sanity check cache_store.yml and warn of misconfig

test-plan:
 - create a cache_store.yml that's not a hash
 - try to boot canvas, get an appropriate error
 - create a cache_store.yml with non-hash values for the top level keys
   (e.g. entire file is "cache_store: redis_store")
 - try to boot canvas, get an appropriate error

Change-Id: I4e8965a61c11c64e1d81894e8ae316761ac12f1b
Reviewed-on: https://gerrit.instructure.com/29663
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>
QA-Review: Jacob Fugal <jacob@instructure.com>

config/cache_store.yml.example

@@ -1,3 +1,5 @@
+# Configure the cache_store by environment. Should be a hash at top level, one
+# key per environment, with configuration hashes as values.
 test:
   cache_store: redis_store
 development:

lib/canvas.rb

@@ -61,6 +61,17 @@ module Canvas
       require_dependency 'app/models/setting'
       @cache_stores = {}
       configs = Setting.from_config('cache_store', nil) || {}
+
+      # sanity check the file
+      unless configs.is_a?(Hash)
+        raise "Invalid config/cache_store.yml: Root is not a hash. See comments in config/cache_store.yml.example"
+      end
+
+      unless configs.values.all? { |cfg| cfg.is_a?(Hash) }
+        broken = configs.keys.select{ |k| !configs[k].is_a?(Hash) }.map(&:to_s).join(', ')
+        raise "Invalid config/cache_store.yml: Some keys are not hashes: #{broken}. See comments in config/cache_store.yml.example"
+      end
+
       configs.each do |env, config|
         config = {'cache_store' => 'mem_cache_store'}.merge(config)
         case config.delete('cache_store')