tx_check: pass read tx into freelist load to avoid nested beginTx

Check already runs on a read transaction (often from View on another
goroutine). Rebuilding an unsynced freelist called beginTx from freepages,
which could block when combined with the outer View transaction.

Pass the active Tx into loadFreelist so reconstruction uses freepagesWithTx
and skips opening a nested read transaction (fixes #877).

Signed-off-by: Vedant Madane <vedantmadane@users.noreply.github.com>

Author: VedantMadane

db.go

@@ -301,7 +301,7 @@ func Open(path string, mode os.FileMode, options *Options) (db *DB, err error) {
 	}
 
 	if db.PreLoadFreelist {
-		db.loadFreelist()
+		db.loadFreelist(nil)
 	}
 
 	if db.readOnly {
@@ -419,12 +419,23 @@ func (db *DB) getPageSizeFromSecondMeta() (int, bool, error) {
 // loadFreelist reads the freelist if it is synced, or reconstructs it
 // by scanning the DB if it is not synced. It assumes there are no
 // concurrent accesses being made to the freelist.
-func (db *DB) loadFreelist() {
+//
+// When sharedReadTx is non-nil, an unsynced freelist is reconstructed by
+// scanning using that transaction instead of opening a nested read-only
+// transaction. Tx.check passes the active read transaction so freelist
+// reconstruction does not call beginTx while another goroutine may still
+// hold the outer read transaction from View, which previously could block
+// indefinitely (see https://github.com/etcd-io/bbolt/issues/877).
+func (db *DB) loadFreelist(sharedReadTx *Tx) {
 	db.freelistLoad.Do(func() {
 		db.freelist = newFreelist(db.FreelistType)
 		if !db.hasSyncedFreelist() {
 			// Reconstruct free list by scanning the DB.
-			db.freelist.Init(db.freepages())
+			if sharedReadTx != nil {
+				db.freelist.Init(db.freepagesWithTx(sharedReadTx))
+			} else {
+				db.freelist.Init(db.freepages())
+			}
 		} else {
 			// Read free list from freelist page.
 			db.freelist.Read(db.page(db.meta().Freelist()))
@@ -1250,6 +1261,13 @@ func (db *DB) freepages() []common.Pgid {
 		panic("freepages: failed to open read only tx")
 	}
 
+	return db.freepagesWithTx(tx)
+}
+
+// freepagesWithTx lists page IDs that are not reachable from the bucket tree
+// for the given read-only transaction. The transaction must not be used
+// concurrently while this function runs.
+func (db *DB) freepagesWithTx(tx *Tx) []common.Pgid {
 	reachable := make(map[common.Pgid]*common.Page)
 	nofreed := make(map[common.Pgid]bool)
 	ech := make(chan error)
@@ -1267,7 +1285,7 @@ func (db *DB) freepages() []common.Pgid {
 	// TODO: If check bucket reported any corruptions (ech) we shouldn't proceed to freeing the pages.
 
 	var fids []common.Pgid
-	for i := common.Pgid(2); i < db.meta().Pgid(); i++ {
+	for i := common.Pgid(2); i < tx.meta.Pgid(); i++ {
 		if _, ok := reachable[i]; !ok {
 			fids = append(fids, i)
 		}

tx_check.go

@@ -41,8 +41,9 @@ func (tx *Tx) check(cfg checkConfig, ch chan error) {
 			ch <- panicked{r}
 		}
 	}()
-	// Force loading free list if opened in ReadOnly mode.
-	tx.db.loadFreelist()
+	// Force loading free list if opened in ReadOnly mode. Pass this tx so
+	// freelist reconstruction does not open a nested read transaction.
+	tx.db.loadFreelist(tx)
 
 	// Check if any pages are double freed.
 	freed := make(map[common.Pgid]bool)