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// SPDX-License-Identifier: Apache-2.0
// Copyright (c) 2024-present, fjall-rs
// Copyright (c) 2026-present, Structured World Foundation
use super::{Block, DataBlock};
use crate::{
CompressionType, InternalValue, SeqNo,
comparator::SharedComparator,
encryption::EncryptionProvider,
fs::{FileHint, FsFile},
table::{block::BlockType, iter::OwnedDataBlockIter},
};
use std::{fs::File, io::BufReader, path::Path, sync::Arc};
/// Table reader that is optimized for consuming an entire table
pub struct Scanner {
reader: BufReader<File>,
iter: OwnedDataBlockIter,
compression: CompressionType,
block_count: usize,
read_count: usize,
global_seqno: SeqNo,
encryption: Option<Arc<dyn EncryptionProvider>>,
comparator: SharedComparator,
/// Per-SST Page-ECC scheme from table metadata: data blocks omit the
/// `block_flags` byte, so the scanner's block-read transform is told
/// here the parity scheme (sizing + recovery). `None` = no parity.
ecc: Option<crate::table::block::EccParams>,
/// Per-SST per-KV-footer flag (`kv_checksum_algo.is_some()`): supplied to
/// `from_loaded` so it strips the footer (data blocks omit the byte).
has_kv_footer: bool,
#[cfg(zstd_any)]
zstd_dictionary: Option<Arc<crate::compression::ZstdDictionary>>,
/// Table id of the SST being scanned; threaded through to
/// per-block reads via `BlockIdentity`.
table_id: crate::TableId,
}
impl Scanner {
#[expect(
clippy::too_many_arguments,
reason = "scanner ctor takes one local per piece of state it needs to thread \
through fetch_next_block; collapsing them into a config struct would \
add an indirection without removing any per-call decision the caller \
makes about the values"
)]
pub fn new(
path: &Path,
block_count: usize,
compression: CompressionType,
global_seqno: SeqNo,
encryption: Option<Arc<dyn EncryptionProvider>>,
ecc: Option<crate::table::block::EccParams>,
has_kv_footer: bool,
#[cfg(zstd_any)] zstd_dictionary: Option<Arc<crate::compression::ZstdDictionary>>,
comparator: SharedComparator,
table_id: crate::TableId,
) -> crate::Result<Self> {
// 2 MiB buffer matches RocksDB's `compaction_readahead_size`
// default and is large enough that the kernel can fold the
// sequential-scan readahead heuristic into a single big
// userspace fill per ~500 typical (4 KiB) data blocks instead
// of one syscall every 8 blocks. Picked to dominate any
// pre-#133-Phase1c micro-cost: the loss is at most a few MB
// of allocator overhead per concurrent compaction, and
// compaction concurrency is bounded by the scheduler. HDD-
// tuning beyond this would benefit from a configurable knob,
// tracked as the configurable-readahead follow-up under #133.
const SCANNER_READAHEAD_BYTES: usize = 2 * 1024 * 1024;
let file = File::open(path)?;
// The scanner walks every block in order — tell the kernel so
// it can ramp readahead aggressively and evict already-read
// pages instead of pinning them. Best-effort: hint() is
// advisory and any failure here would just leave the kernel
// on the default heuristic, so drop the error rather than
// failing the open.
let _ = file.hint(FileHint::Sequential);
let mut reader = BufReader::with_capacity(SCANNER_READAHEAD_BYTES, file);
let block = Self::fetch_next_block(
&mut reader,
table_id,
compression,
encryption.as_deref(),
ecc,
has_kv_footer,
#[cfg(zstd_any)]
zstd_dictionary.as_deref(),
)?;
let cmp = comparator.clone();
let iter = OwnedDataBlockIter::try_new(block, |b| b.try_iter(cmp))?;
Ok(Self {
reader,
iter,
compression,
block_count,
read_count: 1,
global_seqno,
encryption,
comparator,
ecc,
has_kv_footer,
#[cfg(zstd_any)]
zstd_dictionary,
table_id,
})
}
fn fetch_next_block(
reader: &mut BufReader<File>,
table_id: crate::TableId,
compression: CompressionType,
encryption: Option<&dyn EncryptionProvider>,
ecc: Option<crate::table::block::EccParams>,
has_kv_footer: bool,
#[cfg(zstd_any)] zstd_dict: Option<&crate::compression::ZstdDictionary>,
) -> crate::Result<DataBlock> {
let block = Block::from_reader(
reader,
crate::table::block::BlockIdentity {
table_id,
block_type: BlockType::Data,
dict_id: compression.dict_id(),
window_log: 0,
},
&{
// SST blocks omit the block_flags byte, so ECC presence is a
// per-SST property: upgrade the transform to its `*Ecc`
// variant when this table was written with Page ECC, so the
// reader expects the parity trailer. Identity without the
// `page_ecc` feature.
let t = crate::table::block::BlockTransform::from_parts(
compression,
encryption,
#[cfg(zstd_any)]
zstd_dict,
)?;
if let Some(ecc) = ecc {
t.with_ecc(ecc)
} else {
t
}
},
);
match block {
Ok(block) => {
// A data block is always BlockType::Data; `from_loaded`
// strips the per-KV checksum footer when this SST carries one
// (per-SST `has_kv_footer`, since data blocks omit the byte),
// so the scan path is unchanged.
if block.header.block_type != BlockType::Data {
return Err(crate::Error::InvalidTag((
"BlockType",
block.header.block_type.into(),
)));
}
DataBlock::from_loaded(block, has_kv_footer)
}
Err(e) => Err(e),
}
}
}
impl Iterator for Scanner {
type Item = crate::Result<InternalValue>;
fn next(&mut self) -> Option<Self::Item> {
loop {
if let Some(mut item) = self.iter.next() {
item.key.seqno += self.global_seqno;
return Some(Ok(item));
}
if self.read_count >= self.block_count {
return None;
}
// Init new block
let block = match Self::fetch_next_block(
&mut self.reader,
self.table_id,
self.compression,
self.encryption.as_deref(),
self.ecc,
self.has_kv_footer,
#[cfg(zstd_any)]
self.zstd_dictionary.as_deref(),
) {
Ok(block) => block,
Err(e) => {
self.read_count = self.block_count;
return Some(Err(e));
}
};
let cmp = self.comparator.clone();
match OwnedDataBlockIter::try_new(block, |b| b.try_iter(cmp)) {
Ok(iter) => {
self.iter = iter;
self.read_count += 1;
}
Err(e) => {
// Poison the scanner so callers cannot silently skip
// the corrupt block and resume on later blocks.
self.read_count = self.block_count;
return Some(Err(e));
}
}
}
}
}