Sort dock clients by class and instance

This is similar to #3820 but does not use qsort but an insertion sort in
con_attach.

Since each bar block automatically gets its own incremental bar id,
bards end up being sorted according to their definition order in the
config file.

For i3bar, the WM_CLASS is modified to include an instance name which
depends on the bar_id. This could be useful for other reason, e.g. users
targeting a specific bar instance.

Fixes #3491

i3bar/src/xcb.c

@@ -685,19 +685,6 @@ static void handle_visibility_notify(xcb_visibility_notify_event_t *event) {
     }
 }
 
-static int strcasecmp_nullable(const char *a, const char *b) {
-    if (a == b) {
-        return 0;
-    }
-    if (a == NULL) {
-        return -1;
-    }
-    if (b == NULL) {
-        return 1;
-    }
-    return strcasecmp(a, b);
-}
-
 /*
  * Comparison function to sort trayclients in ascending alphanumeric order
  * according to their class.

include/libi3.h

@@ -181,6 +181,12 @@ ssize_t writeall_nonblock(int fd, const void *buf, size_t count);
  */
 ssize_t swrite(int fd, const void *buf, size_t count);
 
+/**
+ * Like strcasecmp but considers the case where either string is NULL.
+ *
+ */
+int strcasecmp_nullable(const char *a, const char *b);
+
 /**
  * Build an i3String from an UTF-8 encoded string.
  * Returns the newly-allocated i3String.

libi3/safewrappers.c

@@ -110,3 +110,20 @@ ssize_t swrite(int fd, const void *buf, size_t count) {
     else
         return n;
 }
+
+/*
+ * Like strcasecmp but considers the case where either string is NULL.
+ *
+ */
+int strcasecmp_nullable(const char *a, const char *b) {
+    if (a == b) {
+        return 0;
+    }
+    if (a == NULL) {
+        return -1;
+    }
+    if (b == NULL) {
+        return 1;
+    }
+    return strcasecmp(a, b);
+}

src/con.c

@@ -132,6 +132,30 @@ static void _con_attach(Con *con, Con *parent, Con *previous, bool ignore_focus)
         goto add_to_focus_head;
     }
 
+    if (parent->type == CT_DOCKAREA) {
+        /* Insert dock client, sorting alphanumerically by class and then
+         * instance name. This makes dock client order deterministic. As a side
+         * effect, bars without a custom bar id will be sorted according to
+         * their declaration order in the config file. See #3491. */
+        current = NULL;
+        TAILQ_FOREACH (loop, nodes_head, nodes) {
+            int result = strcasecmp_nullable(con->window->class_class, loop->window->class_class);
+            if (result == 0) {
+                result = strcasecmp_nullable(con->window->class_instance, loop->window->class_instance);
+            }
+            if (result < 0) {
+                current = loop;
+                break;
+            }
+        }
+        if (current) {
+            TAILQ_INSERT_BEFORE(loop, con, nodes);
+        } else {
+            TAILQ_INSERT_TAIL(nodes_head, con, nodes);
+        }
+        goto add_to_focus_head;
+    }
+
     if (con->type == CT_FLOATING_CON) {
         DLOG("Inserting into floating containers\n");
         TAILQ_INSERT_TAIL(&(parent->floating_head), con, floating_windows);

src/handlers.c

@@ -1147,9 +1147,7 @@ static bool handle_strut_partial_change(Con *con, xcb_get_property_reply_t *prop
 
     /* attach the dock to the dock area */
     con_detach(con);
-    con->parent = dockarea;
-    TAILQ_INSERT_HEAD(&(dockarea->focus_head), con, focused);
-    TAILQ_INSERT_HEAD(&(dockarea->nodes_head), con, nodes);
+    con_attach(con, dockarea, true);
 
     tree_render();