win32: add a method to fetch the available pipe data size

This will be used in the next patch to do nonblocking reads from the child
process, like on posix platforms.  See that for why os.fstat() is insufficient.

mercurial/win32.py

@@ -5,7 +5,7 b''
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
-import ctypes, errno, os, subprocess, random
+import ctypes, errno, msvcrt, os, subprocess, random
 
 _kernel32 = ctypes.windll.kernel32
 _advapi32 = ctypes.windll.advapi32
@@ -26,6 +26,7 b' import ctypes, errno, os, subprocess, ra'
 _ERROR_SUCCESS = 0
 _ERROR_NO_MORE_FILES = 18
 _ERROR_INVALID_PARAMETER = 87
+_ERROR_BROKEN_PIPE = 109
 _ERROR_INSUFFICIENT_BUFFER = 122
 
 # WPARAM is defined as UINT_PTR (unsigned type)
@@ -211,6 +212,10 b' except AttributeError:'
 _kernel32.CreateToolhelp32Snapshot.argtypes = [_DWORD, _DWORD]
 _kernel32.CreateToolhelp32Snapshot.restype = _BOOL
 
+_kernel32.PeekNamedPipe.argtypes = [_HANDLE, ctypes.c_void_p, _DWORD,
+    ctypes.c_void_p, ctypes.c_void_p, ctypes.c_void_p]
+_kernel32.PeekNamedPipe.restype = _BOOL
+
 _kernel32.Process32First.argtypes = [_HANDLE, ctypes.c_void_p]
 _kernel32.Process32First.restype = _BOOL
 
@@ -260,6 +265,19 b' def samedevice(path1, path2):'
     res2 = _getfileinfo(path2)
     return res1.dwVolumeSerialNumber == res2.dwVolumeSerialNumber
 
+def peekpipe(pipe):
+    handle = msvcrt.get_osfhandle(pipe.fileno())
+    avail = _DWORD()
+
+    if not _kernel32.PeekNamedPipe(handle, None, 0, None, ctypes.byref(avail),
+                                   None):
+        err = _kernel32.GetLastError()
+        if err == _ERROR_BROKEN_PIPE:
+            return 0
+        raise ctypes.WinError(err)
+
+    return avail.value
+
 def testpid(pid):
     '''return True if pid is still running or unable to
     determine, False otherwise'''