Add info logging via callback to C api.

Summary:
Adds a very minimal derive of Logger around a C callback useful for FFI
interfaces from other languages to integrate infolog.

The infolog capabilities for C embeddings are quite spartan. LOG files were
generated involuntarily until redirection to stderr was added by #12262;
still insufficient for apps which cannot tolerate pollution of their stdio
and tend to have existing logging frameworks to tie into for that.

Signed-off-by: Jason Volk <jason@zemos.net>

db/c.cc

@@ -531,6 +531,42 @@ struct rocksdb_universal_compaction_options_t {
   ROCKSDB_NAMESPACE::CompactionOptionsUniversal* rep;
 };
 
+struct rocksdb_callback_logger_t : public Logger {
+  static const ssize_t STACK_BUFSZ = 512;
+  rocksdb_callback_logger_t(InfoLogLevel log_level,
+                            void (*logv_cb)(void*, unsigned, char*, size_t),
+                            void* priv)
+      : Logger(log_level), logv_cb_(logv_cb), priv_(priv) {}
+
+  using Logger::Logv;
+  void Logv(const InfoLogLevel level, const char* fmt, va_list ap0) override {
+    char stack_buf[STACK_BUFSZ];
+    char* alloc_buf = nullptr;
+    char* buf = stack_buf;
+    int len = 0;
+    va_list ap1;
+    if (!logv_cb_) return;
+    va_copy(ap1, ap0);
+    if ((len = vsnprintf(buf, STACK_BUFSZ, fmt, ap0)) <= 0)
+      return;
+    else if (len >= STACK_BUFSZ) {
+      if (!(buf = alloc_buf = reinterpret_cast<char*>(malloc(len + 1)))) return;
+      if ((len = vsnprintf(buf, len + 1, fmt, ap1)) <= 0) {
+        va_end(ap1);
+        free(alloc_buf);
+        return;
+      }
+    }
+    logv_cb_(priv_, unsigned(level), buf, size_t(len));
+    va_end(ap1);
+    free(alloc_buf);
+  }
+
+ private:
+  void (*logv_cb_)(void*, unsigned, char*, size_t) = nullptr;
+  void* priv_ = nullptr;
+};
+
 static bool SaveError(char** errptr, const Status& s) {
   assert(errptr != nullptr);
   if (s.ok()) {
@@ -2945,6 +2981,15 @@ rocksdb_logger_t* rocksdb_logger_create_stderr_logger(int log_level,
   return logger;
 }
 
+rocksdb_logger_t* rocksdb_logger_create_callback_logger(
+    int log_level, void (*callback)(void*, unsigned, char*, size_t),
+    void* priv) {
+  rocksdb_logger_t* logger = new rocksdb_logger_t;
+  logger->rep = std::make_shared<rocksdb_callback_logger_t>(
+      static_cast<InfoLogLevel>(log_level), callback, priv);
+  return logger;
+}
+
 void rocksdb_logger_destroy(rocksdb_logger_t* logger) { delete logger; }
 
 void rocksdb_options_set_env(rocksdb_options_t* opt, rocksdb_env_t* env) {

include/rocksdb/c.h

@@ -1163,6 +1163,11 @@ extern ROCKSDB_LIBRARY_API int rocksdb_options_get_info_log_level(
     rocksdb_options_t*);
 extern ROCKSDB_LIBRARY_API rocksdb_logger_t*
 rocksdb_logger_create_stderr_logger(int log_level, const char* prefix);
+extern ROCKSDB_LIBRARY_API rocksdb_logger_t*
+rocksdb_logger_create_callback_logger(int log_level,
+                                      void (*)(void* priv, unsigned lev,
+                                               char* msg, size_t len),
+                                      void* priv);
 extern ROCKSDB_LIBRARY_API void rocksdb_logger_destroy(
     rocksdb_logger_t* logger);
 extern ROCKSDB_LIBRARY_API void rocksdb_options_set_write_buffer_size(