change type of parameter for logv (#471)

Signed-off-by: Wangweizhen <hawking.rei@gmail.com>

UCP https://github.com/tikv/tikv/issues/6496

Fix this error from clippy.

```
this public function dereferences a raw pointer but is not marked `unsafe`
help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#not_unsafe_ptr_arg_deref

```

src/logger.rs

@@ -3,10 +3,11 @@
 use crocksdb_ffi;
 use libc::{c_char, c_void};
 use librocksdb_sys::{DBEnv, DBInfoLogLevel as InfoLogLevel, DBLogger};
-use std::ffi::{CString, VaList};
+use std::ffi::{CStr, CString, VaList};
+use std::str;
 
 pub trait Logger: Send + Sync {
-    fn logv(&self, log_level: InfoLogLevel, format: *const c_char, ap: VaList);
+    fn logv(&self, log_level: InfoLogLevel, format: &str, ap: VaList);
 }
 
 extern "C" fn destructor(ctx: *mut c_void) {
@@ -18,7 +19,8 @@ extern "C" fn destructor(ctx: *mut c_void) {
 extern "C" fn logv(ctx: *mut c_void, log_level: InfoLogLevel, format: *const c_char, ap: VaList) {
     unsafe {
         let logger = &*(ctx as *mut Box<dyn Logger>);
-        logger.logv(log_level, format, ap);
+        let format = CStr::from_ptr(format);
+        logger.logv(log_level, format.to_str().unwrap(), ap);
     }
 }
 

tests/cases/test_logger.rs

 use std::ffi::VaList;
-use std::sync::atomic::*;
-use std::sync::Arc;
-use std::thread;
+use std::sync::{atomic::*, Arc};
 use std::time::Duration;
+use std::{str, thread};
 
 use super::tempdir_with_prefix;
-use libc::c_char;
+
 use rocksdb::{DBInfoLogLevel as InfoLogLevel, DBOptions, Logger, DB};
 
 #[derive(Default, Clone)]
@@ -26,7 +25,7 @@ struct TestLogger {
 }
 
 impl Logger for TestLogger {
-    fn logv(&self, _log_level: InfoLogLevel, _format: *const c_char, _ap: VaList) {
+    fn logv(&self, _log_level: InfoLogLevel, _format: &str, _ap: VaList) {
         self.print.fetch_add(1, Ordering::SeqCst);
     }
 }