Fix GitHub issue #3716: gcc-8 warnings

Summary:
Fix the following gcc-8 warnings:
- conflicting C language linkage declaration [-Werror]
- writing to an object with no trivial copy-assignment [-Werror=class-memaccess]
- array subscript -1 is below array bounds [-Werror=array-bounds]

Solves https://github.com/facebook/rocksdb/issues/3716
Closes https://github.com/facebook/rocksdb/pull/3736

Differential Revision: D7684161

Pulled By: yiwu-arbug

fbshipit-source-id: 47c0423d26b74add251f1d3595211eee1e41e54a

db/c.cc

@@ -1407,11 +1407,6 @@ void rocksdb_writebatch_put_log_data(
   b->rep.PutLogData(Slice(blob, len));
 }
 
-void rocksdb_writebatch_iterate(
-    rocksdb_writebatch_t* b,
-    void* state,
-    void (*put)(void*, const char* k, size_t klen, const char* v, size_t vlen),
-    void (*deleted)(void*, const char* k, size_t klen)) {
 class H : public WriteBatch::Handler {
  public:
   void* state_;
@@ -1424,6 +1419,12 @@ void rocksdb_writebatch_iterate(
     (*deleted_)(state_, key.data(), key.size());
   }
 };
+
+void rocksdb_writebatch_iterate(
+    rocksdb_writebatch_t* b,
+    void* state,
+    void (*put)(void*, const char* k, size_t klen, const char* v, size_t vlen),
+    void (*deleted)(void*, const char* k, size_t klen)) {
   H handler;
   handler.state_ = state;
   handler.put_ = put;
@@ -1668,18 +1669,6 @@ void rocksdb_writebatch_wi_iterate(
     void* state,
     void (*put)(void*, const char* k, size_t klen, const char* v, size_t vlen),
     void (*deleted)(void*, const char* k, size_t klen)) {
-  class H : public WriteBatch::Handler {
-   public:
-    void* state_;
-    void (*put_)(void*, const char* k, size_t klen, const char* v, size_t vlen);
-    void (*deleted_)(void*, const char* k, size_t klen);
-    virtual void Put(const Slice& key, const Slice& value) override {
-      (*put_)(state_, key.data(), key.size(), value.data(), value.size());
-    }
-    virtual void Delete(const Slice& key) override {
-      (*deleted_)(state_, key.data(), key.size());
-    }
-  };
   H handler;
   handler.state_ = state;
   handler.put_ = put;
@@ -3298,7 +3287,6 @@ void rocksdb_slicetransform_destroy(rocksdb_slicetransform_t* st) {
   delete st;
 }
 
-rocksdb_slicetransform_t* rocksdb_slicetransform_create_fixed_prefix(size_t prefixLen) {
 struct Wrapper : public rocksdb_slicetransform_t {
   const SliceTransform* rep_;
   ~Wrapper() { delete rep_; }
@@ -3312,6 +3300,8 @@ rocksdb_slicetransform_t* rocksdb_slicetransform_create_fixed_prefix(size_t pref
   bool InRange(const Slice& src) const override { return rep_->InRange(src); }
   static void DoNothing(void*) { }
 };
+
+rocksdb_slicetransform_t* rocksdb_slicetransform_create_fixed_prefix(size_t prefixLen) {
   Wrapper* wrapper = new Wrapper;
   wrapper->rep_ = rocksdb::NewFixedPrefixTransform(prefixLen);
   wrapper->state_ = nullptr;
@@ -3320,19 +3310,6 @@ rocksdb_slicetransform_t* rocksdb_slicetransform_create_fixed_prefix(size_t pref
 }
 
 rocksdb_slicetransform_t* rocksdb_slicetransform_create_noop() {
-  struct Wrapper : public rocksdb_slicetransform_t {
-    const SliceTransform* rep_;
-    ~Wrapper() { delete rep_; }
-    const char* Name() const override { return rep_->Name(); }
-    Slice Transform(const Slice& src) const override {
-      return rep_->Transform(src);
-    }
-    bool InDomain(const Slice& src) const override {
-      return rep_->InDomain(src);
-    }
-    bool InRange(const Slice& src) const override { return rep_->InRange(src); }
-    static void DoNothing(void*) { }
-  };
   Wrapper* wrapper = new Wrapper;
   wrapper->rep_ = rocksdb::NewNoopTransform();
   wrapper->state_ = nullptr;

memtable/inlineskiplist.h

@@ -287,7 +287,7 @@ struct InlineSkipList<Comparator>::Node {
   // next_[0].  This is used for passing data from AllocateKey to Insert.
   void StashHeight(const int height) {
     assert(sizeof(int) <= sizeof(next_[0]));
-    memcpy(&next_[0], &height, sizeof(int));
+    memcpy(static_cast<void*>(&next_[0]), &height, sizeof(int));
   }
 
   // Retrieves the value passed to StashHeight.  Undefined after a call
@@ -307,30 +307,30 @@ struct InlineSkipList<Comparator>::Node {
     assert(n >= 0);
     // Use an 'acquire load' so that we observe a fully initialized
     // version of the returned Node.
-    return (next_[-n].load(std::memory_order_acquire));
+    return ((&next_[0] - n)->load(std::memory_order_acquire));
   }
 
   void SetNext(int n, Node* x) {
     assert(n >= 0);
     // Use a 'release store' so that anybody who reads through this
     // pointer observes a fully initialized version of the inserted node.
-    next_[-n].store(x, std::memory_order_release);
+    (&next_[0] - n)->store(x, std::memory_order_release);
   }
 
   bool CASNext(int n, Node* expected, Node* x) {
     assert(n >= 0);
-    return next_[-n].compare_exchange_strong(expected, x);
+    return (&next_[0] - n)->compare_exchange_strong(expected, x);
   }
 
   // No-barrier variants that can be safely used in a few locations.
   Node* NoBarrier_Next(int n) {
     assert(n >= 0);
-    return next_[-n].load(std::memory_order_relaxed);
+    return (&next_[0] - n)->load(std::memory_order_relaxed);
   }
 
   void NoBarrier_SetNext(int n, Node* x) {
     assert(n >= 0);
-    next_[-n].store(x, std::memory_order_relaxed);
+    (&next_[0] - n)->store(x, std::memory_order_relaxed);
   }
 
   // Insert node after prev on specific level.