Make RateLimiter::GetTotalPendingRequest() non pure virtual for backward compability (#8938)

Summary:
Context/Summary:
https://github.com/facebook/rocksdb/pull/8890 added a public API `RateLimiter::GetTotalPendingRequest()` but mistakenly marked it as pure virtual, forcing RateLimiter's derived classes to implement this function and breaking backward compatibility.

This PR makes `RateLimiter::GetTotalPendingRequest()` as non-pure virtual method by providing a trivial implementation in rate_limiter.h

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8938

Test Plan: Passing existing tests

Reviewed By: pdillinger

Differential Revision: D31100661

Pulled By: hx235

fbshipit-source-id: 06eff1005156a6e5a881e393b2c5b2ad706897d8

HISTORY.md

@@ -31,10 +31,10 @@
 * Added new callback APIs `OnBlobFileCreationStarted`,`OnBlobFileCreated`and `OnBlobFileDeleted` in `EventListener` class of listener.h. It notifies listeners during creation/deletion of individual blob files in Integrated BlobDB. It also log blob file creation finished event and deletion event in LOG file.
 * Batch blob read requests for `DB::MultiGet` using `MultiRead`.
 * Add support for fallback to local compaction, the user can return `CompactionServiceJobStatus::kUseLocal` to instruct RocksDB to run the compaction locally instead of waiting for the remote compaction result.
+* Add built-in rate limiter's implementation for `RateLimiter::GetTotalPendingRequests()` for the total number of requests that are pending for bytes in the rate limiter.
 
 ### Public API change
 * Remove obsolete implementation details FullKey and ParseFullKey from public API
-* Add a public API RateLimiter::GetTotalPendingRequests() for the total number of requests that are pending for bytes in the rate limiter.
 * Change `SstFileMetaData::size` from `size_t` to `uint64_t`.
 * Made Statistics extend the Customizable class and added a CreateFromString method.  Implementations of Statistics need to be registered with the ObjectRegistry and to implement a Name() method in order to be created via this method.
 * Extended `FlushJobInfo` and `CompactionJobInfo` in listener.h to provide information about the blob files generated by a flush/compaction and garbage collected during compaction in Integrated BlobDB. Added struct members `blob_file_addition_infos` and `blob_file_garbage_infos` that contain this information.

db/db_test.cc

@@ -3937,6 +3937,56 @@ TEST_F(DBTest, DISABLED_RateLimitingTest) {
   ASSERT_LT(ratio, 0.6);
 }
 
+// This is a mocked customed rate limiter without implementing optional APIs
+// (e.g, RateLimiter::GetTotalPendingRequests())
+class MockedRateLimiterWithNoOptionalAPIImpl : public RateLimiter {
+ public:
+  MockedRateLimiterWithNoOptionalAPIImpl() {}
+
+  ~MockedRateLimiterWithNoOptionalAPIImpl() override {}
+
+  void SetBytesPerSecond(int64_t bytes_per_second) override {
+    (void)bytes_per_second;
+  }
+
+  using RateLimiter::Request;
+  void Request(const int64_t bytes, const Env::IOPriority pri,
+               Statistics* stats) override {
+    (void)bytes;
+    (void)pri;
+    (void)stats;
+  }
+
+  int64_t GetSingleBurstBytes() const override { return 200; }
+
+  int64_t GetTotalBytesThrough(
+      const Env::IOPriority pri = Env::IO_TOTAL) const override {
+    (void)pri;
+    return 0;
+  }
+
+  int64_t GetTotalRequests(
+      const Env::IOPriority pri = Env::IO_TOTAL) const override {
+    (void)pri;
+    return 0;
+  }
+
+  int64_t GetBytesPerSecond() const override { return 0; }
+};
+
+// To test that customed rate limiter not implementing optional APIs (e.g,
+// RateLimiter::GetTotalPendingRequests()) works fine with RocksDB basic
+// operations (e.g, Put, Get, Flush)
+TEST_F(DBTest, CustomedRateLimiterWithNoOptionalAPIImplTest) {
+  Options options = CurrentOptions();
+  options.rate_limiter.reset(new MockedRateLimiterWithNoOptionalAPIImpl());
+  DestroyAndReopen(options);
+  ASSERT_OK(Put("abc", "def"));
+  ASSERT_EQ(Get("abc"), "def");
+  ASSERT_OK(Flush());
+  ASSERT_EQ(Get("abc"), "def");
+}
+
 TEST_F(DBTest, TableOptionsSanitizeTest) {
   Options options = CurrentOptions();
   options.create_if_missing = true;

include/rocksdb/rate_limiter.h

@@ -90,8 +90,15 @@ class RateLimiter {
       const Env::IOPriority pri = Env::IO_TOTAL) const = 0;
 
   // Total # of requests that are pending for bytes in rate limiter
+  // For convenience, this function is implemented by the RateLimiter returned
+  // by NewGenericRateLimiter but is not required by RocksDB. The default
+  // implementation indicates "not supported".
   virtual int64_t GetTotalPendingRequests(
-      const Env::IOPriority pri = Env::IO_TOTAL) const = 0;
+      const Env::IOPriority pri = Env::IO_TOTAL) const {
+    assert(false);
+    (void)pri;
+    return -1;
+  }
 
   virtual int64_t GetBytesPerSecond() const = 0;