Josh Kang 4b983da055 Spell out special member functions (rule of five) (#15387) ...

Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/15387

## Summary

RocksDB's `.clang-tidy` enables `cppcoreguidelines-special-member-functions`,
`cppcoreguidelines-virtual-class-destructor` and the noexcept-move checks,
and clang warns `-Wdeprecated-copy-with-user-provided-dtor`. A full scan
reports 476 findings. In most of them, a class declares a destructor (or
copy operations) but leaves the rest of copy/move implicit. That has two
costs. Readers can't tell whether a class may be copied or moved. And
resource-owning classes keep an implicit copy that would double-free if
anyone ever used it. This diff resolves 453 of the 476.

Approach, picked per class from least to most invasive:
1. An empty user-provided destructor becomes `= default`. This covers most
   interface classes, such as `virtual ~X() {}` -> `= default`. If the
   class is still copied, that would only rename the deprecated-copy
   warning, so the destructor is removed instead when nothing needs it
   (`Customizable`, `TableFactory` and `InternalKeyComparator` inherit a
   virtual one; `IteratorWrapperBase` isn't polymorphic), and
   `MemTableRep::KeyComparator` gets defaulted copy and move.
2. Classes whose copy is already deleted get explicitly deleted move
   operations too. Move was already unavailable, so this only documents
   it.
3. Resource-owning classes that are never copied get deleted copy and move
   operations. Examples: the C API wrapper structs, which run a
   `destructor_` callback (edited in `tools/c_api_gen/c_base.cc` and
   regenerated into `db/c.cc`); `ShardedCache`, which frees its shards;
   iterator wrappers; and RAII handles.
4. Value types that are copied or moved get the missing operations as
   `= default`, which keeps their current behavior.
5. Exception specifications of existing functions are unchanged, except
   for `WriteBatch`'s move assignment (below): no other `noexcept` is
   added or removed. Only the move operations this diff
   newly declares (`Comparator`, `FileSampledStats` and the thread-local
   `Entry`) are `noexcept`; they copy only scalars or atomics.

`WriteBatch`'s copy and move assignment destroyed `*this` and
placement-new'ed a new `WriteBatch` over it. That is undefined behavior
when `*this` is a subclass, and if the copy constructor threw, `*this`
was left destroyed and was later destroyed again. Move assignment now
assigns member-wise and ends in the same state as the move constructor,
which leaves `has_key_with_ts_`, the timestamp-size tracking and the
other fields it doesn't copy at their defaults. `rep_` is swapped through
a temporary, because a plain string move-assignment can keep the old
buffer (e.g. a large batch reset with `batch = WriteBatch()` would keep
its memory). Nothing in it can throw, so it is now `noexcept`. Copy
assignment copies into a temporary and moves it in, so a throwing copy
leaves `*this` unchanged.

Bugs this exposed and fixed:
- `RestoreAfterCopyOrCreateWorkItem` (backup engine): the hand-written
  move constructor and assignment moved only some fields, so `from_file`
  and `to_file` were empty after `push_back(std::move(...))`. A restore
  that hit a checksum mismatch then reported "While restoring  -> : ..."
  without file names. It now uses the compiler-generated moves, which
  are still `noexcept`.
- `ExternalSstFileIngestionJob` lived in a `std::vector`, and its
  destructor frees raw-owned pointers, so the implicit copy used on
  vector reallocation would double-free. Its file batches also point into
  its own inline file storage, so neither a copy nor a move is safe after
  `Prepare()`. Copy and move are now deleted, and `DBImpl` holds jobs
  through `std::unique_ptr`.

Compatibility: no public class loses an operation users can call. Public
interfaces mostly only get `virtual ~X() = default;`.
`WriteBatch`'s move assignment becomes `noexcept`, which is not part of
the mangled name, so the ABI doesn't change.
`SubcompactionJobInfo` and `CompactionJobInfo` intentionally stay C++20
aggregates, so users' aggregate and designated initializers keep
compiling. `Configurable`, `CompareInterface`, `WriteBatchBase`,
`ObjectLibrary::Entry` and `MemTableRep::KeyComparator` declare defaulted
copy and move, and `Customizable` no longer declares a destructor. As a
result, `Configurable` and `Customizable` subclasses that declare no
destructor or copy operations of their own now move their
registered-options list instead of copying it. In-tree, these are the
mutable-options adapters, the table factories, the memory allocators and
test classes; all are held through smart pointers or used in place, and
`Clone()` copies, so none is moved by value.

Not changed (23): `IODebugContext` (custom deep copy plus a
`std::shared_mutex`), `DBWithColumnFamilies` in db_bench (stored in a
`std::vector` with non-movable members), `Snapshot`'s protected virtual
destructor (intended), `ManagedSnapshot`, the `SubcompactionJobInfo` and
`CompactionJobInfo` aggregates, and the 16 noexcept-move-constructor and
noexcept-swap findings on existing functions, including `autovector`'s
move assignment (adding `noexcept` would change their exception
specifications). Copying `ManagedSnapshot` already double-releases the
snapshot. Fixing that needs a real move implementation and is a behavior
change, so it is left for a separate diff.

Reviewed By: rban1

Differential Revision: D123186309

fbshipit-source-id: 6f8e57e233e12199a007407130bb9a1f6d52e8d1

2026-10-12 16:00:09 -07:00

3.5 KiB

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