chgserver: add utilities to calculate confighash

confighash is the hash of sensitive config items like [extensions], and
sensitive environment variables like HG*, LD_*, etc. The config items
can come from global, user, repo config, and command line flags.

For chgserver, it is designed that once confighash changes, the server is
not qualified to serve its client and should redirect the client to a new
server. The server does not need to exit in this case, since it can still
be valid (have a matched confighash) to serve other chg clients.

hgext/chgserver.py

@@ -58,6 +58,45 b" testedwith = 'internal'"
 
 _log = commandserver.log
 
+def _hashlist(items):
+    """return sha1 hexdigest for a list"""
+    return util.sha1(str(items)).hexdigest()
+
+# sensitive config sections affecting confighash
+_configsections = ['extensions']
+
+# sensitive environment variables affecting confighash
+_envre = re.compile(r'''\A(?:
+                    CHGHG
+                    |HG.*
+                    |LANG(?:UAGE)?
+                    |LC_.*
+                    |LD_.*
+                    |PATH
+                    |PYTHON.*
+                    |TERM(?:INFO)?
+                    |TZ
+                    )\Z''', re.X)
+
+def _confighash(ui):
+    """return a quick hash for detecting config/env changes
+
+    confighash is the hash of sensitive config items and environment variables.
+
+    for chgserver, it is designed that once confighash changes, the server is
+    not qualified to serve its client and should redirect the client to a new
+    server. different from mtimehash, confighash change will not mark the
+    server outdated and exit since the user can have different configs at the
+    same time.
+    """
+    sectionitems = []
+    for section in _configsections:
+        sectionitems.append(ui.configitems(section))
+    sectionhash = _hashlist(sectionitems)
+    envitems = [(k, v) for k, v in os.environ.iteritems() if _envre.match(k)]
+    envhash = _hashlist(sorted(envitems))
+    return sectionhash[:6] + envhash[:6]
+
 # copied from hgext/pager.py:uisetup()
 def _setuppagercmd(ui, options, cmd):
     if not ui.formatted():