pager: don't run pager if nothing is written to stdout/stderr

This decides when to run the pager based on the first call to
ui.write() and ui.write_err(). This has the side effect of not the
output of subprocesses that write output before hg does.

hgext/pager.py

@@ -1,113 +1,140
 # 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='FSRX' less
 
 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.
 
+By default, the pager is only executed if a command has output. To
+force the pager to run even if a command prints nothing, set::
+
+  [pager]
+  force = True
+
 If you notice "BROKEN PIPE" error messages, you can disable them by
 setting::
 
   [pager]
   quiet = True
 
 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.
 
 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 sys, os, signal, shlex, errno
 from mercurial import commands, dispatch, util, extensions
 from mercurial.i18n import _
 
-def _runpager(p):
+def _runpager(p, sigpipe=False):
     if not hasattr(os, 'fork'):
         sys.stderr = sys.stdout = util.popen(p, 'wb')
         return
     fdin, fdout = os.pipe()
     pid = os.fork()
     if pid == 0:
         os.close(fdin)
         os.dup2(fdout, sys.stdout.fileno())
         os.dup2(fdout, sys.stderr.fileno())
         os.close(fdout)
+        if sigpipe:
+            signal.signal(signal.SIGPIPE, signal.SIG_DFL)
         return
     os.dup2(fdin, sys.stdin.fileno())
     os.close(fdin)
     os.close(fdout)
     try:
         os.execvp('/bin/sh', ['/bin/sh', '-c', p])
     except OSError, e:
         if e.errno == errno.ENOENT:
             # no /bin/sh, try executing the pager directly
             args = shlex.split(p)
             os.execvp(args[0], args)
         else:
             raise
 
 def uisetup(ui):
     if ui.plain():
         return
 
+    class pagerui(ui.__class__):
+        _pager = None
+        _pagerstarted = False
+
+        def write(self, *args, **opts):
+            if self._pager and not self._pagerstarted:
+                self._pagerstarted = True
+                self._pager()
+            return super(pagerui, self).write(*args, **opts)
+
+        def write_err(self, *args, **opts):
+            if self._pager and not self._pagerstarted:
+                self._pagerstarted = True
+                self._pager()
+            return super(pagerui, self).write(*args, **opts)
+    ui.__class__ = pagerui
+
     def pagecmd(orig, ui, options, cmd, cmdfunc):
         p = ui.config("pager", "pager", os.environ.get("PAGER"))
         if p and sys.stdout.isatty() and '--debugger' not in sys.argv:
             attend = ui.configlist('pager', 'attend', attended)
             auto = options['pager'] == 'auto'
             always = util.parsebool(options['pager'])
             if (always or auto and
                 (cmd in attend or
                  (cmd not in ui.configlist('pager', 'ignore') and not attend))):
                 ui.setconfig('ui', 'formatted', ui.formatted())
                 ui.setconfig('ui', 'interactive', False)
-                _runpager(p)
-                if ui.configbool('pager', 'quiet'):
-                    signal.signal(signal.SIGPIPE, signal.SIG_DFL)
+                sigpipe = ui.configbool('pager', 'quiet')
+                if ui.configbool('pager', 'force'):
+                    _runpager(p, sigpipe)
+                else:
+                    ui._pager = lambda: _runpager(p, sigpipe)
         return orig(ui, options, cmd, cmdfunc)
 
     extensions.wrapfunction(dispatch, '_runcommand', pagecmd)
 
 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']