUERY_INFORMATION, False, pid)
         if handle:
             status = win32process.GetExitCodeProcess(handle)
             return status == win32con.STILL_ACTIVE
     except pywintypes.error, details:
         return details[0] != winerror.ERROR_INVALID_PARAMETER
     return True
 
 def lookup_reg(key, valname=None, scope=None):
     ''' Look up a key/value name in the Windows registry.
 
     valname: value name. If unspecified, the default value for the key
     is used.
     scope: optionally specify scope for registry lookup, this can be
     a sequence of scopes to look up in order. Default (CURRENT_USER,
     LOCAL_MACHINE).
     '''
     try:
         from _winreg import HKEY_CURRENT_USER, HKEY_LOCAL_MACHINE, \
             QueryValueEx, OpenKey
     except ImportError:
         return None
 
     if scope is None:
         scope = (HKEY_CURRENT_USER, HKEY_LOCAL_MACHINE)
     elif not isinstance(scope, (list, tuple)):
         scope = (scope,)
     for s in scope:
         try:
             val = QueryValueEx(OpenKey(s, key), valname)[0]
             # never let a Unicode string escape into the wild
             return encoding.tolocal(val.encode('UTF-8'))
         except EnvironmentError:
             pass
 
 def system_rcpath_win32():
     '''return default os-specific hgrc search path'''
     proc = win32api.GetCurrentProcess()
     try:
         # This will fail on windows < NT
         filename = win32process.GetModuleFileNameEx(proc, 0)
     except:
         filename = win32api.GetModuleFileName(0)
     # Use mercurial.ini found in directory with hg.exe
     progrc = os.path.join(os.path.dirname(filename), 'mercurial.ini')
     if os.path.isfile(progrc):
         return [progrc]
     # Use hgrc.d found in directory with hg.exe
     progrcd = os.path.join(os.path.dirname(filename), 'hgrc.d')
     if os.path.isdir(progrcd):
         rcpath = []
         for f, kind in osutil.listdir(progrcd):
             if f.endswith('.rc'):
                 rcpath.append(os.path.join(progrcd, f))
         return rcpath
     # else look for a system rcpath in the registry
     try:
         value = win32api.RegQueryValue(
                 win32con.HKEY_LOCAL_MACHINE, 'SOFTWARE\\Mercurial')
         rcpath = []
         for p in value.split(os.pathsep):
             if p.lower().endswith('mercurial.ini'):
                 rcpath.append(p)
             elif os.path.isdir(p):
                 for f, kind in osutil.listdir(p):
                     if f.endswith('.rc'):
                         rcpath.append(os.path.join(p, f))
         return rcpath
     except pywintypes.error:
         return []
 
 def user_rcpath_win32():
     '''return os-specific hgrc search path to the user dir'''
     userdir = os.path.expanduser('~')
     if sys.getwindowsversion()[3] != 2 and userdir == '~':
         # We are on win < nt: fetch the APPDATA directory location and use
         # the parent directory as the user home dir.
         appdir = shell.SHGetPathFromIDList(
             shell.SHGetSpecialFolderLocation(0, shellcon.CSIDL_APPDATA))
         userdir = os.path.dirname(appdir)
     return [os.path.join(userdir, 'mercurial.ini'),
             os.path.join(userdir, '.hgrc')]
 
 def getuser():
     '''return name of current user'''
     return win32api.GetUserName()
 
 def set_signal_handler_win32():
     """Register a termination handler for console events including
     CTRL+C. python signal handlers do not work well with socket
     operations.
     """
     def handler(event):
         win32process.ExitProcess(1)
     win32api.SetConsoleCtrlHandler(handler)
 
 def hidewindow():
     def callback(*args, **kwargs):
         hwnd, pid = args
         wpid = win32process.GetWindowThreadProcessId(hwnd)[1]
         if pid == wpid:
             win32gui.ShowWindow(hwnd, win32con.SW_HIDE)
 
     pid =  win32process.GetCurrentProcessId()
     win32gui.EnumWindows(callback, pid)
 
 def termwidth_():
     try:
         # Query stderr to avoid problems with redirections
         screenbuf = win32console.GetStdHandle(win32console.STD_ERROR_HANDLE)
         try:
             window = screenbuf.GetConsoleScreenBufferInfo()['Window']
             width = window.Right - window.Left
             return width
         finally:
             screenbuf.Detach()
     except pywintypes.error:
         return 79