sstable: remove ResetForReuse

Perform necessary zeroing as a part of Close.

```
goos: darwin
goarch: arm64
pkg: github.com/cockroachdb/pebble/internal/crdbtest
cpu: Apple M1 Pro
                                 │   old.txt   │              new.txt               │
                                 │   sec/op    │   sec/op     vs base               │
RandSeekInSST/v4/single-level-10   724.9n ± 4%   692.2n ± 2%  -4.52% (p=0.001 n=10)
RandSeekInSST/v4/two-level-10      1.454µ ± 7%   1.453µ ± 7%       ~ (p=0.492 n=10)
RandSeekInSST/v5/single-level-10   508.0n ± 5%   489.0n ± 3%  -3.74% (p=0.011 n=10)
RandSeekInSST/v5/two-level-10      897.5n ± 2%   894.9n ± 7%       ~ (p=0.739 n=10)
geomean                            832.6n        814.4n       -2.18%
```

sstable/block/block.go

@@ -270,7 +270,9 @@ type IndexBlockIterator interface {
 	// false if the index block is exhausted in the reverse direction. A call to
 	// Prev while already exhausted in the reverse direction is a no-op.
 	Prev() bool
-	// Close closes the iterator, releasing any resources it holds.
+	// Close closes the iterator, releasing any resources it holds. After Close,
+	// the iterator must be reset such that it could be reused after a call to
+	// Init or InitHandle.
 	Close() error
 }
 

sstable/colblk/data_block.go

@@ -1051,13 +1051,6 @@ func (i *DataBlockIter) IsDataInvalidated() bool {
 	return i.d == nil
 }
 
-// ResetForReuse resets the iterator for reuse, retaining buffers and
-// configuration supplied to InitOnce, to avoid future allocations.
-func (i *DataBlockIter) ResetForReuse() {
-	i.d = nil
-	i.kv = base.InternalKV{}
-}
-
 // IsLowerBound implements the block.DataBlockIterator interface.
 func (i *DataBlockIter) IsLowerBound(k []byte) bool {
 	if i.transforms.SyntheticPrefix.IsSet() {

sstable/colblk/index_block.go

@@ -246,17 +246,6 @@ func (i *IndexIter) RowIndex() int {
 	return i.row
 }
 
-// ResetForReuse resets the IndexIter for reuse, retaining buffers to avoid
-// future allocations.
-func (i *IndexIter) ResetForReuse() {
-	if invariants.Enabled && i.h != (block.BufferHandle{}) {
-		panic(errors.AssertionFailedf("IndexIter reset for reuse with non-empty handle"))
-	}
-	i.d = nil
-	i.syntheticPrefix = nil
-	i.syntheticSuffix = nil
-}
-
 // Valid returns true if the iterator is currently positioned at a valid block
 // handle.
 func (i *IndexIter) Valid() bool {
@@ -393,6 +382,9 @@ func (i *IndexIter) Prev() bool {
 // Close closes the iterator, releasing any resources it holds.
 func (i *IndexIter) Close() error {
 	i.h.Release()
-	*i = IndexIter{}
+	i.h = block.BufferHandle{}
+	i.d = nil
+	i.syntheticPrefix = nil
+	i.syntheticSuffix = nil
 	return nil
 }

sstable/reader_iter.go

@@ -26,10 +26,6 @@ import (
 type dataBlockIterator[D any] interface {
 	block.DataBlockIterator
 
-	// ResetForReuse resets the data block iterator for reuse, retaining buffers
-	// to avoid future allocations.
-	ResetForReuse()
-
 	*D // non-interface type constraint element
 }
 
@@ -43,10 +39,6 @@ type dataBlockIterator[D any] interface {
 type indexBlockIterator[I any] interface {
 	block.IndexBlockIterator
 
-	// ResetForReuse resets the index iterator for reuse, retaining buffers to
-	// avoid future allocations.
-	ResetForReuse()
-
 	*I // non-interface type constraint element
 }
 

sstable/reader_iter_single_lvl.go

@@ -399,8 +399,6 @@ const _ uintptr = clearLenColBlocks - clearLen
 
 func (i *singleLevelIterator[I, PI, D, PD]) resetForReuse() {
 	*(*[clearLen]byte)(unsafe.Pointer(i)) = [clearLen]byte{}
-	PI(&i.index).ResetForReuse()
-	PD(&i.data).ResetForReuse()
 	i.inPool = true
 }
 

sstable/reader_iter_two_lvl.go

@@ -1047,7 +1047,6 @@ func (i *twoLevelIterator[I, PI, D, PD]) Close() error {
 	err := i.secondLevel.closeInternal()
 	i.secondLevel.resetForReuse()
 	err = firstError(err, PI(&i.topLevelIndex).Close())
-	PI(&i.topLevelIndex).ResetForReuse()
 	i.useFilterBlock = false
 	i.lastBloomFilterMatched = false
 	if pool != nil {

sstable/rowblk/rowblk_fragment_iter.go

@@ -285,14 +285,13 @@ func (i *fragmentIter) Close() {
 		// pool.
 		return
 	}
-
-	i.blockIter.ResetForReuse()
-	*i = fragmentIter{
-		blockIter:   i.blockIter,
-		closeCheck:  i.closeCheck,
-		startKeyBuf: i.startKeyBuf[:0],
-		endKeyBuf:   i.endKeyBuf[:0],
-	}
+	i.span = keyspan.Span{}
+	i.dir = 0
+	i.fileNum = 0
+	i.syntheticSuffix = nil
+	i.syntheticPrefix = nil
+	i.startKeyBuf = i.startKeyBuf[:0]
+	i.endKeyBuf = i.endKeyBuf[:0]
 	fragmentBlockIterPool.Put(i)
 }
 

sstable/rowblk/rowblk_index_iter.go

@@ -32,12 +32,6 @@ func (i *IndexIter) InitHandle(
 	return i.iter.InitHandle(cmp, split, block, transforms)
 }
 
-// ResetForReuse resets the index iterator for reuse, retaining buffers to avoid
-// future allocations.
-func (i *IndexIter) ResetForReuse() {
-	i.iter.ResetForReuse()
-}
-
 // Valid returns true if the iterator is currently positioned at a valid block
 // handle.
 func (i *IndexIter) Valid() bool {

sstable/rowblk/rowblk_iter.go

@@ -301,21 +301,6 @@ func (i *Iter) IsDataInvalidated() bool {
 	return i.data == nil
 }
 
-// ResetForReuse resets the blockIter for reuse, retaining buffers to avoid
-// future allocations.
-func (i *Iter) ResetForReuse() {
-	fullKey := i.fullKey[:0]
-	cached := i.cached[:0]
-	cachedBuf := i.cachedBuf[:0]
-	firstUserKeyWithPrefixBuf := i.firstUserKeyWithPrefixBuf[:0]
-	*i = Iter{
-		fullKey:                   fullKey,
-		cached:                    cached,
-		cachedBuf:                 cachedBuf,
-		firstUserKeyWithPrefixBuf: firstUserKeyWithPrefixBuf,
-	}
-}
-
 func (i *Iter) readEntry() {
 	ptr := unsafe.Pointer(uintptr(i.ptr) + uintptr(i.offset))
 
@@ -1568,10 +1553,16 @@ func (i *Iter) Error() error {
 // package.
 func (i *Iter) Close() error {
 	i.handle.Release()
-	i.handle = block.BufferHandle{}
-	i.val = nil
-	i.ikv = base.InternalKV{}
-	i.lazyValueHandling.getValue = nil
+	fullKey := i.fullKey[:0]
+	cached := i.cached[:0]
+	cachedBuf := i.cachedBuf[:0]
+	firstUserKeyWithPrefixBuf := i.firstUserKeyWithPrefixBuf[:0]
+	*i = Iter{
+		fullKey:                   fullKey,
+		cached:                    cached,
+		cachedBuf:                 cachedBuf,
+		firstUserKeyWithPrefixBuf: firstUserKeyWithPrefixBuf,
+	}
 	return nil
 }
 

sstable/rowblk/rowblk_rewrite.go

@@ -89,6 +89,6 @@ func (r *Rewriter) RewriteSuffixes(
 	end.Trailer = r.scratchKey.Trailer
 	r.keyAlloc, end.UserKey = r.keyAlloc.Copy(r.scratchKey.UserKey)
 
-	r.iter.ResetForReuse()
+	_ = r.iter.Close() // infallible
 	return start, end, r.writer.Finish(), nil
 }