db: fix estimatedCompactionDebt metric

We should set bytesAddedToNextLevel to 0, if there will be no bytes
added to the next level.

This pr also takes the opportunity to make the loop which computes
compaction debt easier to reason about.

compaction_picker.go

@@ -805,19 +805,21 @@ func (p *compactionPickerByScore) estimatedCompactionDebt(l0ExtraSize uint64) ui
 	// We assume that all the bytes in L0 need to be compacted to Lbase. This is
 	// unlike the RocksDB logic that figures out whether L0 needs compaction.
 	bytesAddedToNextLevel := l0ExtraSize + uint64(p.levelSizes[0])
-	nextLevelSize := uint64(p.levelSizes[p.baseLevel])
+	lbaseSize := uint64(p.levelSizes[p.baseLevel])
 
 	var compactionDebt uint64
-	if bytesAddedToNextLevel > 0 && nextLevelSize > 0 {
+	if bytesAddedToNextLevel > 0 && lbaseSize > 0 {
 		// We only incur compaction debt if both L0 and Lbase contain data. If L0
 		// is empty, no compaction is necessary. If Lbase is empty, a move-based
 		// compaction from L0 would occur.
-		compactionDebt += bytesAddedToNextLevel + nextLevelSize
+		compactionDebt += bytesAddedToNextLevel + lbaseSize
 	}
 
+	// loop invariant: At the beginning of the loop, bytesAddedToNextLevel is the
+	// bytes added to `level` in the loop.
 	for level := p.baseLevel; level < numLevels-1; level++ {
-		levelSize := nextLevelSize + bytesAddedToNextLevel
-		nextLevelSize = uint64(p.levelSizes[level+1])
+		levelSize := uint64(p.levelSizes[level]) + bytesAddedToNextLevel
+		nextLevelSize := uint64(p.levelSizes[level+1])
 		if levelSize > uint64(p.levelMaxBytes[level]) {
 			bytesAddedToNextLevel = levelSize - uint64(p.levelMaxBytes[level])
 			if nextLevelSize > 0 {
@@ -828,9 +830,11 @@ func (p *compactionPickerByScore) estimatedCompactionDebt(l0ExtraSize uint64) ui
 				// The next level contributes levelRatio * bytesAddedToNextLevel.
 				compactionDebt += uint64(float64(bytesAddedToNextLevel) * (levelRatio + 1))
 			}
+		} else {
+			// We're not moving any bytes to the next level.
+			bytesAddedToNextLevel = 0
 		}
 	}
-
 	return compactionDebt
 }