Do not depend on folly::Hash values stability in OriginalClientHashRoute

Summary:
We are planning to randomize values of `folly::Hash` between binary
invocations. Seems like `OriginalClientHashRoute` relies on the values to be
stable.

Use `folly::hash::SpookyHashV2::Hash64` with fixed seed instead of
`folly::Hash` to preserve old semantic.

Reviewed By: yfeldblum

Differential Revision: D79348475

fbshipit-source-id: e01e90b1e18713a2e44e187d4e4d4570daffc667

Author: ilvokhin

mcrouter/routes/OriginalClientHashRoute.h

@@ -7,10 +7,11 @@
 
 #pragma once
 
-#include <folly/hash/Hash.h>
 #include <memory>
 #include <string>
 
+#include <folly/hash/SpookyHashV2.h>
+
 #include "mcrouter/CarbonRouterInstanceBase.h"
 #include "mcrouter/McrouterFiberContext.h"
 #include "mcrouter/ProxyRequestContext.h"
@@ -27,6 +28,14 @@ namespace facebook {
 namespace memcache {
 namespace mcrouter {
 
+struct StableHasher {
+  constexpr size_t operator()(folly::StringPiece sp) const {
+    constexpr uint64_t kSeed = 0;
+    return static_cast<size_t>(
+        folly::hash::SpookyHashV2::Hash64(sp.begin(), sp.size(), kSeed));
+  }
+};
+
 /**
  * Use userIpAddress to hash request among children
  * Optional offset param to pick different child
@@ -57,7 +66,7 @@ class OriginalClientHashRoute {
 
     if (auto ip = req.getSourceIpAddr()) {
       return t(
-          *children_[(folly::Hash()(ip->str()) + offset_) % children_.size()],
+          *children_[(StableHasher()(ip->str()) + offset_) % children_.size()],
           req);
     }
     return false;
@@ -69,7 +78,7 @@ class OriginalClientHashRoute {
     assert(children_.size() > 1);
 
     if (auto ip = req.getSourceIpAddr()) {
-      return children_[(folly::Hash()(ip->str()) + offset_) % children_.size()]
+      return children_[(StableHasher()(ip->str()) + offset_) % children_.size()]
           ->route(req);
     } else {
       return createReply<Request>(