1316: support resilient redis mode for cache util

Tests run:
$ bash-4.4$ make busted-util
EXTRA_CFLAGS="-DHAVE_EVP_KDF_CTX=1" /usr/local/openresty/luajit/bin/rover install --roverfile=gateway/Roverfile > /dev/null
/usr/local/openresty/luajit/bin/rover exec bin/busted "spec/threescale_utils_spec.lua"
●●
2 successes / 0 failures / 0 errors / 0 pending : 0.029941 seconds

gateway/src/apicast/threescale_utils.lua

@@ -124,7 +124,7 @@ function _M.connect_redis(options)
   local opts = {}
 
   local url = options and options.url or env.get('REDIS_URL')
-
+  local resilient = options.resilient or false
 
   if url then
     url = resty_url.split(url, 'redis')
@@ -152,22 +152,22 @@ function _M.connect_redis(options)
 
   local ok, err = red:connect(_M.resolve(host, port))
   if not ok then
-    return nil, _M.error("failed to connect to redis on ", host, ":", port, ": ", err)
+    return nil, _M.error(resilient, "failed to connect to redis on ", host, ":", port, ": ", err)
   end
 
   if opts.password then
     ok = red:auth(opts.password)
 
     if not ok then
-      return nil, _M.error("failed to auth on redis ", host, ":", port)
+      return nil, _M.error(resilient, "failed to auth on redis ", host, ":", port)
     end
   end
 
   if opts.db then
     ok = red:select(opts.db)
 
     if not ok then
-      return nil, _M.error("failed to select db ", opts.db, " on redis ", host, ":", port)
+      return nil, _M.error(resilient, "failed to select db ", opts.db, " on redis ", host, ":", port)
     end
   end
 
@@ -188,13 +188,15 @@ function _M.match_xml_element(xml, element, value)
 end
 
 -- error and exit
-function _M.error(...)
+function _M.error(resilient, ...)
   if ngx.get_phase() == 'timer' then
     return table.concat(table.pack(...))
   else
     ngx.status = ngx.HTTP_INTERNAL_SERVER_ERROR
     ngx.say(...)
-    ngx.exit(ngx.status)
+    if not resilient then
+      ngx.exit(ngx.status)
+    end
   end
 end
 

spec/threescale_utils_spec.lua

@@ -4,11 +4,20 @@ describe('3scale utils', function()
     describe('.error', function()
         it('returns concatenated error in timer phase', function()
             local get_phase = spy.on(ngx, 'get_phase', function() return 'timer' end)
-            local error = _M.error('one', ' two', ' three')
+            local error = _M.error(nil, 'one', ' two', ' three')
 
             assert.spy(get_phase).was_called(1)
 
             assert.equal('one two three', error)
         end)
+
+        it('does not terminate with nginx.exit if resilient flag is set', function()
+            local exit = spy.on(ngx, 'exit', function() return 'exited!' end)
+            local error = _M.error(true, 'redis is not reachable')
+
+            assert.spy(exit).was_not_called()
+
+            assert.equal('redis is not reachable', error)
+        end)
     end)
 end)