Shard vars file between multiple plugin configurations

By hashing the `BUILDKITE_PLUGIN_CONFIGURATION` envvar, we can
effectively disambiguate multiple instances of the `metahook` plugin
within a single pipeline.  This allows us to seprate the `vars` file, as
it otherwise can get confused if you have multiple invocations of the
plugin that each use different hooks.

hooks/environment

@@ -2,12 +2,17 @@
 set -euo pipefail
 
 # If users add multiple `metahook` instances, we'll just store our `vars` in the same directory.
+# We store the `vars` in a file that is namespaced by a hash of the plugin configuration, which
+# allows us to avoid having multiple metahook instances clobber eachother.
 if [[ ! -v "BUILDKITE_METAHOOK_HOOKS_PATH" ]]; then
   export BUILDKITE_METAHOOK_HOOKS_PATH="$(mktemp -d)"
-  env | sort | grep "BUILDKITE_PLUGIN_METAHOOK" | uniq >"${BUILDKITE_METAHOOK_HOOKS_PATH}/vars"
 fi
 
-if grep -E 'BUILDKITE_PLUGIN_METAHOOK_.+(\.BAT|\.CMD)=' "${BUILDKITE_METAHOOK_HOOKS_PATH}/vars"; then
+PLUGIN_CONFIG_HASH=$(cksum <<<"${BUILDKITE_PLUGIN_CONFIGURATION}" | awk '{ print $1 }')
+VARS_FILENAME="vars-${PLUGIN_CONFIG_HASH}"
+env | sort | grep "BUILDKITE_PLUGIN_METAHOOK" | uniq >"${BUILDKITE_METAHOOK_HOOKS_PATH}/${VARS_FILENAME}"
+
+if grep -E 'BUILDKITE_PLUGIN_METAHOOK_.+(\.BAT|\.CMD)=' "${BUILDKITE_METAHOOK_HOOKS_PATH}/${VARS_FILENAME}"; then
   echo "Sorry, we had to remove Windows Batch file support in 0.4.0."
   echo "Please refer to https://github.com/improbable-eng/metahook-buildkite-plugin/tree/master/changelog.md#0.4.0"
   echo ""

hooks/pre-exit

@@ -1,8 +1,12 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+PLUGIN_CONFIG_HASH=$(cksum <<<"${BUILDKITE_PLUGIN_CONFIGURATION}" | awk '{ print $1 }')
+VARS_FILENAME="vars-${PLUGIN_CONFIG_HASH}"
 cleanup() {
-  rm -rf "${BUILDKITE_METAHOOK_HOOKS_PATH}"
+  # Delete our `vars` directory, then if the directory itself is empty, delete that too
+  rm -rf "${BUILDKITE_METAHOOK_HOOKS_PATH}/${VARS_FILENAME}"
+  rmdir "${BUILDKITE_METAHOOK_HOOKS_PATH}" 2>/dev/null || true
 }
 trap cleanup EXIT
 

hooks/run-hook.sh

@@ -5,6 +5,8 @@ hook_name="${1:?1st arg needs to be hook name}"
 upperd="$(echo "${hook_name}" | tr "[:lower:]" "[:upper:]" | sed "s:-:_:")"
 var_name="BUILDKITE_PLUGIN_METAHOOK_${upperd}"
 
+PLUGIN_CONFIG_HASH=$(cksum <<<"${BUILDKITE_PLUGIN_CONFIGURATION}" | awk '{ print $1 }')
+VARS_FILENAME="vars-${PLUGIN_CONFIG_HASH}"
 if grep -q "${var_name}" <"${BUILDKITE_METAHOOK_HOOKS_PATH}/vars"; then
   hook_file="${BUILDKITE_METAHOOK_HOOKS_PATH}/${hook_name}"