Make it clearer why checksum checks not needed

db/db.go

@@ -40,6 +40,8 @@ const (
 	numETx               = "execute_transactions"
 	numQTx               = "query_transactions"
 	numRTx               = "request_transactions"
+
+	CheckpointQuery = "PRAGMA wal_checkpoint(TRUNCATE)" // rqlite WAL compaction requires truncation
 )
 
 var (
@@ -485,7 +487,7 @@ func (db *DB) CheckpointWithTimeout(dur time.Duration) (err error) {
 	var nMoved int
 
 	f := func() error {
-		err := db.rwDB.QueryRow("PRAGMA wal_checkpoint(TRUNCATE)").Scan(&ok, &nPages, &nMoved)
+		err := db.rwDB.QueryRow(CheckpointQuery).Scan(&ok, &nPages, &nMoved)
 		stats.Add(numCheckpointedPages, int64(nPages))
 		stats.Add(numCheckpointedMoves, int64(nMoved))
 		if err != nil {

db/wal/compacting_scanner.go

@@ -49,7 +49,8 @@ func NewFastCompactingScanner(r io.ReadSeeker) (*CompactingScanner, error) {
 // If fullScan is true, the scanner will perform a full scan of the WAL file, performing
 // a checksum on each frame. If fullScan is false, the scanner will only scan the file
 // sufficiently to find the last valid frame for each page. This is faster when the
-// caller knows that the WAL file is valid and does not need to be checked.
+// caller knows that the entire WAL file is valid, and will not contain pages from a
+// previous checkpointing operation.
 func NewCompactingScanner(r io.ReadSeeker, fullScan bool) (*CompactingScanner, error) {
 	walReader := NewReader(r)
 	err := walReader.ReadHeader()