CT: Introduce SECONDARY_DIST_VSN for easier testing

SECONDARY_DIST_VSN takes a version of RabbitMQ to download and
use for the SECONDARY_DIST environment. For example:

  SECONDARY_DIST_VSN=4.2.3

The dist environment is only setup once and is not downloaded
again as long as it remains available (ie not deleted).

By default the dist environments are in the secondary_dist/
folder at the root of the RabbitMQ repository. This location
can be changed using the SECONDARY_DIST_DIR variable which
must contain an absolute path to the directory. For example:

  SECONDARY_DIST_DIR=/home/`whoami`/rabbitmq/secondary_dist/

Combined we can run mixed version tests using a command like:

  SECONDARY_DIST_VSN=4.2.0 SECONDARY_DIST_DIR=/home/`whoami`/rabbitmq/secondary_dist/ make -C deps/rabbit ct

Although SECONDARY_DIST_DIR is best exported from your own
shell's rc file if you're going to use it, as this allows
sharing the same secondary_dist/ directory across all RabbitMQ
clones and worktrees.

Author: lhoguin

.gitignore

@@ -116,6 +116,7 @@ rabbitmq-server-*.tar.gz
 rabbitmq-server-*.tar.bz2
 rabbitmq-server-*.tar.xz
 rabbitmq-server-*.zip
+secondary_dist/
 
 # Trace tools output.
 *-ttb

deps/rabbit_common/mk/rabbitmq-early-plugin.mk

@@ -42,3 +42,21 @@ dep_cth_styledout = git https://github.com/rabbitmq/cth_styledout.git master
 ifneq ($(strip $(CT_HOOKS)),)
 CT_OPTS += -ct_hooks $(CT_HOOKS)
 endif
+
+# We fetch a SECONDARY_DIST if SECONDARY_DIST_VSN is set and
+# we setup the SECONDARY_DIST variable. The location of the
+# secondary dists can be customized using SECONDARY_DIST_DIR
+# but note that it only works with an absolute path.
+ifneq ($(strip $(SECONDARY_DIST_VSN)),)
+SECONDARY_DIST_DIR ?= $(abspath $(CURDIR)/../../)/secondary_dist
+SECONDARY_DIST=$(SECONDARY_DIST_DIR)/rabbitmq_server-$(SECONDARY_DIST_VSN)
+export SECONDARY_DIST
+
+SECONDARY_DIST_DOWNLOAD_URL = https://github.com/rabbitmq/rabbitmq-server/releases/download/v$(SECONDARY_DIST_VSN)/rabbitmq-server-generic-unix-$(SECONDARY_DIST_VSN).tar.xz
+
+ifeq ($(wildcard $(SECONDARY_DIST)),)
+$(shell curl -L -o $(ERLANG_MK_TMP)/rabbitmq_server-$(SECONDARY_DIST_VSN).tar.xz $(SECONDARY_DIST_DOWNLOAD_URL) \
+    && mkdir -p $(SECONDARY_DIST) \
+    && tar -xf $(ERLANG_MK_TMP)/rabbitmq_server-$(SECONDARY_DIST_VSN).tar.xz -C $(dir $(SECONDARY_DIST)))
+endif
+endif

deps/rabbitmq_ct_helpers/src/rabbit_ct_helpers.erl

@@ -339,7 +339,6 @@ ensure_application_srcdir(Config, App, _Lang, Module) ->
         P ->
             P
     end,
-    %% @todo Doesn't account for SECONDARY_DIST.
     SecondaryPath = case ?config(secondary_umbrella, Config) of
                         false ->
                             Path;