make: Easy code reloading for run-broker/start-cluster

This enables code reloading for single nodes and
for clusters. For single nodes using `make run-broker`
at least two terminals must be used: the one with
the broker, and the one doing the reloading.

When only using a single node (`make run-broker`),
the `RELOAD=1` variable can be passed to Make when
recompiling. This will lead to code reloading at
the end of compilation.

  make RELOAD=1

Alternatively the "reload-broker" target can be used:

  make all reload-broker

Special care must be given when the -j flag is used.
In that case it may be needed to separate compilation
and reloading in two separate calls:

  make && make reload-broker

The same considerations apply to clusters, only there
isn't a shorthand variable for them. Instead, the
"reload-cluster" target must be used:

  make all reload-cluster

Or:

  make && make reload-cluster

Either of these will compile and reload the modules
on all nodes of the cluster.

It is also possible to reload modules only on some nodes,
using `make reload-broker` with the variable "RABBITMQ_NODENAME"
set to the node you want to trigger a reload on. This could
be used to test scenarios where the code differs between
the nodes in the cluster.

Author: Loïc Hoguin

deps/rabbit_common/mk/rabbitmq-run.mk

@@ -416,6 +416,42 @@ stop-brokers stop-cluster:
 	done; \
 	wait
 
+# --------------------------------------------------------------------
+# Code reloading.
+#
+# For `make run-broker` either do:
+# * make RELOAD=1
+# * make all reload-broker        (can't do this alongside -j flag)
+# * make && make reload-broker    (fine with -j flag)
+#
+# Or if recompiling a specific application:
+# * make -C deps/rabbit RELOAD=1
+#
+# For `make start-cluster` use the `reload-cluster` target.
+# Same constraints apply as with `reload-broker`:
+# * make all reload-cluster
+# * make && make reload-cluster
+# --------------------------------------------------------------------
+
+reload-broker:
+	$(exec_verbose) ERL_LIBS="$(DIST_ERL_LIBS)" \
+		$(RABBITMQCTL) -n $(RABBITMQ_NODENAME) \
+		eval "io:format(\"~p~n\", [c:lm()])."
+
+ifeq ($(MAKELEVEL),0)
+ifdef RELOAD
+all:: reload-broker
+endif
+endif
+
+reload-cluster:
+	@for n in $$(seq $(NODES) -1 1); do \
+		nodename="rabbit-$$n@$(HOSTNAME)"; \
+		$(MAKE) reload-broker \
+			RABBITMQ_NODENAME="$$nodename" & \
+	done; \
+	wait
+
 # --------------------------------------------------------------------
 # Used by testsuites.
 # --------------------------------------------------------------------