Implement os.waitpid() for Windows, in a way that's compatible with Linux

where their capabilities intersect.  Would be nice if people using non-
MSVC compilers (Borland etc) took a whack at doing something similar for
them (this code relies on the MS _cwait function).

Doc/lib/libos.tex

@@ -1166,12 +1166,14 @@ Availability: \UNIX.
 \end{funcdesc}
 
 \begin{funcdesc}{waitpid}{pid, options}
+The details of this function differ on \UNIX and Windows.
+
+On \UNIX:
 Wait for completion of a child process given by process id \var{pid},
 and return a tuple containing its process id and exit status
 indication (encoded as for \function{wait()}).  The semantics of the
 call are affected by the value of the integer \var{options}, which
 should be \code{0} for normal operation.
-Availability: \UNIX.
 
 If \var{pid} is greater than \code{0}, \function{waitpid()} requests
 status information for that specific process.  If \var{pid} is
@@ -1180,6 +1182,19 @@ group of the current process.  If \var{pid} is \code{-1}, the request
 pertains to any child of the current process.  If \var{pid} is less
 than \code{-1}, status is requested for any process in the process
 group \code{-\var{pid}} (the absolute value of \var{pid}).
+
+On Windows:
+Wait for completion of a process given by process id \var{pid},
+and return a tuple containing \var{pid},
+and its exit status shifted left by 8 bits (shifting makes cross-platform
+use of the function easier).
+A \var{pid} less than or equal to \code{0} has no special meaning on
+Windows, and raises an exception.
+The value of integer \var{options} has no effect.
+\var{pid} can refer to any process whose id is known, not necessarily a
+child process.
+The \function{spawn()} functions called with \constant{P_NOWAIT}
+return suitable process ids.
 \end{funcdesc}
 
 \begin{datadesc}{WNOHANG}

Misc/NEWS

@@ -59,6 +59,13 @@ Tests
 
 Windows
 
+- os.waitpid() is now implemented for Windows, and can be used to block
+  until a specified process exits.  This is similar to, but not exactly
+  the same as, os.waitpid() on POSIX systems.  If you're waiting for
+  a specific process whose pid was obtained from one of the spawn()
+  functions, the same Python os.waitpid() code works across platforms.
+  See the docs for details.
+
 - New tempfile.TemporaryFile implementation for Windows:  this doesn't
   need a TemproraryFileWrapper wrapper anymore, and should be immune
   to a nasty problem:  before 2.3, if you got a temp file on Windows, it

Modules/posixmodule.c

@@ -78,6 +78,7 @@ corresponding Unix manual entries for more information on calls.";
 #define HAVE_PIPE       1
 #define HAVE_POPEN      1
 #define HAVE_SYSTEM	1
+#define HAVE_CWAIT	1
 #else /* 16-bit Windows */
 #endif /* !MS_WIN32 */
 #else			/* all other compilers */
@@ -3333,8 +3334,33 @@ posix_waitpid(PyObject *self, PyObject *args)
 	else
 		return Py_BuildValue("ii", pid, status_i);
 }
-#endif /* HAVE_WAITPID */
 
+#elif defined(HAVE_CWAIT)
+
+/* MS C has a variant of waitpid() that's usable for most purposes. */
+static char posix_waitpid__doc__[] =
+"waitpid(pid, options) -> (pid, status << 8)\n"
+"Wait for completion of a given process.  options is ignored on Windows.";
+
+static PyObject *
+posix_waitpid(PyObject *self, PyObject *args)
+{
+	int pid, options;
+	int status;
+
+	if (!PyArg_ParseTuple(args, "ii:waitpid", &pid, &options))
+		return NULL;
+	Py_BEGIN_ALLOW_THREADS
+	pid = _cwait(&status, pid, options);
+	Py_END_ALLOW_THREADS
+	if (pid == -1)
+		return posix_error();
+	else
+		/* shift the status left a byte so this is more like the
+		   POSIX waitpid */
+		return Py_BuildValue("ii", pid, status << 8);
+}
+#endif /* HAVE_WAITPID || HAVE_CWAIT */
 
 #ifdef HAVE_WAIT
 static char posix_wait__doc__[] =
@@ -5678,7 +5704,7 @@ static PyMethodDef posix_methods[] = {
 #ifdef HAVE_WAIT
 	{"wait",	posix_wait, METH_VARARGS, posix_wait__doc__},
 #endif /* HAVE_WAIT */
-#ifdef HAVE_WAITPID
+#if defined(HAVE_WAITPID) || defined(HAVE_CWAIT)
 	{"waitpid",	posix_waitpid, METH_VARARGS, posix_waitpid__doc__},
 #endif /* HAVE_WAITPID */
 #ifdef HAVE_SETSID