Add a flavor option to disable flavor override

Summary:
Adds a `--disable-flavor-override` standalone CLI toggle and a corresponding `disable_flavor_override` option that can be set in flavor JSON to skip reading `.override` files from `/etc/mcrouter/`.

The disable mechanism works through two paths: a global atomic flag set during standalone CLI init (via `disableFlavorOverride()`), and a per-opts-map check for the key in the flavor JSON `options` section. The global flag covers the standalone binary case, while the opts-map check covers the library embedding case where the option can be set directly in the flavor JSON.

This is helpful to add to unit test and integration test for our devservers.  Otherwise we won't be properly testing the prod routing changes and instead test the devproxy routes instead.

The server side already protect against direct connections from dev servers.

Reviewed By: disylh, ghostonhuang

Differential Revision: D92323131

fbshipit-source-id: a18e0f241d69009ba0a54828c83b3ca7a8224005

Author: David Huynh (Infra)

mcrouter/StandaloneUtils.cpp

@@ -417,12 +417,25 @@ void getFlavorOptionsAndApplyOverrides(
   // may affect the way we read from flavors (fb-only).
   standalonePreInitFromCommandLineOpts(cmdLineOpts.standaloneOptionsOverrides);
 
+  // Extract disable_flavor_override from CLI opts to pass through the
+  // flavor loading call chain (fb-only: controls override file reading).
+  bool disableFlavorOverride = false;
+  auto disableIt =
+      cmdLineOpts.standaloneOptionsOverrides.find("disable_flavor_override");
+  if (disableIt != cmdLineOpts.standaloneOptionsOverrides.end()) {
+    disableFlavorOverride =
+        (disableIt->second == "1" || disableIt->second == "true");
+  }
+
   if (cmdLineOpts.flavor.empty()) {
     libmcrouterOptionsDict = cmdLineOpts.libmcrouterOptionsOverrides;
     standaloneOptionsDict = cmdLineOpts.standaloneOptionsOverrides;
   } else {
     read_standalone_flavor(
-        cmdLineOpts.flavor, libmcrouterOptionsDict, standaloneOptionsDict);
+        cmdLineOpts.flavor,
+        libmcrouterOptionsDict,
+        standaloneOptionsDict,
+        disableFlavorOverride);
     for (auto& it : cmdLineOpts.libmcrouterOptionsOverrides) {
       libmcrouterOptionsDict[it.first] = it.second;
     }

mcrouter/mcrouter_config.cpp

@@ -34,7 +34,8 @@ bool readLibmcrouterFlavor(
 bool read_standalone_flavor(
     const std::string& flavor,
     std::unordered_map<std::string, std::string>& option_dict,
-    std::unordered_map<std::string, std::string>& st_option_dict) {
+    std::unordered_map<std::string, std::string>& st_option_dict,
+    bool /*disableFlavorOverride*/) {
   if (!readFlavor(flavor, st_option_dict, option_dict)) {
     LOG(ERROR) << "CRITICAL: Couldn't initialize from standalone flavor file "
                << flavor;

mcrouter/mcrouter_config.h

@@ -120,7 +120,8 @@ bool readLibmcrouterFlavor(
 bool read_standalone_flavor(
     const std::string& flavor,
     std::unordered_map<std::string, std::string>& option_dict,
-    std::unordered_map<std::string, std::string>& st_option_dict);
+    std::unordered_map<std::string, std::string>& st_option_dict,
+    bool disableFlavorOverride = false);
 
 std::unique_ptr<ConfigApi> createConfigApi(const McrouterOptions& opts);