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Summary: format_version background: SST files are shared across RocksDB versions. format_version is a critical tool in ensuring prior (and later) versioned readers do not misinterpret a file created by some version. Opt-in features can sometimes rely on previously-rejected corners of the schema space, but not always. This change: lands much of the format_version=8 (fv8) format surface as an internal-only draft -- readable and testable but not user-writable -- so upcoming features can build on a stable base. fv8 is NOT published: kLatestBbtFormatVersion stays 7, and reading or writing fv8 is gated behind the existing TEST-only TEST_AllowUnsupportedFormatVersion() hook. Publishing fv8 (raising the ceiling) is deferred to a later change. The pieces of fv8 added here, and what each unlocks: - Index value-delta escape. Existing behavior: index blocks (format_version >= 4) store each block handle as a delta against the previous entry's handle, and signal "this value is a full handle instead" by reusing the key sentinel `shared == 0`. Because the two are coupled, an index entry whose handle is not contiguous with the previous one must be forced to a restart, costing a full key as well as a full value. fv8 decouples them with an in-value escape codepoint: varint 0 means "a full BlockHandle follows," and genuine size deltas are remapped to zigzag(delta)+1 so that 0 stays free. A non-contiguous entry can then remain an ordinary key-delta-encoded, on-cadence entry. The escape applies to every index-shaped block, which includes the top-level index of index partitions and the index of filter partitions, not just the index over data blocks. Unlocks: the upcoming optimize_key_common_prefix index feature (removes its need to special-case super_block_alignment) and value-delta encoding for embedded-blob SSTs (previously disabled outright because interleaved blob records break handle contiguity). Guarded strictly on fv8; format_version <= 7 encoding and decoding are byte-for-byte unchanged. Note that restart points must not delta-encode, so they still have full values without a sentinel. - Data block footer bit 30 is reserved as an "extended metadata present" escape. Decode rejects a set bit as unsupported/corrupt at all versions. Unlocks: adding block-level metadata in the future through the one shared footer decoder, without another format_version bump or plumbing format_version into every block parser. (Older reader versions relied on this bit being zero for some integrity/consistency checking; format_version bump now makes it safe to use.) - Footer metaindex gap. For fv8, the top 16 bits of the footer's 4-byte metaindex-size field record the gap (in bytes) between the end of the metaindex block and the start of the footer; the low 16 bits hold the metaindex block size. Unlocks: a space very near the end of the file to save full file checksum state, for a "files carry their own checksums" feature that doesn't require SST parsing to extract the checksum states (to run them to completion on the trivial file tail). - Reserved "user_key_common_prefix" table property. The reader returns NotSupported when it is present and non-empty. Unlocks: a future whole-file optimization that stores the common prefix of all user keys once instead of in every restart key; until implemented, fv8 readers refuse such a file rather than mis-read it. - Because of the problem of inferring such a prefix at the start of constructing an SST file, we expect it will rely on SstPartitioner. Here we've added SstPartitioner::ShouldPartitionByPrefix() (not yet used) that will enable this optimization, and avoid the overhead of a virtual callback to SstPartitioner on every key written to an SST file. Supporting cleanups: the Footer's "unset" format_version sentinel is named (kInvalidFormatVersion) and the FormatVersionUses*() predicates assert against it, as each is a `version >= N` test that an unpopulated Footer would silently satisfy. Index block reader entry points (Block::NewIndexIterator() and friends) drop their defaulted trailing arguments so each caller states which codec it expects, and BlockBuilder::Add() takes delta_value by const reference rather than a pointer whose null case no caller used. Pull Request resolved: https://github.com/facebook/rocksdb/pull/15104 Test Plan: New unit tests, all exercising fv8 through TEST_AllowUnsupportedFormatVersion(): - block_test: value-delta escape codec round-trip (delta 0, negative, large, and the escape; asserts format_version <= 7 bytes match the legacy encoding and a legacy 0x00 still decodes as delta 0), and rejection of footer bit 30 at all versions. - db_flush_test: fv7-vs-fv8 read equivalence for super_block_alignment, including the separate_key_value_in_data_block intersection, confirming padding events (hence escape entries) occur and fv8 index values are delta encoded. - sst_file_reader_test: fv7-vs-fv8 read equivalence for embedded-blob SSTs, confirming fv8 (unlike fv7) delta-encodes their index values and actually emits escape entries (counted through a sync point). - block_based_table_reader_test: a builder+reader test that injects a footer metaindex gap via a sync point (verifies the gap bytes are written and the reader locates the metaindex via the recorded gap, and that too large a gap surfaces as a builder error), and that a non-empty reserved user_key_common_prefix property makes the reader return NotSupported. - table_test: two-level index with partitioned filters and index_block_restart_interval > 1, the configuration in which both top-level indexes delta-encode their values; verifies scan and point lookup round trips at each tested format_version including fv8. - (Not strictly fv8) Basic SstPartitionerFixedPrefix tests in slice_test (ShouldPartition, ShouldPartitionByPrefix, their mutual consistency, and CanDoTrivialMove), which had no non-DB coverage before. fv8 is also added to the table_test harness (GenerateArgList) and the shared footer version set (test::kFooterFormatVersionsToTest), so the existing table-level and DB-level property/checksum suites now also run at fv8. And added to crash test (minor revisions to db_stress). ## Performance details Release db_bench (PORTABLE=0, -march=native) A/B/C on the read/write fast path, comparing (a) parent, (b) this commit @ fv7, (c) this commit @ fv8. All DBs on tmpfs, 20M keys, block_size=2048, compression=none. Note index_block_restart_ interval=16 (not the default 1) is required to actually exercise the fv7<->fv8 index value-delta codec -- with restart interval 1 every index entry is a full- handle restart and the changed path never runs (confirmed delta-value=1 via sst_dump). Reads single-thread + core-pinned, warm cache, interleaved/rotated rounds, paired deltas. Reads (readrandom + seekrandom, non-partitioned single ~1.9MB index block and partitioned index+filters): fv7->fv8 in the *same* binary is within noise on all four workloads (largest -1.0%, CIs cross zero). before->after (fv7 path) is also within noise; perf shows a deterministic +0.4-0.6% instructions/op from the added branches (the format_version>=8 value_delta_escape guard + refactored data-block footer decode), which is below the throughput noise floor. Writes (fillseq + compact total CPU): fv7->fv8 null. compact (which rewrites all index blocks) is null before->after. fillseq showed +3.5% CPU before->after, but perf attributes it entirely to cycles/IPC with *identical* instructions/op (+0.01-0.04%) -- a code-layout effect on the untouched memtable-insert loop, not this change. Sanity: any single throughput/CPU delta <~1% is within the build-to-build noise floor, measured directly by comparing two byte-identical binaries (swings up to ~0.9% reads; the pre-native/PORTABLE=1 build showed a spurious -1.85% that vanished under -march=native). Net: no measurable regression from fv8 draft; fv7 encoding/decoding byte-for-byte unchanged, only the noted sub-1% instruction bump on the shared read path. Reviewed By: xingbowang Differential Revision: D116105558 Pulled By: pdillinger fbshipit-source-id: ead776cdd396d497d47872c0956ea8c651c0d13f