storage: add MVCC point synthesizing iterator

This patch adds `pointSynthesizingIter`, an MVCC iterator which wraps an
arbitrary MVCC iterator and synthesizes point keys for range keys at
their start key and where they overlap point keys. It can optionally
synthesize around the SeekGE seek key too, which is useful for point
operations like `MVCCGet` where we may want to return a synthetic
tombstone for an MVCC range tombstone if there is no existing point key.

This will primarily be used to handle MVCC range tombstones in MVCC
scans and gets, as well as during MVCC conflict checks, which allows
much of this logic to remain unchanged and simplified (in particular,
`pebbleMVCCScanner` will not need any changes).

However, this patch does not make use of the iterator yet, since both it
and Pebble will need further performance optimizations for use in hot
paths. For now, correctness is sufficient, and only basic attempts at
performance optimization have been made.

Release note: None

pkg/storage/BUILD.bazel

@@ -29,6 +29,7 @@ go_library(
         "pebble_iterator.go",
         "pebble_merge.go",
         "pebble_mvcc_scanner.go",
+        "point_synthesizing_iter.go",
         "read_as_of_iterator.go",
         "replicas_storage.go",
         "resource_limiter.go",

pkg/storage/mvcc_history_test.go

@@ -69,7 +69,7 @@ import (
 // get            [t=<name>] [ts=<int>[,<int>]]                         [resolve [status=<txnstatus>]] k=<key> [inconsistent] [tombstones] [failOnMoreRecent] [localUncertaintyLimit=<int>[,<int>]] [globalUncertaintyLimit=<int>[,<int>]]
 // scan           [t=<name>] [ts=<int>[,<int>]]                         [resolve [status=<txnstatus>]] k=<key> [end=<key>] [inconsistent] [tombstones] [reverse] [failOnMoreRecent] [localUncertaintyLimit=<int>[,<int>]] [globalUncertaintyLimit=<int>[,<int>]] [max=<max>] [targetbytes=<target>] [avoidExcess] [allowEmpty]
 //
-// iter_new       [k=<key>] [end=<key>] [prefix] [kind=key|keyAndIntents] [types=pointsOnly|pointsWithRanges|pointsAndRanges|rangesOnly] [maskBelow=<int>[,<int>]]
+// iter_new       [k=<key>] [end=<key>] [prefix] [kind=key|keyAndIntents] [types=pointsOnly|pointsWithRanges|pointsAndRanges|rangesOnly] [pointSynthesis [emitOnSeekGE]] [maskBelow=<int>[,<int>]]
 // iter_seek_ge   k=<key> [ts=<int>[,<int>]]
 // iter_seek_lt   k=<key> [ts=<int>[,<int>]]
 // iter_seek_intent_ge k=<key> txn=<name>
@@ -1027,6 +1027,9 @@ func cmdIterNew(e *evalCtx) error {
 		MVCCIterator: r.NewMVCCIterator(kind, opts),
 		closeReader:  closeReader,
 	}
+	if e.hasArg("pointSynthesis") {
+		e.iter = newPointSynthesizingIter(e.iter, e.hasArg("emitOnSeekGE"))
+	}
 	return nil
 }
 

pkg/storage/mvcc_key.go

@@ -68,6 +68,12 @@ func (k MVCCKey) Next() MVCCKey {
 	}
 }
 
+// Clone returns a copy of the key.
+func (k MVCCKey) Clone() MVCCKey {
+	k.Key = k.Key.Clone()
+	return k
+}
+
 // Compare returns -1 if this key is less than the given key, 0 if they're
 // equal, or 1 if this is greater. Comparison is by key,timestamp, where larger
 // timestamps sort before smaller ones except empty ones which sort first (like