kvserver/gc,rowcontainer: fix misuse of ByteAllocator API

`bufalloc.ByteAllocator.Copy` has an option to allocate extra capacity,
meaning that it copies the passed-in byte slice into a new memory region
while also adding extra capacity to be utilized by caller. Crucially,
that extra capacity lives in the returned byte slice containing the
copy. We use this feature in two places throughout the codebase, and in
both places it was used incorrectly, resulting in a wasteful allocation.

Consider the following setup:

  k = [1, 2], v = [3, 4, 5, 6]

With invocation like `alloc, k = alloc.Copy(k, len(v))`, we would get

  len(k) = 2, cap(k) = 6

However, in both spots we copied `v` via `alloc, v = alloc.Copy(v, 0)`,
which would actually create another allocation.

This commit fixes this oversight (both were introduced in 2022 by different
authors), and the following commit will clean up the API to avoid possible
confusion.

Release note: None

Author: yuzefovich

pkg/kv/kvserver/gc/gc_iterator.go

@@ -239,7 +239,12 @@ func (b *gcIteratorRingBuf) pushBack(
 	b.allocs[i] = b.allocs[i].Truncate()
 	b.allocs[i], k.Key = b.allocs[i].Copy(k.Key, len(metaValue))
 	if len(metaValue) > 0 {
-		b.allocs[i], metaValue = b.allocs[i].Copy(metaValue, 0)
+		// k.Key now contains the capacity for copy of metaValue - separate it
+		// out.
+		buf := k.Key[len(k.Key):cap(k.Key)]
+		k.Key = k.Key[:len(k.Key):len(k.Key)]
+		copy(buf, metaValue)
+		metaValue = buf
 	}
 	b.buf[i] = mvccKeyValue{
 		key:                  k,

pkg/sql/rowcontainer/disk_row_container.go

@@ -524,7 +524,11 @@ func (r *diskRowIterator) EncRow() (rowenc.EncDatumRow, error) {
 	// reuse the same byte slice across EncRow() calls because it would lead to
 	// modification of the EncDatums (which is not allowed).
 	r.rowBuf, k = r.rowBuf.Copy(k, len(v))
-	r.rowBuf, v = r.rowBuf.Copy(v, 0 /* extraCap */)
+	// k now contains the capacity for copy of v - separate it out.
+	buf := k[len(k):cap(k)]
+	k = k[:len(k):len(k)]
+	copy(buf, v)
+	v = buf
 
 	for i, orderInfo := range r.rowContainer.ordering {
 		// Types with composite key encodings are decoded from the value.