balancer: raise error when trying to set the peer twice

With the latest changes, the balancer phase can be retried, and the
upstream might be already set. This change is needed to only raise an
error when the peer has not been set for the current retry.

gateway/src/apicast/balancer.lua

@@ -22,7 +22,7 @@ local function get_upstream(context)
     return nil, 'missing upstream'
   end
 
-  if context[upstream] then
+  if context.peer_set_in_current_balancer_try then
     return nil, 'already set peer'
   end
 
@@ -114,7 +114,7 @@ function _M.call(_, context, bal)
 
     -- I wish there would be a nicer way, but unfortunately ngx.exit(ngx.OK) does not
     -- terminate the current phase handler and will evaluate all remaining balancer phases.
-    context[upstream] = peer
+    context.peer_set_in_current_balancer_try = true
     return peer
   else
     ngx.log(ngx.ERR, 'failed to set current backend peer: ', err)

gateway/src/apicast/executor.lua

@@ -119,6 +119,7 @@ do
     function _M:balancer()
         local context = self:context('balancer')
         context.balancer_retries = (context.balancer_retries and context.balancer_retries + 1) or 0
+        context.peer_set_in_current_balancer_try = false
         return self.policy_chain.balancer(self.policy_chain, context)
     end
 

spec/executor_spec.lua

@@ -170,6 +170,12 @@ describe('executor', function()
       assert.same(3, ngx.ctx.context.balancer_retries)
     end)
 
+    it('sets a var in the context that marks that the peer has not been set yet in the current try', function()
+      executor:balancer()
+
+      assert.is_false(ngx.ctx.context.peer_set_in_current_balancer_try)
+    end)
+
     it('forwards the call to the policy chain', function()
       executor:balancer()