Remove unnecessary checks before releasing resources

Summary:
Continuation from the exploration of heap usage under `storage/rocksdb`. Essence of these changes:

  - Remove the code we don't need. Less code, less problems, better comprehension and readability.
  - Where appropriate set the released pointers to `nullptr` to make sure that all the "use after delete/free" type of bugs will manifest ASAP. Depending on OS, allocators used, what particular deallocator //really// does under the hood, the memory location may contain valid data for indeterminate amount of time and we won't be able to catch invalid usage till it's too late.

Why change this? Because there are better things in life than sleeping in your office for three days and trying to trace down that heap corruption which blocks shipping the product, causes random `SIGSEGV`-s, and happens once per week on a specific configuration when you happen to wear a certain brand of trail running shoes :-)

Test Plan:
`mysqlbuild.sh
mysqltest.sh --testset=RocksDB

mysqlbuild.sh --release
mysqltest.sh --release --testset=RocksDB`

Reviewers: yoshinorim, jtolmer

Reviewed By: jtolmer

Subscribers: jkedgar, webscalesql-eng

Differential Revision: https://reviews.facebook.net/D53439

storage/rocksdb/ha_rocksdb.cc

@@ -2705,11 +2705,8 @@ void ha_rocksdb::load_auto_incr_value()
     (Why don't we use index_init/index_end? class handler defines index_init
      as private, for some reason).
   */
-  if (scan_it)
-  {
-    delete scan_it;
-    scan_it= NULL;
-  }
+  delete scan_it;
+  scan_it= NULL;
 }
 
 
@@ -3403,44 +3400,34 @@ int ha_rocksdb::close(void)
   pk_descr= NULL;
   key_descr= NULL;
 
-  if (pk_tuple)
-    my_free(pk_tuple);
+  my_free(pk_tuple);
   pk_tuple= NULL;
 
-  if (pk_packed_tuple)
-    my_free(pk_packed_tuple);
+  my_free(pk_packed_tuple);
   pk_packed_tuple= NULL;
 
-  if (sec_key_packed_tuple)
-    my_free(sec_key_packed_tuple);
+  my_free(sec_key_packed_tuple);
   sec_key_packed_tuple= NULL;
 
-  if (sec_key_match_prefix_buf)
-    my_free(sec_key_match_prefix_buf);
+  my_free(sec_key_match_prefix_buf);
   sec_key_match_prefix_buf= NULL;
 
-  if (sec_key_packed_tuple_old)
-    my_free(sec_key_packed_tuple_old);
+  my_free(sec_key_packed_tuple_old);
   sec_key_packed_tuple_old= NULL;
 
-  if (end_key_packed_tuple)
-    my_free(end_key_packed_tuple);
+  my_free(end_key_packed_tuple);
   end_key_packed_tuple= NULL;
 
-  if (sec_key_tails)
-    my_free(sec_key_tails);
+  my_free(sec_key_tails);
   sec_key_tails= NULL;
 
-  if (sec_key_tails_old)
-    my_free(sec_key_tails_old);
+  my_free(sec_key_tails_old);
   sec_key_tails_old= NULL;
 
-  if (pack_buffer)
-    my_free(pack_buffer);
+  my_free(pack_buffer);
   pack_buffer= NULL;
 
-  if (field_enc)
-    my_free(field_enc);
+  my_free(field_enc);
   field_enc= NULL;
 
   DBUG_RETURN(free_share(share));
@@ -3636,6 +3623,7 @@ int ha_rocksdb::create_key_defs(TABLE *table_arg, const char *db_table,
 error:
   /* Delete what we have allocated so far */
   delete tbl_def;
+  tbl_def = nullptr;
 
   DBUG_RETURN(HA_ERR_INTERNAL_ERROR);
 }
@@ -5702,11 +5690,9 @@ int ha_rocksdb::rnd_end()
 {
   DBUG_ENTER("ha_rocksdb::rnd_end");
 
-  if (scan_it)
-  {
-    delete scan_it;
-    scan_it= NULL;
-  }
+  delete scan_it;
+  scan_it= NULL;
+
   DBUG_RETURN(0);
 }
 
@@ -5726,12 +5712,8 @@ int ha_rocksdb::index_init(uint idx, bool sorted)
 
 int ha_rocksdb::index_end()
 {
-
-  if (scan_it)
-  {
-    delete scan_it;
-    scan_it= NULL;
-  }
+  delete scan_it;
+  scan_it= NULL;
 
   active_index= MAX_KEY;
   in_range_check_pushed_down= FALSE;

storage/rocksdb/rdb_datadic.cc

@@ -155,10 +155,11 @@ RDBSE_KEYDEF::~RDBSE_KEYDEF()
 {
   mysql_mutex_destroy(&mutex);
 
-  if (pk_part_no)
-    my_free(pk_part_no);
-  if (pack_info)
-    my_free(pack_info);
+  my_free(pk_part_no);
+  pk_part_no = nullptr;
+
+  my_free(pack_info);
+  pack_info = nullptr;
 }
 
 void RDBSE_KEYDEF::setup(TABLE *tbl)
@@ -1466,9 +1467,13 @@ RDBSE_TABLE_DEF::~RDBSE_TABLE_DEF()
       if (ddl_manager && key_descr[i]) {
         ddl_manager->erase_index_num(key_descr[i]->get_gl_index_id());
       }
+
       delete key_descr[i];
+      key_descr[i] = nullptr;
     }
+
     delete[] key_descr;
+    key_descr = nullptr;
   }
 }
 

storage/rocksdb/rdb_locks.cc

@@ -18,12 +18,9 @@
 #pragma implementation        // gcc: Class implementation
 #endif
 
-#include <mysql/plugin.h>
-
-#include "ha_rocksdb.h"  // TODO: this is too much
-
 #include "rdb_locks.h"
 
+#include <mysql/plugin.h>
 
 typedef struct
 {
@@ -306,8 +303,8 @@ Row_lock* LockTable::get_lock(LF_PINS *pins, const uchar* key, size_t keylen,
   locked= do_locking_action(pins, found_lock, timeout_sec, is_write_lock);
 
 err:
-  if (key_copy)
-    my_free(key_copy);
+  my_free(key_copy);
+  key_copy = nullptr;
 
   return locked? found_lock->get_lock_handle(is_write_lock) : NULL;
 }
@@ -469,6 +466,7 @@ bool LockTable::do_locking_action(LF_PINS *pins, Row_lock_impl *found_lock,
       res= lf_hash_delete(&lf_hash, pins, found_lock->rowkey, found_lock->len);
       DBUG_ASSERT(res == 0);
       my_free(rowkey);
+      rowkey = nullptr;
     }
   }
 
@@ -579,6 +577,7 @@ void LockTable::release_lock(LF_PINS *pins, Row_lock *own_lock_handle)
     res= lf_hash_delete(&lf_hash, pins, own_lock->rowkey, own_lock->len);
     DBUG_ASSERT(res == 0);
     my_free(rowkey);
+    rowkey = nullptr;
   }
   else
   {