Summary:
The `creation_time` table property has long stored the file's oldest ancestor time (the oldest key time among the file's ancestors, falling back to flush/compaction time), not the file's creation time, which is `file_creation_time`. A TODO in table_properties.h acknowledged the misleading name. This change starts the API migration: `TableProperties::oldest_ancestor_time` is the new name, and `creation_time` remains as a deprecated alias sharing storage via an anonymous union, so existing external reads and writes continue to compile and behave identically.
An anonymous union is used rather than a `uint64_t&` alias member. A reference member would delete TableProperties copy/move assignment, make the implicit copy constructor bind the copy's alias to the source object (dangling), permit writes through a const TableProperties, add 8 bytes, and break the uint64_t-range walk in TEST_SetRandomTableProperties. The union leaves size, layout, copying, constness, and offsetof unchanged. Reading the other union member is formally UB in ISO C++, but both members are uint64_t and this is supported in practice by GCC, Clang, and MSVC.
All in-repo uses of the old name now use `oldest_ancestor_time`. Names that are externally observable or persisted are intentionally unchanged:
* On-disk property name `rocksdb.creation.time` (`TablePropertiesNames::kCreationTime`)
* The `"creation_time"` key in TableProperties serialization (used by remote compaction)
* The event-log JSON key and `TableProperties::ToString()` label
* Java `TableProperties.getCreationTime()` and C `rocksdb_table_properties_creation_time()`
C API: the auto-binding generator (tools/c_api_gen/auto_simple_bindings.py) only considered direct FieldDecls and failed on the anonymous union's implicit unnamed field. It now treats anonymous union members as direct fields. Regenerating adds a new C getter
`rocksdb_table_properties_oldest_ancestor_time()` alongside the existing `rocksdb_table_properties_creation_time()`.
Follow-up: remove the deprecated `creation_time` alias in a future release.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/15304
Test Plan: New unit test DBTablePropertiesTest.CreationTimeAliasesOldestAncestorTime verifies the two names share an address and that a write through the old name is visible through the new one. options_settable_test and compaction_service_test cover TableProperties serialization round trips through the unchanged "creation_time" key, now mapped to the renamed field.
Reviewed By: jaykorean
Differential Revision: D123079083
Pulled By: pdillinger
fbshipit-source-id: 18b07d91f303a8f6de58da0c730f80d8c371505c
c83a84e98c