Record the amount of free pages instead of hashmapFreeCount calculation

Signed-off-by: xing0821 <54933318+xing0821@users.noreply.github.com>

db.go

@@ -15,6 +15,9 @@ import (
 	"go.etcd.io/bbolt/internal/common"
 )
 
+// When enabled, the database will perform assert function to check the slow-path code
+var assertVerify = os.Getenv("BBOLT_VERIFY") == "true"
+
 // The time elapsed between consecutive file locking attempts.
 const flockRetryTimeout = 50 * time.Millisecond
 
@@ -1309,3 +1312,9 @@ type Info struct {
 	Data     uintptr
 	PageSize int
 }
+
+func _assertVerify(conditionFunc func() bool, msg string, v ...interface{}) {
+	if assertVerify && !conditionFunc() {
+		panic(fmt.Sprintf("assertion failed: "+msg, v...))
+	}
+}

freelist.go

@@ -30,6 +30,7 @@ type freelist struct {
 	freemaps       map[uint64]pidSet                         // key is the size of continuous pages(span), value is a set which contains the starting pgids of same size
 	forwardMap     map[common.Pgid]uint64                    // key is start pgid, value is its span size
 	backwardMap    map[common.Pgid]uint64                    // key is end pgid, value is its span size
+	freePagesCount uint64                                    // count of free pages(hashmap version)
 	allocate       func(txid common.Txid, n int) common.Pgid // the freelist allocate func
 	free_count     func() int                                // the function which gives you free page number
 	mergeSpans     func(ids common.Pgids)                    // the mergeSpan func

freelist_hmap.go

@@ -8,7 +8,11 @@ import (
 
 // hashmapFreeCount returns count of free pages(hashmap version)
 func (f *freelist) hashmapFreeCount() int {
-	// use the forwardMap to get the total count
+	_assertVerify(func() bool { return int(f.freePagesCount) == f.hashmapFreeCountSlow() }, "freePagesCount is out of sync with free pages map")
+	return int(f.freePagesCount)
+}
+
+func (f *freelist) hashmapFreeCountSlow() int {
 	count := 0
 	for _, size := range f.forwardMap {
 		count += int(size)
@@ -142,6 +146,7 @@ func (f *freelist) addSpan(start common.Pgid, size uint64) {
 	}
 
 	f.freemaps[size][start] = struct{}{}
+	f.freePagesCount += size
 }
 
 func (f *freelist) delSpan(start common.Pgid, size uint64) {
@@ -151,6 +156,7 @@ func (f *freelist) delSpan(start common.Pgid, size uint64) {
 	if len(f.freemaps[size]) == 0 {
 		delete(f.freemaps, size)
 	}
+	f.freePagesCount -= size
 }
 
 // initial from pgids using when use hashmap version
@@ -162,6 +168,8 @@ func (f *freelist) init(pgids []common.Pgid) {
 
 	size := uint64(1)
 	start := pgids[0]
+	// reset the counter when freelist init
+	f.freePagesCount = 0
 
 	if !sort.SliceIsSorted([]common.Pgid(pgids), func(i, j int) bool { return pgids[i] < pgids[j] }) {
 		panic("pgids not sorted")