Fix flaky mcrouter PreprocessedConfigDumpTest ms-timestamp collision

Summary:
`mcrouter:preprocessed_config_dump_test` flaked after dumping five configs in one tight loop. Backup filenames use millisecond timestamps, so sub-ms dumps collided and the test sometimes kept 2 files instead of 3. Prod dumps are naturally spaced; the bug is test-only. This injects deterministic timestamps in the test while leaving prod on wall clock.

Sentinel-Harness: claude

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Model used: Claude Opus 4.8

Reviewed By: ghostonhuang

Differential Revision: D112753398

fbshipit-source-id: 5a3858ec4230d33db77e8e9d02dcec1d056fd406

Author: generatedunixname26860585020226206

mcrouter/CarbonRouterInstanceBase.cpp

@@ -345,7 +345,8 @@ int32_t CarbonRouterInstanceBase::getStatsEnabledPoolIndex(
 
 struct PreprocessedConfigDumpTag;
 
-void CarbonRouterInstanceBase::dumpPreprocessedConfigToDisk() {
+void CarbonRouterInstanceBase::dumpPreprocessedConfigToDisk(
+    std::optional<int64_t> timestampMsOverride) {
   if (!isDumpPreprocessedConfigEnabled(opts_)) {
     return;
   }
@@ -383,10 +384,10 @@ void CarbonRouterInstanceBase::dumpPreprocessedConfigToDisk() {
         // Create timestamped backup only if max_preprocessed_config_history > 0
         if (opts_.max_preprocessed_config_history > 0) {
           try {
-            auto timestampMs =
+            auto timestampMs = timestampMsOverride.value_or(
                 std::chrono::duration_cast<std::chrono::milliseconds>(
                     std::chrono::system_clock::now().time_since_epoch())
-                    .count();
+                    .count());
             auto timestampFilename = getPpcFilename(
                 opts_.service_name, opts_.flavor_name, timestampMs);
             auto timestampFilePath = (directory / timestampFilename).string();

mcrouter/CarbonRouterInstanceBase.h

@@ -9,7 +9,9 @@
 
 #include <atomic>
 #include <chrono>
+#include <cstdint>
 #include <memory>
+#include <optional>
 #include <unordered_map>
 
 #include <folly/Synchronized.h>
@@ -283,8 +285,10 @@ class CarbonRouterInstanceBase {
    */
   void deregisterForStatsUpdates();
 
-  // Dumps preprocessed config to disk on reconfiguration
-  void dumpPreprocessedConfigToDisk();
+  // Dumps preprocessed config to disk on reconfiguration. timestampMsOverride
+  // is a test-only seam to fix the backup filename timestamp; unset in prod.
+  void dumpPreprocessedConfigToDisk(
+      std::optional<int64_t> timestampMsOverride = std::nullopt);
 
   const McrouterOptions opts_;
   const pid_t pid_;
@@ -351,6 +355,10 @@ class CarbonRouterInstanceBase {
     dumpPreprocessedConfigToDisk();
   }
 
+  void dumpPreprocessedConfigToDiskForTesting(int64_t timestampMs) {
+    dumpPreprocessedConfigToDisk(timestampMs);
+  }
+
  private:
   // Track preprocessed config files to avoid unnecessary rewrites
   std::string preprocessedConfigFileMD5Hash_;

mcrouter/test/PreprocessedConfigDumpTest.cpp

@@ -486,6 +486,11 @@ TEST_F(PreprocessedConfigDumpTest, TimestampedFilesCleanupWorksCorrectly) {
 
   TestableRouterInstance instance(std::move(opts), std::move(mockConfigApi));
 
+  // Backups are named ppc_<ms>.json. Pass an explicit, increasing timestamp per
+  // dump so each writes a distinct file deterministically, without relying on
+  // wall-clock spacing between rapid dumps.
+  constexpr int64_t kBaseTimestampMs = 1700000000000;
+
   // Create 5 different configs to trigger 5 timestamped files
   for (int i = 0; i < 5; ++i) {
     std::string config = fmt::format(
@@ -508,7 +513,7 @@ TEST_F(PreprocessedConfigDumpTest, TimestampedFilesCleanupWorksCorrectly) {
                 ::testing::SetArgReferee<1>("test_config_path"),
                 ::testing::Return(true)));
 
-    instance.dumpPreprocessedConfigToDiskForTesting();
+    instance.dumpPreprocessedConfigToDiskForTesting(kBaseTimestampMs + i);
   }
 
   // Should only have 3 timestamped files (oldest 2 should be deleted)