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Author SHA1 Message Date
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
Levi Tamasi
3becc9409e Some small improvements around allow_unprepared_value and multi-CF iterators (#13113) 
Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/13113

The patch makes some small improvements related to `allow_unprepared_value` and multi-CF iterators as groundwork for further changes:

1) Similarly to `BaseDeltaIterator`'s base iterator, `MultiCfIteratorImpl` gets passed its child iterators by the client. Even though they are currently guaranteed to have been created using the same read options as each other and the multi-CF iterator, it is safer to not assume this and call `PrepareValue` unconditionally before using any child iterator's `value()` or `columns()`.
2) Again similarly to `BaseDeltaIterator`, it makes sense to pass the entire `ReadOptions` structure to `MultiCfIteratorImpl` in case it turns out to require other read options in the future.
3) The constructors of the various multi-CF iterator classes now take an rvalue reference to a vector of column family handle + `unique_ptr` to child iterator pairs and use move semantics to take ownership of this vector (instead of taking two separate vectors of column family handles and raw iterator pointers).
4) Constructor arguments and the members of `MultiCfIteratorImpl` are reordered for consistency.

Reviewed By: jowlyzhang

Differential Revision: D65407521

fbshipit-source-id: 66c2c689ec8b036740bd98641b7b5c0ff7e777f2
 2024-11-04 18:06:07 -08:00
Levi Tamasi
c0be6a4b90 Support allow_unprepared_value for multi-CF iterators (#13079) 
Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/13079

The patch adds support for the new read option `allow_unprepared_value` to the multi-column-family iterators `CoalescingIterator` and `AttributeGroupIterator`. When this option is set, these iterators populate their value (`value()` + `columns()` or `attribute_groups()`) in an on-demand fashion when `PrepareValue()` is called. Calling `PrepareValue()` on the child iterators is similarly deferred until `PrepareValue()` is called on the main iterator.

Reviewed By: jowlyzhang

Differential Revision: D64570587

fbshipit-source-id: 783c8d408ad10074417dabca7b82c5e1fe5cab36
 2024-10-20 20:53:08 -07:00
Levi Tamasi
917e98ff9e Templatize MultiCfIteratorImpl to avoid std::function's overhead (#13052) 
Summary:
Pull Request resolved: https://github.com/facebook/rocksdb/pull/13052

Currently, `MultiCfIteratorImpl` uses `std::function`s for `reset_func_` and `populate_func_`, which uses type erasure and has a performance overhead. The patch turns `MultiCfIteratorImpl` into a template that takes the two function object types as template parameters, and changes `AttributeGroupIteratorImpl` and `CoalescingIterator` so they pass in function objects of named types (as opposed to lambdas).

Reviewed By: jaykorean

Differential Revision: D63802598

fbshipit-source-id: e202f6d80c9054335e5b2571051a67a9e012c2d0
 2024-10-02 19:18:15 -07:00
Jay Huh
d34712e0ac MultiCfIterator - AttributeGroupIter Impl & CoalescingIter Optimization (#12534) 
Summary:
Continuing from the previous MultiCfIterator Implementations - (https://github.com/facebook/rocksdb/issues/12422, https://github.com/facebook/rocksdb/issues/12480 #12465), this PR completes the `AttributeGroupIterator` by implementing `AttributeGroupIteratorImpl::AddToAttributeGroups()`. While implementing the `AttributeGroupIterator`, we had to make some changes in `MultiCfIteratorImpl` and found an opportunity to improve `Coalesce()` in `CoalescingIterator`.

Lifting `UNDER CONSTRUCTION - DO NOT USE` comment by replacing it with `EXPERIMENTAL`

Here are some implementation details:
- `IteratorAttributeGroups` is introduced to avoid having to copy all `WideColumn` objects during iteration.
- `PopulateIterator()` no longer advances non-top iterators that have the same key as the top iterator in the heap.
- `AdvanceIterator()` needs to advance the non-top iterators when they have the same key as the top iterator in the heap.
- Instead of populating one by one, `PopulateIterator()` now collects all items with the same key and calls `populate_func(items)` at once.
- This allowed optimization in `Coalesce()` such that we no longer do K-1 rounds of 2-way merge, but do one K-way merge instead.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/12534

Test Plan:
Uncommented the assertions in `verifyAttributeGroupIterator()`

```
./multi_cf_iterator_test
```

Reviewed By: ltamasi

Differential Revision: D56089019

Pulled By: jaykorean

fbshipit-source-id: 6b0b4247e221f69b40b147d41492008cc9b15054
 2024-04-16 08:45:38 -07:00
Jay Huh
58a98bded9 MultiCFIterator Refactor - CoalescingIterator & AttributeGroupIterator (#12480) 
Summary:
There are a couple of reasons to modify the current implementation of the MultiCfIterator, which implements the generic `Iterator` interface.
- The default behavior of `value()`/`columns()` returning data from different Column Families for different keys can be prone to errors, even though there might be valid use cases where users do not care about the origin of the value/columns.
- The `attribute_groups()` API, which is not yet implemented, will not be useful for a single-CF iterator.

In this PR, we are implementing the following changes:
- `IteratorBase` introduced, which includes all basic iterator functions except `value()` and `columns()`.
- `Iterator`, which now inherits from `IteratorBase`, includes `value()` and `columns()`.
- New public interface `AttributeGroupIterator` inherits from `IteratorBase` and additionally includes `attribute_groups()` (to be implemented).
- Renamed former `MultiCfIterator` to `CoalescingIterator` which inherits from `Iterator`
- Existing MultiCfIteratorTest has been split into two - `CoalescingIteratorTest` and `AttributeGroupIteratorTest`.
- Moved AttributeGroup related code from `wide_columns.h` to a new file, `attribute_groups.h`.

Some Implementation Details
- `MultiCfIteratorImpl` takes two functions - `populate_func` and `reset_func` and use them to populate `value_` and `columns_` in CoalescingIterator and `attribute_groups_` in AttributeGroupIterator. In CoalescingIterator, populate_func is `Coalesce()`, in AttributeGroupIterator populate_func is `AddToAttributeGroups()`. `reset_func` clears populated value_, columns_ and attribute_groups_ accordingly.
- `Coalesce()` merge sorts columns from multiple CFs when a key exists in more than on CFs. column that appears in later CF overwrites the prior ones.

For example, if CF1 has `"key_1" ==> {"col_1": "foo",  "col_2", "baz"}` and CF2 has `"key_1" ==> {"col_2": "quux", "col_3", "bla"}`, and when the iterator is at `key_1`, `columns()` will return `{"col_1": "foo", "col_2", "quux", "col_3", "bla"}`

In this example, `value()` will be empty, because none of them have values for `kDefaultColumnName`

Pull Request resolved: https://github.com/facebook/rocksdb/pull/12480

Test Plan:
## Unit Test
```
./multi_cf_iterator_test
```

## Performance Test

To make sure this change does not impact existing `Iterator` performance

**Build**
```
$> make -j64 release
```
**Setup**
```
$> TEST_TMPDIR=/dev/shm/db_bench ./db_bench -benchmarks="filluniquerandom" -key_size=32 -value_size=512 -num=1000000 -compression_type=none
```
**Run**
```
TEST_TMPDIR=/dev/shm/db_bench ./db_bench -use_existing_db=1 -benchmarks="newiterator,seekrandom" -cache_size=10485760000
```

**Before the change**
```
DB path: [/dev/shm/db_bench/dbbench]
newiterator  :       0.519 micros/op 1927904 ops/sec 0.519 seconds 1000000 operations;
DB path: [/dev/shm/db_bench/dbbench]
seekrandom   :       5.302 micros/op 188589 ops/sec 5.303 seconds 1000000 operations; (0 of 1000000 found)
```
**After the change**
```
DB path: [/dev/shm/db_bench/dbbench]
newiterator  :       0.497 micros/op 2011012 ops/sec 0.497 seconds 1000000 operations;
DB path: [/dev/shm/db_bench/dbbench]
seekrandom   :       5.252 micros/op 190405 ops/sec 5.252 seconds 1000000 operations; (0 of 1000000 found)
```

Reviewed By: ltamasi

Differential Revision: D55353909

Pulled By: jaykorean

fbshipit-source-id: 8d7786ffee09e022261ce34aa60e8633685e1946
 2024-04-11 11:34:04 -07:00