Require only one `Logger::Logv()` implementation (#7605)

Summary:
A user who extended `Logger` recently pointed out it is unusual to
require they implement the two-argument `Logv()` overload when they've
already implemented the three-argument `Logv()` overload. I agree with
that and think we can fix it by only calling the two-argument overload
from the default implementation of the three-argument overload. Then
when the three-argument overload is overridden, RocksDB would not
rely on the two-argument overload. Only `Logger::LogHeader()` needed
adjustment to achieve this.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7605

Reviewed By: riversand963

Differential Revision: D24584749

Pulled By: ajkr

fbshipit-source-id: 9aabe040ac761c4c0dbebc4be046967403ecaf21

env/env_test.cc

@@ -2051,6 +2051,26 @@ TEST_F(EnvTest, Close) {
   delete env;
 }
 
+class LogvWithInfoLogLevelLogger : public Logger {
+ public:
+  using Logger::Logv;
+  void Logv(const InfoLogLevel /* log_level */, const char* /* format */,
+            va_list /* ap */) override {}
+};
+
+TEST_F(EnvTest, LogvWithInfoLogLevel) {
+  // Verifies the log functions work on a `Logger` that only overrides the
+  // `Logv()` overload including `InfoLogLevel`.
+  const std::string kSampleMessage("sample log message");
+  LogvWithInfoLogLevelLogger logger;
+  ROCKS_LOG_HEADER(&logger, "%s", kSampleMessage.c_str());
+  ROCKS_LOG_DEBUG(&logger, "%s", kSampleMessage.c_str());
+  ROCKS_LOG_INFO(&logger, "%s", kSampleMessage.c_str());
+  ROCKS_LOG_WARN(&logger, "%s", kSampleMessage.c_str());
+  ROCKS_LOG_ERROR(&logger, "%s", kSampleMessage.c_str());
+  ROCKS_LOG_FATAL(&logger, "%s", kSampleMessage.c_str());
+}
+
 INSTANTIATE_TEST_CASE_P(DefaultEnvWithoutDirectIO, EnvPosixTestWithParam,
                         ::testing::Values(std::pair<Env*, bool>(Env::Default(),
                                                                 false)));

include/rocksdb/env.h

@@ -1051,11 +1051,17 @@ class Logger {
   virtual void LogHeader(const char* format, va_list ap) {
     // Default implementation does a simple INFO level log write.
     // Please override as per the logger class requirement.
-    Logv(format, ap);
+    Logv(InfoLogLevel::INFO_LEVEL, format, ap);
   }
 
   // Write an entry to the log file with the specified format.
-  virtual void Logv(const char* format, va_list ap) = 0;
+  //
+  // Users who override the `Logv()` overload taking `InfoLogLevel` do not need
+  // to implement this, unless they explicitly invoke it in
+  // `Logv(InfoLogLevel, ...)`.
+  virtual void Logv(const char* /* format */, va_list /* ap */) {
+    assert(false);
+  }
 
   // Write an entry to the log file with the specified log level
   // and format.  Any log with level under the internal log level