database/sql: use slices rather than container/list

Significantly reduces the number of allocations, while also
simplifying the code and increasing performance by a 1-2%.

benchmark                          old ns/op     new ns/op     delta
BenchmarkConcurrentDBExec          13290567      13026236      -1.99%
BenchmarkConcurrentStmtQuery       13249399      13008879      -1.82%
BenchmarkConcurrentStmtExec        8806237       8680182       -1.43%
BenchmarkConcurrentTxQuery         13628379      12756293      -6.40%
BenchmarkConcurrentTxExec          4794800       4722440       -1.51%
BenchmarkConcurrentTxStmtQuery     5040804       5200721       +3.17%
BenchmarkConcurrentTxStmtExec      1366574       1336626       -2.19%
BenchmarkConcurrentRandom          11119120      10926113      -1.74%

benchmark                          old allocs     new allocs     delta
BenchmarkConcurrentDBExec          14191          13684          -3.57%
BenchmarkConcurrentStmtQuery       16020          15514          -3.16%
BenchmarkConcurrentStmtExec        4179           3672           -12.13%
BenchmarkConcurrentTxQuery         16025          15518          -3.16%
BenchmarkConcurrentTxExec          12717          12709          -0.06%
BenchmarkConcurrentTxStmtQuery     15532          15525          -0.05%
BenchmarkConcurrentTxStmtExec      2175           2168           -0.32%
BenchmarkConcurrentRandom          12320          11997          -2.62%

benchmark                          old bytes     new bytes     delta
BenchmarkConcurrentDBExec          2164827       2139760       -1.16%
BenchmarkConcurrentStmtQuery       2418070       2394030       -0.99%
BenchmarkConcurrentStmtExec        1728782       1704371       -1.41%
BenchmarkConcurrentTxQuery         2477144       2452620       -0.99%
BenchmarkConcurrentTxExec          588920        588343        -0.10%
BenchmarkConcurrentTxStmtQuery     790866        796578        +0.72%
BenchmarkConcurrentTxStmtExec      98502         98143         -0.36%
BenchmarkConcurrentRandom          1725906       1710220       -0.91%

LGTM=ruiu, dave, bradfitz
R=golang-codereviews, ruiu, gobot, bradfitz, dave, minux
CC=bradfitz, golang-codereviews
https://golang.org/cl/107020044

src/pkg/database/sql/sql.go

@@ -13,7 +13,6 @@
 package sql
 
 import (
-	"container/list"
 	"database/sql/driver"
 	"errors"
 	"fmt"
@@ -198,8 +197,8 @@ type DB struct {
 	dsn    string
 
 	mu           sync.Mutex // protects following fields
-	freeConn     *list.List // of *driverConn
-	connRequests *list.List // of connRequest
+	freeConn     []*driverConn
+	connRequests []chan *connRequest
 	numOpen      int
 	pendingOpens int
 	// Used to signal the need for new connections
@@ -232,9 +231,6 @@ type driverConn struct {
 	inUse      bool
 	onPut      []func() // code (with db.mu held) run when conn is next returned
 	dbmuClosed bool     // same as closed, but guarded by db.mu, for connIfFree
-	// This is the Element returned by db.freeConn.PushFront(conn).
-	// It's used by connIfFree to remove the conn from the freeConn list.
-	listElem *list.Element
 }
 
 func (dc *driverConn) releaseConn(err error) {
@@ -437,8 +433,6 @@ func Open(driverName, dataSourceName string) (*DB, error) {
 		openerCh: make(chan struct{}, connectionRequestQueueSize),
 		lastPut:  make(map[*driverConn]string),
 	}
-	db.freeConn = list.New()
-	db.connRequests = list.New()
 	go db.connectionOpener()
 	return db, nil
 }
@@ -469,17 +463,13 @@ func (db *DB) Close() error {
 	}
 	close(db.openerCh)
 	var err error
-	fns := make([]func() error, 0, db.freeConn.Len())
-	for db.freeConn.Front() != nil {
-		dc := db.freeConn.Front().Value.(*driverConn)
-		dc.listElem = nil
+	fns := make([]func() error, 0, len(db.freeConn))
+	for _, dc := range db.freeConn {
 		fns = append(fns, dc.closeDBLocked())
-		db.freeConn.Remove(db.freeConn.Front())
 	}
+	db.freeConn = nil
 	db.closed = true
-	for db.connRequests.Front() != nil {
-		req := db.connRequests.Front().Value.(connRequest)
-		db.connRequests.Remove(db.connRequests.Front())
+	for _, req := range db.connRequests {
 		close(req)
 	}
 	db.mu.Unlock()
@@ -527,11 +517,11 @@ func (db *DB) SetMaxIdleConns(n int) {
 		db.maxIdle = db.maxOpen
 	}
 	var closing []*driverConn
-	for db.freeConn.Len() > db.maxIdleConnsLocked() {
-		dc := db.freeConn.Back().Value.(*driverConn)
-		dc.listElem = nil
-		db.freeConn.Remove(db.freeConn.Back())
-		closing = append(closing, dc)
+	idleCount := len(db.freeConn)
+	maxIdle := db.maxIdleConnsLocked()
+	if idleCount > maxIdle {
+		closing = db.freeConn[maxIdle:]
+		db.freeConn = db.freeConn[:maxIdle]
 	}
 	db.mu.Unlock()
 	for _, c := range closing {
@@ -564,7 +554,7 @@ func (db *DB) SetMaxOpenConns(n int) {
 // If there are connRequests and the connection limit hasn't been reached,
 // then tell the connectionOpener to open new connections.
 func (db *DB) maybeOpenNewConnections() {
-	numRequests := db.connRequests.Len() - db.pendingOpens
+	numRequests := len(db.connRequests) - db.pendingOpens
 	if db.maxOpen > 0 {
 		numCanOpen := db.maxOpen - (db.numOpen + db.pendingOpens)
 		if numRequests > numCanOpen {
@@ -616,7 +606,10 @@ func (db *DB) openNewConnection() {
 // connRequest represents one request for a new connection
 // When there are no idle connections available, DB.conn will create
 // a new connRequest and put it on the db.connRequests list.
-type connRequest chan<- interface{} // takes either a *driverConn or an error
+type connRequest struct {
+	conn *driverConn
+	err  error
+}
 
 var errDBClosed = errors.New("sql: database is closed")
 
@@ -630,32 +623,24 @@ func (db *DB) conn() (*driverConn, error) {
 
 	// If db.maxOpen > 0 and the number of open connections is over the limit
 	// and there are no free connection, make a request and wait.
-	if db.maxOpen > 0 && db.numOpen >= db.maxOpen && db.freeConn.Len() == 0 {
+	if db.maxOpen > 0 && db.numOpen >= db.maxOpen && len(db.freeConn) == 0 {
 		// Make the connRequest channel. It's buffered so that the
 		// connectionOpener doesn't block while waiting for the req to be read.
-		ch := make(chan interface{}, 1)
-		req := connRequest(ch)
-		db.connRequests.PushBack(req)
+		req := make(chan *connRequest, 1)
+		db.connRequests = append(db.connRequests, req)
 		db.maybeOpenNewConnections()
 		db.mu.Unlock()
-		ret, ok := <-ch
-		if !ok {
+		ret := <-req
+		if ret == nil {
 			return nil, errDBClosed
 		}
-		switch ret.(type) {
-		case *driverConn:
-			return ret.(*driverConn), nil
-		case error:
-			return nil, ret.(error)
-		default:
-			panic("sql: Unexpected type passed through connRequest.ch")
-		}
+		return ret.conn, ret.err
 	}
 
-	if f := db.freeConn.Front(); f != nil {
-		conn := f.Value.(*driverConn)
-		conn.listElem = nil
-		db.freeConn.Remove(f)
+	if c := len(db.freeConn); c > 0 {
+		conn := db.freeConn[0]
+		copy(db.freeConn, db.freeConn[1:])
+		db.freeConn = db.freeConn[:c-1]
 		conn.inUse = true
 		db.mu.Unlock()
 		return conn, nil
@@ -702,9 +687,15 @@ func (db *DB) connIfFree(wanted *driverConn) (*driverConn, error) {
 	if wanted.inUse {
 		return nil, errConnBusy
 	}
-	if wanted.listElem != nil {
-		db.freeConn.Remove(wanted.listElem)
-		wanted.listElem = nil
+	idx := -1
+	for ii, v := range db.freeConn {
+		if v == wanted {
+			idx = ii
+			break
+		}
+	}
+	if idx >= 0 {
+		db.freeConn = append(db.freeConn[:idx], db.freeConn[idx+1:]...)
 		wanted.inUse = true
 		return wanted, nil
 	}
@@ -793,18 +784,20 @@ func (db *DB) putConn(dc *driverConn, err error) {
 // If a connRequest was fulfilled or the *driverConn was placed in the
 // freeConn list, then true is returned, otherwise false is returned.
 func (db *DB) putConnDBLocked(dc *driverConn, err error) bool {
-	if db.connRequests.Len() > 0 {
-		req := db.connRequests.Front().Value.(connRequest)
-		db.connRequests.Remove(db.connRequests.Front())
-		if err != nil {
-			req <- err
-		} else {
+	if c := len(db.connRequests); c > 0 {
+		req := db.connRequests[0]
+		copy(db.connRequests, db.connRequests[1:])
+		db.connRequests = db.connRequests[:c-1]
+		if err == nil {
 			dc.inUse = true
-			req <- dc
+		}
+		req <- &connRequest{
+			conn: dc,
+			err:  err,
 		}
 		return true
-	} else if err == nil && !db.closed && db.maxIdleConnsLocked() > db.freeConn.Len() {
-		dc.listElem = db.freeConn.PushFront(dc)
+	} else if err == nil && !db.closed && db.maxIdleConnsLocked() > len(db.freeConn) {
+		db.freeConn = append(db.freeConn, dc)
 		return true
 	}
 	return false

src/pkg/database/sql/sql_test.go

@@ -24,7 +24,14 @@ func init() {
 	}
 	freedFrom := make(map[dbConn]string)
 	putConnHook = func(db *DB, c *driverConn) {
-		if c.listElem != nil {
+		idx := -1
+		for i, v := range db.freeConn {
+			if v == c {
+				idx = i
+				break
+			}
+		}
+		if idx >= 0 {
 			// print before panic, as panic may get lost due to conflicting panic
 			// (all goroutines asleep) elsewhere, since we might not unlock
 			// the mutex in freeConn here.
@@ -79,15 +86,14 @@ func closeDB(t testing.TB, db *DB) {
 			t.Errorf("Error closing fakeConn: %v", err)
 		}
 	})
-	for node, i := db.freeConn.Front(), 0; node != nil; node, i = node.Next(), i+1 {
-		dc := node.Value.(*driverConn)
+	for i, dc := range db.freeConn {
 		if n := len(dc.openStmt); n > 0 {
 			// Just a sanity check. This is legal in
 			// general, but if we make the tests clean up
 			// their statements first, then we can safely
 			// verify this is always zero here, and any
 			// other value is a leak.
-			t.Errorf("while closing db, freeConn %d/%d had %d open stmts; want 0", i, db.freeConn.Len(), n)
+			t.Errorf("while closing db, freeConn %d/%d had %d open stmts; want 0", i, len(db.freeConn), n)
 		}
 	}
 	err := db.Close()
@@ -105,10 +111,10 @@ func closeDB(t testing.TB, db *DB) {
 // numPrepares assumes that db has exactly 1 idle conn and returns
 // its count of calls to Prepare
 func numPrepares(t *testing.T, db *DB) int {
-	if n := db.freeConn.Len(); n != 1 {
+	if n := len(db.freeConn); n != 1 {
 		t.Fatalf("free conns = %d; want 1", n)
 	}
-	return (db.freeConn.Front().Value.(*driverConn)).ci.(*fakeConn).numPrepare
+	return db.freeConn[0].ci.(*fakeConn).numPrepare
 }
 
 func (db *DB) numDeps() int {
@@ -133,7 +139,7 @@ func (db *DB) numDepsPollUntil(want int, d time.Duration) int {
 func (db *DB) numFreeConns() int {
 	db.mu.Lock()
 	defer db.mu.Unlock()
-	return db.freeConn.Len()
+	return len(db.freeConn)
 }
 
 func (db *DB) dumpDeps(t *testing.T) {
@@ -650,10 +656,10 @@ func TestQueryRowClosingStmt(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
-	if db.freeConn.Len() != 1 {
+	if len(db.freeConn) != 1 {
 		t.Fatalf("expected 1 free conn")
 	}
-	fakeConn := (db.freeConn.Front().Value.(*driverConn)).ci.(*fakeConn)
+	fakeConn := db.freeConn[0].ci.(*fakeConn)
 	if made, closed := fakeConn.stmtsMade, fakeConn.stmtsClosed; made != closed {
 		t.Errorf("statement close mismatch: made %d, closed %d", made, closed)
 	}
@@ -878,13 +884,13 @@ func TestMaxIdleConns(t *testing.T) {
 		t.Fatal(err)
 	}
 	tx.Commit()
-	if got := db.freeConn.Len(); got != 1 {
+	if got := len(db.freeConn); got != 1 {
 		t.Errorf("freeConns = %d; want 1", got)
 	}
 
 	db.SetMaxIdleConns(0)
 
-	if got := db.freeConn.Len(); got != 0 {
+	if got := len(db.freeConn); got != 0 {
 		t.Errorf("freeConns after set to zero = %d; want 0", got)
 	}
 
@@ -893,7 +899,7 @@ func TestMaxIdleConns(t *testing.T) {
 		t.Fatal(err)
 	}
 	tx.Commit()
-	if got := db.freeConn.Len(); got != 0 {
+	if got := len(db.freeConn); got != 0 {
 		t.Errorf("freeConns = %d; want 0", got)
 	}
 }
@@ -1180,10 +1186,10 @@ func TestCloseConnBeforeStmts(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	if db.freeConn.Len() != 1 {
-		t.Fatalf("expected 1 freeConn; got %d", db.freeConn.Len())
+	if len(db.freeConn) != 1 {
+		t.Fatalf("expected 1 freeConn; got %d", len(db.freeConn))
 	}
-	dc := db.freeConn.Front().Value.(*driverConn)
+	dc := db.freeConn[0]
 	if dc.closed {
 		t.Errorf("conn shouldn't be closed")
 	}