chgserver: handle ParseError during validate

Currently the validate command in chgserver expects config can be loaded
without issues but the config can be broken and chg will print a stacktrace
instead of the parsing error, if a chg server is already running.

This patch adds a handler for ParseError in validate and a new instruction
"exit" to make the client exit without abortmsg. A test is also added to make
sure it will behave as expected.

tests/test-chg.t

@@ -0,0 +1,12 b''
+init repo
+
+  $ hg init foo
+  $ cd foo
+
+ill-formed config
+
+  $ hg status
+  $ echo '=brokenconfig' >> $HGRCPATH
+  $ hg status
+  hg: parse error at * (glob)
+  [255]

contrib/chg/chg.c

@@ -460,6 +460,11 b' static void runinstructions(struct cmdse'
 					 "%s", *pinst + 9);
 			if (r < 0 || r >= (int)sizeof(opts->redirectsockname))
 				abortmsg("redirect path is too long (%d)", r);
+		} else if (strncmp(*pinst, "exit ", 5) == 0) {
+			int n = 0;
+			if (sscanf(*pinst + 5, "%d", &n) != 1)
+				abortmsg("cannot read the exit code");
+			exit(n);
 		} else {
 			abortmsg("unknown instruction: %s", *pinst);
 		}

hgext/chgserver.py

@@ -429,9 +429,17 b' class chgcmdserver(commandserver.server)'
               outdated server.
             - "redirect $path", the client should try to connect to another
               server instead.
+            - "exit $n", the client should exit directly with code n.
+              This may happen if we cannot parse the config.
         """
         args = self._readlist()
-        self.ui = _renewui(self.ui, args)
+        try:
+            self.ui = _renewui(self.ui, args)
+        except error.ParseError as inst:
+            dispatch._formatparse(self.ui.warn, inst)
+            self.ui.flush()
+            self.cresult.write('exit 255')
+            return
         newhash = hashstate.fromui(self.ui, self.hashstate.mtimepaths)
         insts = []
         if newhash.mtimehash != self.hashstate.mtimehash: