util: add a simple poll utility

We'll use it to detect when a sshpeer have server output to be displayed.

The implementation is super basic because all case support is not the focus of
this series.

mercurial/posix.py

@@ -8,6 +8,7 b''
 from i18n import _
 import encoding
 import os, sys, errno, stat, getpass, pwd, grp, socket, tempfile, unicodedata
+import select
 import fcntl, re
 
 posixfile = open
@@ -594,6 +595,19 b' def statisexec(st):'
     '''check whether a stat result is an executable file'''
     return st and (st.st_mode & 0100 != 0)
 
+def poll(fds):
+    """block until something happens on any file descriptor
+
+    This is a generic helper that will check for any activity
+    (read, write.  exception) and return the list of touched files.
+
+    In unsupported cases, it will raise a NotImplementedError"""
+    try:
+        res = select.select(fds, fds, fds)
+    except ValueError: # out of range file descriptor
+        raise NotImplementedError()
+    return sorted(list(set(sum(res, []))))
+
 def readpipe(pipe):
     """Read all available data from a pipe."""
     # We can't fstat() a pipe because Linux will always report 0.

mercurial/util.py

@@ -54,6 +54,7 b' openhardlinks = platform.openhardlinks'
 oslink = platform.oslink
 parsepatchoutput = platform.parsepatchoutput
 pconvert = platform.pconvert
+poll = platform.poll
 popen = platform.popen
 posixfile = platform.posixfile
 quotecommand = platform.quotecommand

mercurial/windows.py

@@ -371,6 +371,10 b' def statisexec(st):'
     '''check whether a stat result is an executable file'''
     return False
 
+def poll(fds):
+    # see posix.py for description
+    raise NotImplementedError()
+
 def readpipe(pipe):
     """Read all available data from a pipe."""
     chunks = []