add freelistType as an option

Makefile

@@ -6,6 +6,8 @@ default: build
 
 race:
 	@go test -v -race -test.run="TestSimulate_(100op|1000op)"
+	@echo "array freelist test"
+	@ARRAY_FREELIST=y go test -v -race -test.run="TestSimulate_(100op|1000op)"
 
 fmt:
 	!(gofmt -l -s -d $(shell find . -name \*.go) | grep '[a-z]')
@@ -27,4 +29,10 @@ test:
 	# Note: gets "program not an importable package" in out of path builds
 	go test -v ./cmd/bbolt
 
+	@echo "array freelist test"
+
+	@ARRAY_FREELIST=y go test -timeout 20m -v -coverprofile cover.out -covermode atomic
+	# Note: gets "program not an importable package" in out of path builds
+	@ARRAY_FREELIST=y go test -v ./cmd/bbolt
+
 .PHONY: race fmt errcheck test gosimple unused

allocate_test.go

@@ -6,8 +6,12 @@ import (
 
 func TestTx_allocatePageStats(t *testing.T) {
 	f := newFreelist()
-	ids := []pgid{2, 3}
-	f.init(ids)
+	if !f.IsFreelistTypeArray {
+		ids := []pgid{2, 3}
+		f.init(ids)
+	} else {
+		f.ids = []pgid{2, 3}
+	}
 
 	tx := &Tx{
 		db: &DB{

db.go

@@ -70,6 +70,13 @@ type DB struct {
 	// re-sync during recovery.
 	NoFreelistSync bool
 
+	// IsFreelistTypeArray sets the backend freelist type to array which is simple but endures
+	// dramatic performance degradation if database is large and framentation in freelist is common.
+	// The alternative one is using segregated freelist, it is faster in almost all circumstances
+	// but it doesn't guarantee that it offers the smallest page id available. In normal case it is safe.
+	// if you depend on this(it offers the smallest page id available), you may set this to true.
+	IsFreelistTypeArray bool
+
 	// When true, skips the truncate call when growing the database.
 	// Setting this to true is only safe on non-ext3/ext4 systems.
 	// Skipping truncation avoids preallocation of hard drive space and
@@ -169,6 +176,7 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 	db.NoGrowSync = options.NoGrowSync
 	db.MmapFlags = options.MmapFlags
 	db.NoFreelistSync = options.NoFreelistSync
+	db.IsFreelistTypeArray = options.IsFreelistTypeArray
 
 	// Set default values for later DB operations.
 	db.MaxBatchSize = DefaultMaxBatchSize
@@ -283,6 +291,11 @@ func Open(path string, mode os.FileMode, options *Options) (*DB, error) {
 // concurrent accesses being made to the freelist.
 func (db *DB) loadFreelist() {
 	db.freelistLoad.Do(func() {
+		if db.IsFreelistTypeArray {
+			isFreelistTypeArray = true
+		} else {
+			isFreelistTypeArray = false
+		}
 		db.freelist = newFreelist()
 		if !db.hasSyncedFreelist() {
 			// Reconstruct free list by scanning the DB.
@@ -1005,6 +1018,13 @@ type Options struct {
 	// under normal operation, but requires a full database re-sync during recovery.
 	NoFreelistSync bool
 
+	// IsFreelistTypeArray sets the backend freelist type to array which is simple but endures
+	// dramatic performance degradation if database is large and framentation in freelist is common.
+	// The alternative one is using segregated freelist, it is faster in almost all circumstances
+	// but it doesn't guarantee that it offers the smallest page id available. In normal case it is safe.
+	// if you depend on this(it offers the smallest page id available), you may set this to true.
+	IsFreelistTypeArray bool
+
 	// Open database in read-only mode. Uses flock(..., LOCK_SH |LOCK_NB) to
 	// grab a shared lock (UNIX).
 	ReadOnly bool