compact: include expected size in MissizedDeleteCallback

Fix one of the instances where we report a missized DELSIZED tombstone but did
not include the expected tombstone size. If a DELSIZED tombstone with a
non-zero value meets another tombstone during a compaction, it indicates the
more recent DELSIZED was missized--it deleted a key that did not exist (and was
already deleted). Previously when invoking a callback to report this missizing,
the compaction iterator passed expectedSize=0. This was misleading because the
DELSIZED explicitly carried a non-zero size which allowed to us to determine
there was a missizing in the first place.

This commit adapts the compaction iterator to decode the DELSIZED's value in
this case and propagate it into the missized delete callback.

Author: jbowens

internal/compact/iterator.go

@@ -1201,12 +1201,23 @@ func (i *Iter) deleteSizedNext() *base.InternalKV {
 			//
 			// We treat both cases the same functionally, adopting the identity
 			// of the lower-sequence numbered tombstone. However in the second
-			// case, we also increment the stat counting missized tombstones.
+			// case, we also increment the stat counting missized tombstones and
+			// report the missizing via the callback.
 			if i.kv.V.Len() > 0 {
 				// The original DELSIZED key was missized. The key that the user
 				// thought they were deleting does not exist.
+				//
+				// We decode the original DELSIZED's value so that we can invoke
+				// the callback with the additional context. Note that the
+				// tombstone's value must be in-place.
+				v := i.kv.V.InPlaceValue()
+				expectedSize, n := binary.Uvarint(v)
+				if n != len(v) {
+					i.err = base.CorruptionErrorf("DELSIZED holds invalid value: %x", errors.Safe(v))
+					return nil
+				}
 				i.stats.CountMissizedDels++
-				i.cfg.MissizedDeleteCallback(i.kv.K.UserKey, 0, 0)
+				i.cfg.MissizedDeleteCallback(i.kv.K.UserKey, 0, expectedSize)
 			}
 			// If the tombstone has a value, it must be in-place. To save it, we
 			// can just copy the in-place value directly.

internal/compact/testdata/iter_delete_sized

@@ -1877,3 +1877,24 @@ b#7,DEL:
 c#6,DELSIZED:
 missized-dels=2
 missized-delete-info: a (elided=4, expected=20); b (elided=4, expected=15)
+
+# Test a delete sized that doesn't delete a key, because it meets another
+# tombstone.
+
+define
+a.SET.1:hello
+b.DELSIZED.9:varint(6)
+b.DELSIZED.4:varint(6)
+b.SET.2:world
+----
+
+iter print-missized-dels print-missized-del-info
+first
+next
+next
+----
+a#1,SET:hello
+b#9,DELSIZED:
+.
+missized-dels=1
+missized-delete-info: b (elided=0, expected=6)