pager: recreate stdout to make it line-buffered

We want to see partial command results as soon as possible. But the buffering
mode of stdout (= pager's stdin) was set to fully-buffered because it isn't
associated with a tty. So, this patch recreates new stdout object to force its
buffering mode.

Because two file objects are associated with the same stdout fd and their
destructors will call close(), one of them must be closed carefully. Python
expects that the stdout fd never be closed even after sys.stdout.close() [1],
but newstdout has no such hack. So this patch calls newstdout.close()
immediately before duplicating the original stdout fd to sys.stdout.

  operation              sys.stdout  newstdout  fd
  ---------------------  ----------  ---------  --------
  newstdout.close()      open        closed     closed
  os.dup2(stdoutfd, ..)  open        closed     open
  del sys.stdout         closed      closed     open [1]

 [1]: https://hg.python.org/cpython/file/v2.7.10/Python/sysmodule.c#l1391

hgext/pager.py

@@ -1,142 +1,154 b''
 # pager.py - display output using a pager
 #
 # Copyright 2008 David Soria Parra <[e-mail]>
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 #
 # To load the extension, add it to your configuration file:
 #
 #   [extension]
 #   pager =
 #
 # Run "hg help pager" to get info on configuration.
 
 '''browse command output with an external pager
 
 To set the pager that should be used, set the application variable::
 
   [pager]
   pager = less -FRX
 
 If no pager is set, the pager extensions uses the environment variable
 $PAGER. If neither pager.pager, nor $PAGER is set, no pager is used.
 
 You can disable the pager for certain commands by adding them to the
 pager.ignore list::
 
   [pager]
   ignore = version, help, update
 
 You can also enable the pager only for certain commands using
 pager.attend. Below is the default list of commands to be paged::
 
   [pager]
   attend = annotate, cat, diff, export, glog, log, qdiff
 
 Setting pager.attend to an empty value will cause all commands to be
 paged.
 
 If pager.attend is present, pager.ignore will be ignored.
 
 Lastly, you can enable and disable paging for individual commands with
 the attend-<command> option. This setting takes precedence over
 existing attend and ignore options and defaults::
 
   [pager]
   attend-cat = false
 
 To ignore global commands like :hg:`version` or :hg:`help`, you have
 to specify them in your user configuration file.
 
 The --pager=... option can also be used to control when the pager is
 used. Use a boolean value like yes, no, on, off, or use auto for
 normal behavior.
 
 '''
 
 import atexit, sys, os, signal, subprocess
 from mercurial import commands, dispatch, util, extensions, cmdutil
 from mercurial.i18n import _
 
 # Note for extension authors: ONLY specify testedwith = 'internal' for
 # extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
 # be specifying the version(s) of Mercurial they are tested with, or
 # leave the attribute unspecified.
 testedwith = 'internal'
 
 def _runpager(ui, p):
     pager = subprocess.Popen(p, shell=True, bufsize=-1,
                              close_fds=util.closefds, stdin=subprocess.PIPE,
                              stdout=sys.stdout, stderr=sys.stderr)
 
+    # back up original file objects and descriptors
+    olduifout = ui.fout
+    oldstdout = sys.stdout
     stdoutfd = os.dup(sys.stdout.fileno())
     stderrfd = os.dup(sys.stderr.fileno())
+
+    # create new line-buffered stdout so that output can show up immediately
+    ui.fout = sys.stdout = newstdout = os.fdopen(sys.stdout.fileno(), 'wb', 1)
     os.dup2(pager.stdin.fileno(), sys.stdout.fileno())
     if ui._isatty(sys.stderr):
         os.dup2(pager.stdin.fileno(), sys.stderr.fileno())
 
     @atexit.register
     def killpager():
         if util.safehasattr(signal, "SIGINT"):
             signal.signal(signal.SIGINT, signal.SIG_IGN)
         pager.stdin.close()
+        ui.fout = olduifout
+        sys.stdout = oldstdout
+        # close new stdout while it's associated with pager; otherwise stdout
+        # fd would be closed when newstdout is deleted
+        newstdout.close()
+        # restore original fds: stdout is open again
         os.dup2(stdoutfd, sys.stdout.fileno())
         os.dup2(stderrfd, sys.stderr.fileno())
         pager.wait()
 
 def uisetup(ui):
     if '--debugger' in sys.argv or not ui.formatted():
         return
 
     def pagecmd(orig, ui, options, cmd, cmdfunc):
         p = ui.config("pager", "pager", os.environ.get("PAGER"))
         usepager = False
         always = util.parsebool(options['pager'])
         auto = options['pager'] == 'auto'
 
         if not p:
             pass
         elif always:
             usepager = True
         elif not auto:
             usepager = False
         else:
             attend = ui.configlist('pager', 'attend', attended)
             ignore = ui.configlist('pager', 'ignore')
             cmds, _ = cmdutil.findcmd(cmd, commands.table)
 
             for cmd in cmds:
                 var = 'attend-%s' % cmd
                 if ui.config('pager', var):
                     usepager = ui.configbool('pager', var)
                     break
                 if (cmd in attend or
                      (cmd not in ignore and not attend)):
                     usepager = True
                     break
 
         setattr(ui, 'pageractive', usepager)
 
         if usepager:
             ui.setconfig('ui', 'formatted', ui.formatted(), 'pager')
             ui.setconfig('ui', 'interactive', False, 'pager')
             if util.safehasattr(signal, "SIGPIPE"):
                 signal.signal(signal.SIGPIPE, signal.SIG_DFL)
             _runpager(ui, p)
         return orig(ui, options, cmd, cmdfunc)
 
     # Wrap dispatch._runcommand after color is loaded so color can see
     # ui.pageractive. Otherwise, if we loaded first, color's wrapped
     # dispatch._runcommand would run without having access to ui.pageractive.
     def afterloaded(loaded):
         extensions.wrapfunction(dispatch, '_runcommand', pagecmd)
     extensions.afterloaded('color', afterloaded)
 
 def extsetup(ui):
     commands.globalopts.append(
         ('', 'pager', 'auto',
          _("when to paginate (boolean, always, auto, or never)"),
          _('TYPE')))
 
 attended = ['annotate', 'cat', 'diff', 'export', 'glog', 'log', 'qdiff']