preserve clients on old tags when renewing dwm

By default, when dwm is recompiled-restarted all clients will
lose it's current tag and collapse to first tag. This patch preserves
clients on old tags, however note that layout order is not preserved.

dwm.c

@@ -61,7 +61,7 @@ enum { CurNormal, CurResize, CurMove, CurLast }; /* cursor */
 enum { SchemeNorm, SchemeSel }; /* color schemes */
 enum { NetSupported, NetWMName, NetWMState, NetWMCheck,
        NetWMFullscreen, NetActiveWindow, NetWMWindowType,
-       NetWMWindowTypeDialog, NetClientList, NetLast }; /* EWMH atoms */
+       NetWMWindowTypeDialog, NetClientList, NetClientInfo, NetLast }; /* EWMH atoms */
 enum { WMProtocols, WMDelete, WMState, WMTakeFocus, WMLast }; /* default atoms */
 enum { ClkTagBar, ClkLtSymbol, ClkStatusText, ClkWinTitle,
        ClkClientWin, ClkRootWin, ClkLast }; /* clicks */
@@ -197,6 +197,7 @@ static void scan(void);
 static int sendevent(Client *c, Atom proto);
 static void sendmon(Client *c, Monitor *m);
 static void setclientstate(Client *c, long state);
+static void setclienttagprop(Client *c);
 static void setfocus(Client *c);
 static void setfullscreen(Client *c, int fullscreen);
 static void setlayout(const Arg *arg);
@@ -1071,6 +1072,26 @@ manage(Window w, XWindowAttributes *wa)
 	updatewindowtype(c);
 	updatesizehints(c);
 	updatewmhints(c);
+	{
+		int format;
+		unsigned long *data, n, extra;
+		Monitor *m;
+		Atom atom;
+		if (XGetWindowProperty(dpy, c->win, netatom[NetClientInfo], 0L, 2L, False, XA_CARDINAL,
+				&atom, &format, &n, &extra, (unsigned char **)&data) == Success && n == 2) {
+			c->tags = *data;
+			for (m = mons; m; m = m->next) {
+				if (m->num == *(data+1)) {
+					c->mon = m;
+					break;
+				}
+			}
+		}
+		if (n > 0)
+			XFree(data);
+	}
+	setclienttagprop(c);
+
 	XSelectInput(dpy, w, EnterWindowMask|FocusChangeMask|PropertyChangeMask|StructureNotifyMask);
 	grabbuttons(c, 0);
 	if (!c->isfloating)
@@ -1433,6 +1454,7 @@ sendmon(Client *c, Monitor *m)
 	c->tags = m->tagset[m->seltags]; /* assign tags of target monitor */
 	attach(c);
 	attachstack(c);
+	setclienttagprop(c);
 	focus(NULL);
 	arrange(NULL);
 }
@@ -1586,6 +1608,7 @@ setup(void)
 	netatom[NetWMWindowType] = XInternAtom(dpy, "_NET_WM_WINDOW_TYPE", False);
 	netatom[NetWMWindowTypeDialog] = XInternAtom(dpy, "_NET_WM_WINDOW_TYPE_DIALOG", False);
 	netatom[NetClientList] = XInternAtom(dpy, "_NET_CLIENT_LIST", False);
+	netatom[NetClientInfo] = XInternAtom(dpy, "_NET_CLIENT_INFO", False);
 	/* init cursors */
 	cursor[CurNormal] = drw_cur_create(drw, XC_left_ptr);
 	cursor[CurResize] = drw_cur_create(drw, XC_sizing);
@@ -1609,6 +1632,7 @@ setup(void)
 	XChangeProperty(dpy, root, netatom[NetSupported], XA_ATOM, 32,
 		PropModeReplace, (unsigned char *) netatom, NetLast);
 	XDeleteProperty(dpy, root, netatom[NetClientList]);
+	XDeleteProperty(dpy, root, netatom[NetClientInfo]);
 	/* select events */
 	wa.cursor = cursor[CurNormal]->cursor;
 	wa.event_mask = SubstructureRedirectMask|SubstructureNotifyMask
@@ -1695,11 +1719,22 @@ spawn(const Arg *arg)
 	}
 }
 
+void
+setclienttagprop(Client *c)
+{
+	long data[] = { (long) c->tags, (long) c->mon->num };
+	XChangeProperty(dpy, c->win, netatom[NetClientInfo], XA_CARDINAL, 32,
+			PropModeReplace, (unsigned char *) data, 2);
+}
+
 void
 tag(const Arg *arg)
 {
+	Client *c;
 	if (selmon->sel && arg->ui & TAGMASK) {
+		c = selmon->sel;
 		selmon->sel->tags = arg->ui & TAGMASK;
+		setclienttagprop(c);
 		focus(NULL);
 		arrange(selmon);
 	}
@@ -1774,6 +1809,7 @@ toggletag(const Arg *arg)
 	newtags = selmon->sel->tags ^ (arg->ui & TAGMASK);
 	if (newtags) {
 		selmon->sel->tags = newtags;
+		setclienttagprop(selmon->sel);
 		focus(NULL);
 		arrange(selmon);
 	}