[PATCH V2] pager: set some environment variables if they're not set

[Jun Wu]

# HG changeset patch
# User Jun Wu <quark at fb.com>
# Date 1492097239 25200
#      Thu Apr 13 08:27:19 2017 -0700
# Node ID aec3d224f8955e8956bbb2b7d49c76d410f42e19
# Parent  eaf3819631c206722c7594d8f54ed769a95f54b4
# Available At https://bitbucket.org/quark-zju/hg-draft
#              hg pull https://bitbucket.org/quark-zju/hg-draft -r aec3d224f895
pager: set some environment variables if they're not set

Git did this already [1] [2]. We want this behavior too [3].

This provides a better default user experience (like, supporting colors) if
users have things like "PAGER=less" set, which is not uncommon.

The environment variables are provided by a method so extensions can
override them on demand.

[1]: https://github.com/git/git/blob/6a5ff7acb5965718cc7016c0ab6c601454fd7cde/pager.c#L87
[2]: https://github.com/git/git/blob/6a5ff7acb5965718cc7016c0ab6c601454fd7cde/Makefile#L1545
[3]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-March/094780.html

diff --git a/mercurial/chgserver.py b/mercurial/chgserver.py
--- a/mercurial/chgserver.py
+++ b/mercurial/chgserver.py
@@ -191,6 +191,6 @@ def _newchgui(srcui, csystem, attachio):
             return self._csystem(cmd, util.shellenviron(environ), cwd)
 
-        def _runpager(self, cmd):
-            self._csystem(cmd, util.shellenviron(), type='pager',
+        def _runpager(self, cmd, env=None):
+            self._csystem(cmd, util.shellenviron(env), type='pager',
                           cmdtable={'attachio': attachio})
             return True
diff --git a/mercurial/rcutil.py b/mercurial/rcutil.py
--- a/mercurial/rcutil.py
+++ b/mercurial/rcutil.py
@@ -91,2 +91,8 @@ def rccomponents():
         _rccomponents.extend(normpaths(userrcpath()))
     return _rccomponents
+
+def defaultpagerenv():
+    '''return a dict of default environment variables and their values,
+    intended to be set before starting a pager.
+    '''
+    return {'LESS': 'FRX', 'LV': '-c'}
diff --git a/mercurial/ui.py b/mercurial/ui.py
--- a/mercurial/ui.py
+++ b/mercurial/ui.py
@@ -855,4 +855,9 @@ class ui(object):
             return
 
+        pagerenv = {}
+        for name, value in rcutil.defaultpagerenv().items():
+            if name not in encoding.environ:
+                pagerenv[name] = value
+
         self.debug('starting pager for command %r\n' % command)
         self.flush()
@@ -861,5 +866,5 @@ class ui(object):
         if util.safehasattr(signal, "SIGPIPE"):
             signal.signal(signal.SIGPIPE, _catchterm)
-        if self._runpager(pagercmd):
+        if self._runpager(pagercmd, pagerenv):
             self.pageractive = True
             # Preserve the formatted-ness of the UI. This is important
@@ -880,5 +885,5 @@ class ui(object):
             self.disablepager()
 
-    def _runpager(self, command):
+    def _runpager(self, command, env=None):
         """Actually start the pager and set up file descriptors.
 
@@ -913,5 +918,6 @@ class ui(object):
                 command, shell=shell, bufsize=-1,
                 close_fds=util.closefds, stdin=subprocess.PIPE,
-                stdout=util.stdout, stderr=util.stderr)
+                stdout=util.stdout, stderr=util.stderr,
+                env=util.shellenviron(env))
         except OSError as e:
             if e.errno == errno.ENOENT and not shell:
diff --git a/tests/test-pager.t b/tests/test-pager.t
--- a/tests/test-pager.t
+++ b/tests/test-pager.t
@@ -255,2 +255,28 @@ Put annotate in the ignore list for page
    9: a 9
   10: a 10
+
+Environment variables like LESS and LV are set automatically:
+  $ cat > $TESTTMP/printlesslv.py <<EOF
+  > import os, sys
+  > sys.stdin.read()
+  > for name in ['LESS', 'LV']:
+  >     sys.stdout.write(('%s=%s\n') % (name, os.environ.get(name, '-')))
+  > sys.stdout.flush()
+  > EOF
+
+  $ cat >> $HGRCPATH <<EOF
+  > [alias]
+  > noop = log -r 0 -T ''
+  > [ui]
+  > formatted=1
+  > [pager]
+  > pager = $PYTHON $TESTTMP/printlesslv.py
+  > EOF
+  $ unset LESS
+  $ unset LV
+  $ hg noop --pager=on
+  LESS=FRX
+  LV=-c
+  $ LESS=EFGH hg noop --pager=on
+  LESS=EFGH
+  LV=-c