Add terminal attribute to request hash. (#4292)

Add a terminal attribute to request hash policy.

Think about a case where customers want to hash on a cookie if it's present but if it's not present, do best-effort sticky based on something like IP so the customer has a stable hash.

This "terminal" allows request hashing to have the ability of "if A not working, fallback to B.", which also saves time to generate the hash.

Changes:
* Add a terminal attribute to HashMethod, which shortcircuit the hash generating process if a policy is marked terminal and there is a hash computed already.

Signed-off-by: Xin Zhuang stevenzzz@google.com

Description: Add terminal attribute to request hash.
Risk Level: Low
Testing: unit tests.

Mirrored from https://github.com/envoyproxy/envoy @ ee710d0a92b476cc3d7a0f99f11b62675d1a4e01

envoy/api/v2/route/route.proto

@@ -585,6 +585,27 @@ message RouteAction {
       // Connection properties hash policy.
       ConnectionProperties connection_properties = 3;
     }
+
+    // The flag that shortcircuits the hash computing. This field provides a
+    // 'fallback' style of configuration: "if a terminal policy doesn't work,
+    // fallback to rest of the policy list", it saves time when the terminal
+    // policy works.
+    //
+    // If true, and there is already a hash computed, ignore rest of the
+    // list of hash polices.
+    // For example, if the following hash methods are configured:
+    //
+    //  ========= ========
+    //  specifier terminal
+    //  ========= ========
+    //  Header A  true
+    //  Header B  false
+    //  Header C  false
+    //  ========= ========
+    //
+    // The generateHash process ends if policy "header A" generates a hash, as
+    // it's a terminal policy.
+    bool terminal = 4;
   }
 
   // Specifies a list of hash policies to use for ring hash load balancing. Each
@@ -596,7 +617,9 @@ message RouteAction {
   // hash policies fail to generate a hash, no hash will be produced for
   // the route. In this case, the behavior is the same as if no hash policies
   // were specified (i.e. the ring hash load balancer will choose a random
-  // backend).
+  // backend). If a hash policy has the "terminal" attribute set to true, and
+  // there is already a hash generated, the hash is returned immediately,
+  // ignoring the rest of the hash policy list.
   repeated HashPolicy hash_policy = 15;
 
   // Indicates that a HTTP/1.1 client connection to this particular route is allowed to