compaction: avoid arbitrary grandparent overlap

If the compaction output splitters suggest a user key less than the next user
key, use the the splitters' suggestion as the split key. This avoids arbitrary
overlap with grandparents if the next key is far beyond the grandparent limit.

compaction.go

@@ -2330,28 +2330,18 @@ func (d *DB) runCompaction(
 
 		// A splitter requested a split, and we're ready to finish the output.
 		// We need to choose the key at which to split any pending range
-		// tombstones.
-		var splitKey []byte
-		switch {
-		case key != nil:
-			// We hit the size, grandparent, or L0 limit for the sstable.
-			// The next key either has a greater user key than the previous
-			// key, or if not, the previous key must not have had a zero
-			// sequence number.
-
-			// TODO(jackson): If we hit the grandparent limit, the next
-			// grandparent's smallest key may be less than the current key.
-			// Splitting at the current key will cause this output to overlap
-			// a potentially unbounded number of grandparents.
+		// tombstones. There are two options:
+		// 1. splitterSuggestion — The key suggested by the splitter. This key
+		//    is guaranteed to be greater than the last key written to the
+		//    current output.
+		// 2. key.UserKey — the first key of the next sstable output. This user
+		//     key is also guaranteed to be greater than the last user key
+		//     written to the current output (see userKeyChangeSplitter).
+		// Use whichever is smaller.
+		splitKey := splitterSuggestion
+		if key != nil && (splitKey == nil || c.cmp(splitKey, key.UserKey) > 0) {
 			splitKey = key.UserKey
-		case key == nil:
-			// NB: Because of the userKeyChangeSplitter, `splitterSuggestion`
-			// must be > any key previously added to the current output sstable.
-			splitKey = splitterSuggestion
-		default:
-			return nil, nil, errors.New("pebble: not reached")
 		}
-
 		if err := finishOutput(splitKey); err != nil {
 			return nil, pendingOutputs, err
 		}

testdata/manual_compaction

@@ -180,8 +180,8 @@ L3
 compact a-e L1
 ----
 2:
-  000009:[a#3,SET-g#72057594037927935,RANGEDEL]
-  000010:[h#3,SET-h#3,SET]
+  000009:[a#3,SET-c#72057594037927935,RANGEDEL]
+  000010:[c#2,RANGEDEL-h#3,SET]
 3:
   000006:[a#0,SET-b#0,SET]
   000007:[c#0,SET-d#0,SET]
@@ -284,8 +284,8 @@ L3
 compact a-c L1
 ----
 2:
-  000009:[a#3,RANGEDEL-c#72057594037927935,RANGEDEL]
-  000010:[c#2,SET-c#2,SET]
+  000009:[a#3,RANGEDEL-b#72057594037927935,RANGEDEL]
+  000010:[b#3,RANGEDEL-c#2,SET]
 3:
   000006:[b#2,SET-b#2,SET]
   000007:[b#1,SET-b#1,SET]

testdata/manual_compaction_set_with_del

@@ -180,8 +180,8 @@ L3
 compact a-e L1
 ----
 2:
-  000009:[a#3,SETWITHDEL-g#72057594037927935,RANGEDEL]
-  000010:[h#3,SET-h#3,SET]
+  000009:[a#3,SETWITHDEL-c#72057594037927935,RANGEDEL]
+  000010:[c#2,RANGEDEL-h#3,SET]
 3:
   000006:[a#0,SET-b#0,SET]
   000007:[c#0,SET-d#0,SET]
@@ -284,8 +284,8 @@ L3
 compact a-c L1
 ----
 2:
-  000009:[a#3,RANGEDEL-c#72057594037927935,RANGEDEL]
-  000010:[c#2,SET-c#2,SET]
+  000009:[a#3,RANGEDEL-b#72057594037927935,RANGEDEL]
+  000010:[b#3,RANGEDEL-c#2,SET]
 3:
   000006:[b#2,SET-b#2,SET]
   000007:[b#1,SET-b#1,SET]