New "Feature flags" subsystem to ease upgrades with breaking changes

The goal is that each breaking change is implemented in way that a
RabbitMQ node having that change can work in the same cluster as other
nodes which do not know about it.

The feature flags are here to indicate which breaking changes are
"available" (i.e. the node knows about the breaking changes, but it
still runs in a backward-compatible mode) and which ones are currently
activated (i.e. the node uses the new code and older node can not be
part of the cluster anymore).

Enabling a feature flag (i.e. activating a breaking change) is a manual
operation where the user validates that the cluster is ready to switch
to the new world after the breaking change. Therefore the subsystem
ensures RabbitMQ nodes can talk to each other even if they don't have
the same code as long as they agree on using the common subset, and
prevents RabbitMQ nodes from talking to each other once the new code is
being used and one node does not understand it.

The consequence is that if a breaking change is implemented using this
new subsystem, a cluster can be upgraded one node at a time instead of
shutting down the entire cluster to upgrade.

Of course, the ability to implement a breaking change in such a way
entirely depends on the nature of that change. This new subsystem does
not guarantee that a cluster shutdown will never be required again.

[#159298729]

1 files changed, 17 insertions, 4 deletions

diff --git a/scripts/rabbitmq-env b/scripts/rabbitmq-env
index f4b06c171b..2b06dec2cd 100755
--- a/scripts/rabbitmq-env
+++ b/scripts/rabbitmq-env
@@ -278,6 +278,10 @@ rmq_normalize_path_var RABBITMQ_PID_FILE
 
 [ "x" = "x$RABBITMQ_BOOT_MODULE" ] && RABBITMQ_BOOT_MODULE=${BOOT_MODULE}
 
+[ "x" != "x$RABBITMQ_FEATURE_FLAGS_FILE" ] && RABBITMQ_FEATURE_FLAGS_FILE_source=environment
+[ "x" = "x$RABBITMQ_FEATURE_FLAGS_FILE" ] && RABBITMQ_FEATURE_FLAGS_FILE=${RABBITMQ_MNESIA_BASE}/${RABBITMQ_NODENAME}-feature_flags
+rmq_normalize_path_var RABBITMQ_FEATURE_FLAGS_FILE
+
 [ "x" = "x$RABBITMQ_PLUGINS_EXPAND_DIR" ] && RABBITMQ_PLUGINS_EXPAND_DIR=${PLUGINS_EXPAND_DIR}
 [ "x" = "x$RABBITMQ_PLUGINS_EXPAND_DIR" ] && RABBITMQ_PLUGINS_EXPAND_DIR=${RABBITMQ_MNESIA_BASE}/${RABBITMQ_NODENAME}-plugins-expand
 rmq_normalize_path_var RABBITMQ_PLUGINS_EXPAND_DIR
@@ -310,6 +314,7 @@ rmq_check_if_shared_with_mnesia \
     RABBITMQ_CONFIG_FILE \
     RABBITMQ_LOG_BASE \
     RABBITMQ_PID_FILE \
+    RABBITMQ_FEATURE_FLAGS_FILE \
     RABBITMQ_PLUGINS_EXPAND_DIR \
     RABBITMQ_ENABLED_PLUGINS_FILE \
     RABBITMQ_PLUGINS_DIR \
@@ -320,21 +325,29 @@ rmq_check_if_shared_with_mnesia \
 
 ## Development-specific environment.
 if [ "${RABBITMQ_DEV_ENV}" ]; then
-    if [ "$RABBITMQ_PLUGINS_DIR_source" != 'environment' -o \
+    if [ "$RABBITMQ_FEATURE_FLAGS_FILE_source" != 'environment' -o \
+         "$RABBITMQ_PLUGINS_DIR_source" != 'environment' -o \
          "$RABBITMQ_ENABLED_PLUGINS_FILE_source" != 'environment' ]; then
         # We need to query the running node for the plugins directory
         # and the "enabled plugins" file.
         eval $( (${RABBITMQ_SCRIPTS_DIR}/rabbitmqctl eval \
-            '{ok, P} = application:get_env(rabbit, plugins_dir),
+            '{ok, F} = application:get_env(rabbit, feature_flags_file),
+             {ok, P} = application:get_env(rabbit, plugins_dir),
              {ok, E} = application:get_env(rabbit, enabled_plugins_file),
              B = os:getenv("RABBITMQ_MNESIA_BASE"),
              M = os:getenv("RABBITMQ_MNESIA_DIR"),
              io:format(
+               "feature_flags_file=\"~s\"~n"
                "plugins_dir=\"~s\"~n"
                "enabled_plugins_file=\"~s\"~n"
                "mnesia_base=\"~s\"~n"
-               "mnesia_dir=\"~s\"~n", [P, E, B, M]).' \
-            2>/dev/null | grep -E '^(plugins_dir|enabled_plugins_file|mnesia_base|mnesia_dir)=') || :)
+               "mnesia_dir=\"~s\"~n", [F, P, E, B, M]).' \
+            2>/dev/null | grep -E '^(feature_flags_file|plugins_dir|enabled_plugins_file|mnesia_base|mnesia_dir)=') || :)
+
+        if [ "${feature_flags_file}" -a \
+             "$RABBITMQ_FEATURE_FLAGS_FILE_source" != 'environment' ]; then
+            RABBITMQ_FEATURE_FLAGS_FILE="${feature_flags_file}"
+        fi
         if [ "${plugins_dir}" -a \
              "$RABBITMQ_PLUGINS_DIR_source" != 'environment' ]; then
             RABBITMQ_PLUGINS_DIR="${plugins_dir}"