move method freePages into freelist.go

The motivation is to get all freelist related logic included
in freelist.go. We are going to introduce freelist interface
in the next step.

Signed-off-by: Benjamin Wang <benjamin.ahrtr@gmail.com>

Author: ahrtr

db.go

@@ -6,7 +6,6 @@ import (
 	"io"
 	"os"
 	"runtime"
-	"sort"
 	"sync"
 	"time"
 	"unsafe"
@@ -797,6 +796,9 @@ func (db *DB) beginTx() (*Tx, error) {
 	// Keep track of transaction until it closes.
 	db.txs = append(db.txs, t)
 	n := len(db.txs)
+	if db.freelist != nil {
+		db.freelist.addReadonlyTXID(t.meta.Txid())
+	}
 
 	// Unlock the meta pages.
 	db.metalock.Unlock()
@@ -841,36 +843,10 @@ func (db *DB) beginRWTx() (*Tx, error) {
 	t := &Tx{writable: true}
 	t.init(db)
 	db.rwtx = t
-	db.freePages()
+	db.freelist.freePages()
 	return t, nil
 }
 
-// freePages releases any pages associated with closed read-only transactions.
-func (db *DB) freePages() {
-	// Free all pending pages prior to earliest open transaction.
-	sort.Sort(txsById(db.txs))
-	minid := common.Txid(0xFFFFFFFFFFFFFFFF)
-	if len(db.txs) > 0 {
-		minid = db.txs[0].meta.Txid()
-	}
-	if minid > 0 {
-		db.freelist.release(minid - 1)
-	}
-	// Release unused txid extents.
-	for _, t := range db.txs {
-		db.freelist.releaseRange(minid, t.meta.Txid()-1)
-		minid = t.meta.Txid() + 1
-	}
-	db.freelist.releaseRange(minid, common.Txid(0xFFFFFFFFFFFFFFFF))
-	// Any page both allocated and freed in an extent is safe to release.
-}
-
-type txsById []*Tx
-
-func (t txsById) Len() int           { return len(t) }
-func (t txsById) Swap(i, j int)      { t[i], t[j] = t[j], t[i] }
-func (t txsById) Less(i, j int) bool { return t[i].meta.Txid() < t[j].meta.Txid() }
-
 // removeTx removes a transaction from the database.
 func (db *DB) removeTx(tx *Tx) {
 	// Release the read lock on the mmap.
@@ -890,6 +866,9 @@ func (db *DB) removeTx(tx *Tx) {
 		}
 	}
 	n := len(db.txs)
+	if db.freelist != nil {
+		db.freelist.removeReadonlyTXID(tx.meta.Txid())
+	}
 
 	// Unlock the meta pages.
 	db.metalock.Unlock()

freelist.go

@@ -24,6 +24,7 @@ type pidSet map[common.Pgid]struct{}
 type freelist struct {
 	freelistType   FreelistType                              // freelist type
 	ids            []common.Pgid                             // all free and available free page ids.
+	readonlyTXIDs  []common.Txid                             // all readonly transaction IDs.
 	allocs         map[common.Pgid]common.Txid               // mapping of Txid that allocated a pgid.
 	pending        map[common.Txid]*txPending                // mapping of soon-to-be free page ids by tx.
 	cache          map[common.Pgid]struct{}                  // fast lookup of all free and pending page ids.
@@ -326,3 +327,44 @@ func (f *freelist) reindex() {
 		}
 	}
 }
+
+func (f *freelist) addReadonlyTXID(tid common.Txid) {
+	f.readonlyTXIDs = append(f.readonlyTXIDs, tid)
+}
+
+func (f *freelist) removeReadonlyTXID(tid common.Txid) {
+	for i := range f.readonlyTXIDs {
+		if f.readonlyTXIDs[i] == tid {
+			last := len(f.readonlyTXIDs) - 1
+			f.readonlyTXIDs[i] = f.readonlyTXIDs[last]
+			f.readonlyTXIDs = f.readonlyTXIDs[:last]
+			break
+		}
+	}
+}
+
+type txIDx []common.Txid
+
+func (t txIDx) Len() int           { return len(t) }
+func (t txIDx) Swap(i, j int)      { t[i], t[j] = t[j], t[i] }
+func (t txIDx) Less(i, j int) bool { return t[i] < t[j] }
+
+// freePages releases any pages associated with closed read-only transactions.
+func (f *freelist) freePages() {
+	// Free all pending pages prior to earliest open transaction.
+	sort.Sort(txIDx(f.readonlyTXIDs))
+	minid := common.Txid(0xFFFFFFFFFFFFFFFF)
+	if len(f.readonlyTXIDs) > 0 {
+		minid = f.readonlyTXIDs[0]
+	}
+	if minid > 0 {
+		f.release(minid - 1)
+	}
+	// Release unused txid extents.
+	for _, tid := range f.readonlyTXIDs {
+		f.releaseRange(minid, tid-1)
+		minid = tid + 1
+	}
+	f.releaseRange(minid, common.Txid(0xFFFFFFFFFFFFFFFF))
+	// Any page both allocated and freed in an extent is safe to release.
+}