rocksdb/utilities/transactions/optimistic_transaction_db_impl.cc
Peter Dillinger 54cb9c77d9 Prefer static_cast in place of most reinterpret_cast (#12308)
Summary:
The following are risks associated with pointer-to-pointer reinterpret_cast:
* Can produce the "wrong result" (crash or memory corruption). IIRC, in theory this can happen for any up-cast or down-cast for a non-standard-layout type, though in practice would only happen for multiple inheritance cases (where the base class pointer might be "inside" the derived object). We don't use multiple inheritance a lot, but we do.
* Can mask useful compiler errors upon code change, including converting between unrelated pointer types that you are expecting to be related, and converting between pointer and scalar types unintentionally.

I can only think of some obscure cases where static_cast could be troublesome when it compiles as a replacement:
* Going through `void*` could plausibly cause unnecessary or broken pointer arithmetic. Suppose we have
`struct Derived: public Base1, public Base2`.  If we have `Derived*` -> `void*` -> `Base2*` -> `Derived*` through reinterpret casts, this could plausibly work (though technical UB) assuming the `Base2*` is not dereferenced. Changing to static cast could introduce breaking pointer arithmetic.
* Unnecessary (but safe) pointer arithmetic could arise in a case like `Derived*` -> `Base2*` -> `Derived*` where before the Base2 pointer might not have been dereferenced. This could potentially affect performance.

With some light scripting, I tried replacing pointer-to-pointer reinterpret_casts with static_cast and kept the cases that still compile. Most occurrences of reinterpret_cast have successfully been changed (except for java/ and third-party/). 294 changed, 257 remain.

A couple of related interventions included here:
* Previously Cache::Handle was not actually derived from in the implementations and just used as a `void*` stand-in with reinterpret_cast. Now there is a relationship to allow static_cast. In theory, this could introduce pointer arithmetic (as described above) but is unlikely without multiple inheritance AND non-empty Cache::Handle.
* Remove some unnecessary casts to void* as this is allowed to be implicit (for better or worse).

Most of the remaining reinterpret_casts are for converting to/from raw bytes of objects. We could consider better idioms for these patterns in follow-up work.

I wish there were a way to implement a template variant of static_cast that would only compile if no pointer arithmetic is generated, but best I can tell, this is not possible. AFAIK the best you could do is a dynamic check that the void* conversion after the static cast is unchanged.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/12308

Test Plan: existing tests, CI

Reviewed By: ltamasi

Differential Revision: D53204947

Pulled By: pdillinger

fbshipit-source-id: 9de23e618263b0d5b9820f4e15966876888a16e2
2024-02-07 10:44:11 -08:00

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3.9 KiB
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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
#include "utilities/transactions/optimistic_transaction_db_impl.h"
#include <string>
#include <vector>
#include "db/db_impl/db_impl.h"
#include "rocksdb/db.h"
#include "rocksdb/options.h"
#include "rocksdb/utilities/optimistic_transaction_db.h"
#include "utilities/transactions/optimistic_transaction.h"
namespace ROCKSDB_NAMESPACE {
std::shared_ptr<OccLockBuckets> MakeSharedOccLockBuckets(size_t bucket_count,
bool cache_aligned) {
if (cache_aligned) {
return std::make_shared<OccLockBucketsImpl<true>>(bucket_count);
} else {
return std::make_shared<OccLockBucketsImpl<false>>(bucket_count);
}
}
Transaction* OptimisticTransactionDBImpl::BeginTransaction(
const WriteOptions& write_options,
const OptimisticTransactionOptions& txn_options, Transaction* old_txn) {
if (old_txn != nullptr) {
ReinitializeTransaction(old_txn, write_options, txn_options);
return old_txn;
} else {
return new OptimisticTransaction(this, write_options, txn_options);
}
}
Status OptimisticTransactionDB::Open(const Options& options,
const std::string& dbname,
OptimisticTransactionDB** dbptr) {
DBOptions db_options(options);
ColumnFamilyOptions cf_options(options);
std::vector<ColumnFamilyDescriptor> column_families;
column_families.push_back(
ColumnFamilyDescriptor(kDefaultColumnFamilyName, cf_options));
std::vector<ColumnFamilyHandle*> handles;
Status s = Open(db_options, dbname, column_families, &handles, dbptr);
if (s.ok()) {
assert(handles.size() == 1);
// i can delete the handle since DBImpl is always holding a reference to
// default column family
delete handles[0];
}
return s;
}
Status OptimisticTransactionDB::Open(
const DBOptions& db_options, const std::string& dbname,
const std::vector<ColumnFamilyDescriptor>& column_families,
std::vector<ColumnFamilyHandle*>* handles,
OptimisticTransactionDB** dbptr) {
return OptimisticTransactionDB::Open(db_options,
OptimisticTransactionDBOptions(), dbname,
column_families, handles, dbptr);
}
Status OptimisticTransactionDB::Open(
const DBOptions& db_options,
const OptimisticTransactionDBOptions& occ_options,
const std::string& dbname,
const std::vector<ColumnFamilyDescriptor>& column_families,
std::vector<ColumnFamilyHandle*>* handles,
OptimisticTransactionDB** dbptr) {
Status s;
DB* db;
std::vector<ColumnFamilyDescriptor> column_families_copy = column_families;
// Enable MemTable History if not already enabled
for (auto& column_family : column_families_copy) {
ColumnFamilyOptions* options = &column_family.options;
if (options->max_write_buffer_size_to_maintain == 0 &&
options->max_write_buffer_number_to_maintain == 0) {
// Setting to -1 will set the History size to
// max_write_buffer_number * write_buffer_size.
options->max_write_buffer_size_to_maintain = -1;
}
}
s = DB::Open(db_options, dbname, column_families_copy, handles, &db);
if (s.ok()) {
*dbptr = new OptimisticTransactionDBImpl(db, occ_options);
}
return s;
}
void OptimisticTransactionDBImpl::ReinitializeTransaction(
Transaction* txn, const WriteOptions& write_options,
const OptimisticTransactionOptions& txn_options) {
assert(dynamic_cast<OptimisticTransaction*>(txn) != nullptr);
auto txn_impl = static_cast<OptimisticTransaction*>(txn);
txn_impl->Reinitialize(this, write_options, txn_options);
}
} // namespace ROCKSDB_NAMESPACE