database/sql: do not exhaust connection pool on conn request timeout

Previously if a context was canceled while it was waiting for a
connection request, that connection request would leak.

To prevent this remove the pending connection request if the
context is canceled and ensure no connection has been sent on the channel.
This requires a change to how the connection requests are represented in the DB.

Fixes #18995

Change-Id: I9a274b48b8f4f7ca46cdee166faa38f56d030852
Reviewed-on: https://go-review.googlesource.com/36563
Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql.go

@@ -305,8 +305,9 @@ type DB struct {
 
 	mu           sync.Mutex // protects following fields
 	freeConn     []*driverConn
-	connRequests []chan connRequest
-	numOpen      int // number of opened and pending open connections
+	connRequests map[uint64]chan connRequest
+	nextRequest  uint64 // Next key to use in connRequests.
+	numOpen      int    // number of opened and pending open connections
 	// Used to signal the need for new connections
 	// a goroutine running connectionOpener() reads on this chan and
 	// maybeOpenNewConnections sends on the chan (one send per needed connection)
@@ -572,10 +573,11 @@ func Open(driverName, dataSourceName string) (*DB, error) {
 		return nil, fmt.Errorf("sql: unknown driver %q (forgotten import?)", driverName)
 	}
 	db := &DB{
-		driver:   driveri,
-		dsn:      dataSourceName,
-		openerCh: make(chan struct{}, connectionRequestQueueSize),
-		lastPut:  make(map[*driverConn]string),
+		driver:       driveri,
+		dsn:          dataSourceName,
+		openerCh:     make(chan struct{}, connectionRequestQueueSize),
+		lastPut:      make(map[*driverConn]string),
+		connRequests: make(map[uint64]chan connRequest),
 	}
 	go db.connectionOpener()
 	return db, nil
@@ -881,6 +883,14 @@ type connRequest struct {
 
 var errDBClosed = errors.New("sql: database is closed")
 
+// nextRequestKeyLocked returns the next connection request key.
+// It is assumed that nextRequest will not overflow.
+func (db *DB) nextRequestKeyLocked() uint64 {
+	next := db.nextRequest
+	db.nextRequest++
+	return next
+}
+
 // conn returns a newly-opened or cached *driverConn.
 func (db *DB) conn(ctx context.Context, strategy connReuseStrategy) (*driverConn, error) {
 	db.mu.Lock()
@@ -918,12 +928,25 @@ func (db *DB) conn(ctx context.Context, strategy connReuseStrategy) (*driverConn
 		// Make the connRequest channel. It's buffered so that the
 		// connectionOpener doesn't block while waiting for the req to be read.
 		req := make(chan connRequest, 1)
-		db.connRequests = append(db.connRequests, req)
+		reqKey := db.nextRequestKeyLocked()
+		db.connRequests[reqKey] = req
 		db.mu.Unlock()
 
 		// Timeout the connection request with the context.
 		select {
 		case <-ctx.Done():
+			// Remove the connection request and ensure no value has been sent
+			// on it after removing.
+			db.mu.Lock()
+			delete(db.connRequests, reqKey)
+			select {
+			default:
+			case ret, ok := <-req:
+				if ok {
+					db.putConnDBLocked(ret.conn, ret.err)
+				}
+			}
+			db.mu.Unlock()
 			return nil, ctx.Err()
 		case ret, ok := <-req:
 			if !ok {
@@ -1044,12 +1067,12 @@ func (db *DB) putConnDBLocked(dc *driverConn, err error) bool {
 		return false
 	}
 	if c := len(db.connRequests); c > 0 {
-		req := db.connRequests[0]
-		// This copy is O(n) but in practice faster than a linked list.
-		// TODO: consider compacting it down less often and
-		// moving the base instead?
-		copy(db.connRequests, db.connRequests[1:])
-		db.connRequests = db.connRequests[:c-1]
+		var req chan connRequest
+		var reqKey uint64
+		for reqKey, req = range db.connRequests {
+			break
+		}
+		delete(db.connRequests, reqKey) // Remove from pending requests.
 		if err == nil {
 			dc.inUse = true
 		}