perf(ecstore): slice in-memory shards instead of copying them twice (#4687)

* perf(ecstore): slice in-memory shards instead of copying them twice (backlog#1159)

The GET path reads a shard out of the page cache into a `Bytes`, then
`open_disk_reader` erased it behind `Box<dyn AsyncRead>` by wrapping it in
a `Cursor`. Downstream, `BitrotReader` could only get it back by copying:
once out of the `Cursor` into its scratch buffer, and once from there into
the caller's buffer. CPU profiling of a cached 1 MiB GET (device reads = 0)
attributed 8.23% of the whole server to `Cursor::poll_read` alone — a copy
of data that was already sitting in memory.

Keep the source concrete instead of erasing it. `ShardReader` is an enum of
`InMemory(Cursor<Bytes>)` and `Stream(Box<dyn AsyncRead ...>)`, and the new
`ShardSource::try_take_block(n)` lets an in-memory source hand over the
`[hash][data]` block as a slice. `read_appending` uses it to verify the hash
on the slice and `extend_from_slice` the shard straight into the caller's
buffer: one copy instead of two.

`try_take_block` defaults to `None`, so a streaming source keeps the old
path byte for byte, along with its short-read and EOF semantics. A source
that cannot serve `n` bytes declines rather than truncating, so a partial
block still becomes UnexpectedEof rather than a short shard. The hash is
still checked before anything is appended, so a corrupt shard never reaches
the caller's buffer on either path. The deferred parity reader opens its
source lazily and stays on the streaming path; parity is only read when a
data shard fails.

Tests gate equivalence and non-vacuity:
  * `try_take_block` fires for `Cursor<Bytes>`, advances the position exactly
    as a read of the same length would, declines when fewer than `n` bytes
    remain, and returns `None` for a non-`Bytes` source — without this the
    equivalence test below would silently compare one path against itself;
  * both paths return identical bytes for the same shard;
  * a corrupt shard fails on the fast path too, appending nothing.

Verified: clippy --tests -D warnings clean; `erasure::` 215 passed, 0 failed;
`set_disk::core::io_primitives` 49 and `io_support::` 22 pass.

Stacked on #4681 (`read_appending`), which this builds on.

Co-Authored-By: heihutu <heihutu@gmail.com>

* fix(ecstore): implement ShardSource for Cursor<&[u8]> used by the erasure bench

`crates/ecstore/benches/erasure_benchmark.rs` builds
`BitrotReader<Cursor<&[u8]>>`, which the new `ShardSource` bound on
`ParallelReader`/`decode` does not accept. `cargo clippy --tests` does not
compile bench targets, so this only surfaced in CI's `--all-targets` run.

A borrowed slice carries no `Bytes` to hand out, so it takes the default
`try_take_block` and keeps the old streaming copy path — no behavior change.

Verified with the same target set CI uses:
`cargo clippy -p rustfs-ecstore --all-targets -- -D warnings` clean.

Co-Authored-By: heihutu <heihutu@gmail.com>

---------

Co-authored-by: heihutu <heihutu@gmail.com>

crates/ecstore/src/set_disk/core/io_primitives.rs

@@ -48,6 +48,7 @@ use crate::diagnostics::get::{
 };
 use crate::disk::OldCurrentSize;
 use crate::erasure::coding::BitrotReader;
+use crate::io_support::bitrot::ShardReader;
 use crate::io_support::bitrot::{
     BitrotReaderStageMetrics, DeferredReaderStripeHandle, create_bitrot_reader_with_stage_metrics,
     create_deferred_bitrot_reader_with_stripe_handle, object_mmap_read_enabled,
@@ -916,7 +917,7 @@ pub(in crate::set_disk) async fn submit_read_repair_heal_with_submitter(
     });
 }
 
-pub(in crate::set_disk) type ObjectBitrotReader = BitrotReader<Box<dyn AsyncRead + Send + Sync + Unpin>>;
+pub(in crate::set_disk) type ObjectBitrotReader = BitrotReader<ShardReader>;
 pub(in crate::set_disk) type BitrotReaderTask<'a> =
     Pin<Box<dyn Future<Output = (usize, std::result::Result<Option<ObjectBitrotReader>, DiskError>)> + Send + 'a>>;
 
@@ -3954,7 +3955,7 @@ mod tests {
 
     fn test_object_bitrot_reader() -> ObjectBitrotReader {
         BitrotReader::new(
-            Box::new(Cursor::new(vec![1u8, 2, 3, 4])) as Box<dyn AsyncRead + Send + Sync + Unpin>,
+            ShardReader::Stream(Box::new(Cursor::new(vec![1u8, 2, 3, 4]))),
             4,
             HashAlgorithm::None,
             false,

crates/ecstore/src/set_disk/mod.rs

@@ -192,7 +192,7 @@ type ListObjectsV2Info = StorageListObjectsV2Info<ObjectInfo>;
 type ListObjectVersionsInfo = StorageListObjectVersionsInfo<ObjectInfo>;
 type ObjectInfoOrErr = StorageObjectInfoOrErr<ObjectInfo, Error>;
 type WalkOptions = StorageWalkOptions<fn(&FileInfo) -> bool>;
-type InlineBitrotReader = coding::BitrotReader<Box<dyn tokio::io::AsyncRead + Send + Sync + Unpin>>;
+type InlineBitrotReader = coding::BitrotReader<crate::io_support::bitrot::ShardReader>;
 
 const LOG_COMPONENT_ECSTORE: &str = "ecstore";
 const LOG_SUBSYSTEM_SET_DISK: &str = "set_disk";