sstable: reconcile out-of-order key error messages

When keys are written to a sstable writer out-of-order, the writer returns an
error message. The error messages returned from the row-based and column-based
writers varied, in part because the columnar writer did not readily have the
previous key at hand. This commit adapts the column writer to synthesize the
previous user key for the purpose of embedding it within the returned error
message.

sstable/colblk_writer.go

@@ -536,8 +536,13 @@ func (w *RawColumnWriter) evaluatePoint(
 
 	if !w.disableKeyOrderChecks && (eval.kcmp.UserKeyComparison < 0 ||
 		(eval.kcmp.UserKeyComparison == 0 && w.prevPointKey.trailer <= key.Trailer)) {
+		previousKey := base.InternalKey{
+			UserKey: w.dataBlock.MaterializeLastUserKey(nil),
+			Trailer: w.prevPointKey.trailer,
+		}
 		return eval, errors.Errorf(
-			"pebble: keys must be added in strictly increasing order: %s",
+			"pebble: keys must be added in strictly increasing order: %s, %s",
+			previousKey.Pretty(w.comparer.FormatKey),
 			key.Pretty(w.comparer.FormatKey))
 	}
 

sstable/testdata/writer_v5

@@ -142,13 +142,13 @@ build
 a.SET.1:b
 a.SET.2:c
 ----
-pebble: keys must be added in strictly increasing order: a#2,SET
+pebble: keys must be added in strictly increasing order: a#1,SET, a#2,SET
 
 build
 b.SET.1:a
 a.SET.2:b
 ----
-pebble: keys must be added in strictly increasing order: a#2,SET
+pebble: keys must be added in strictly increasing order: b#1,SET, a#2,SET
 
 build
 b.RANGEDEL.1:c