obs(get): add GET read pipeline metrics (#3868)

crates/ecstore/src/erasure_codec/workspace.rs

@@ -0,0 +1,88 @@
+// Copyright 2024 RustFS Team
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#[derive(Debug, Default)]
+pub(crate) struct ShardBufferPool {
+    buffers: Vec<Option<Vec<u8>>>,
+}
+
+impl ShardBufferPool {
+    #[inline]
+    pub(crate) fn new(slot_count: usize) -> Self {
+        let mut buffers = Vec::with_capacity(slot_count);
+        buffers.resize_with(slot_count, || None);
+        Self { buffers }
+    }
+
+    #[inline]
+    pub(crate) fn ensure_slots(&mut self, slot_count: usize) {
+        if self.buffers.len() < slot_count {
+            self.buffers.resize_with(slot_count, || None);
+        }
+    }
+
+    // Returned bytes may contain stale scratch data and must be overwritten before use.
+    #[inline]
+    pub(crate) fn take(&mut self, index: usize, len: usize) -> Vec<u8> {
+        self.ensure_slots(index + 1);
+        let mut buf = self.buffers[index].take().unwrap_or_else(|| Vec::with_capacity(len));
+        if buf.capacity() < len {
+            buf.reserve_exact(len - buf.capacity());
+        }
+        buf.resize(len, 0);
+        buf
+    }
+
+    #[inline]
+    pub(crate) fn put(&mut self, index: usize, buf: Vec<u8>) {
+        self.ensure_slots(index + 1);
+        self.buffers[index] = Some(buf);
+    }
+
+    #[cfg(test)]
+    fn stored_capacity(&self, index: usize) -> Option<usize> {
+        self.buffers.get(index).and_then(|buf| buf.as_ref().map(Vec::capacity))
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn shard_buffer_pool_reuses_slot_without_clearing() {
+        let mut pool = ShardBufferPool::new(2);
+        let mut buf = pool.take(1, 16);
+        assert_eq!(buf.len(), 16);
+        buf[0] = 42;
+        let capacity = buf.capacity();
+
+        pool.put(1, buf);
+        assert_eq!(pool.stored_capacity(1), Some(capacity));
+
+        let reused = pool.take(1, 8);
+        assert_eq!(reused.len(), 8);
+        assert!(reused.capacity() >= capacity);
+        assert_eq!(reused[0], 42);
+    }
+
+    #[test]
+    fn shard_buffer_pool_grows_for_sparse_slot_indexes() {
+        let mut pool = ShardBufferPool::new(0);
+        let buf = pool.take(3, 4);
+
+        assert_eq!(buf.len(), 4);
+        assert_eq!(pool.buffers.len(), 4);
+    }
+}