database/sql: process all Session Resets synchronously

Adds a new interface, driver.ConnectionValidator, to allow
drivers to signal they should not be used again,
separatly from the session resetter interface.
This is done now that the session reset is done
after the connection is put into the connection pool.

Previous behavior attempted to run Session Resets
in a background worker. This implementation had two
problems: untested performance gains for additional
complexity, and failures when the pool size
exceeded the connection reset channel buffer size.

Fixes #31480

Change-Id: I7d483b883c24a362c292471e87a88db5b204d1d0
Reviewed-on: https://go-review.googlesource.com/c/go/+/174122
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/driver/driver.go

@@ -255,15 +255,23 @@ type ConnBeginTx interface {
 // SessionResetter may be implemented by Conn to allow drivers to reset the
 // session state associated with the connection and to signal a bad connection.
 type SessionResetter interface {
-	// ResetSession is called while a connection is in the connection
-	// pool. No queries will run on this connection until this method returns.
-	//
-	// If the connection is bad this should return driver.ErrBadConn to prevent
-	// the connection from being returned to the connection pool. Any other
-	// error will be discarded.
+	// ResetSession is called prior to executing a query on the connection
+	// if the connection has been used before. If the driver returns ErrBadConn
+	// the connection is discarded.
 	ResetSession(ctx context.Context) error
 }
 
+// ConnectionValidator may be implemented by Conn to allow drivers to
+// signal if a connection is valid or if it should be discarded.
+//
+// If implemented, drivers may return the underlying error from queries,
+// even if the connection should be discarded by the connection pool.
+type ConnectionValidator interface {
+	// ValidConnection is called prior to placing the connection into the
+	// connection pool. The connection will be discarded if false is returned.
+	ValidConnection() bool
+}
+
 // Result is the result of a query execution.
 type Result interface {
 	// LastInsertId returns the database's auto-generated ID

src/database/sql/fakedb_test.go

@@ -396,6 +396,12 @@ func (c *fakeConn) ResetSession(ctx context.Context) error {
 	return nil
 }
 
+var _ driver.ConnectionValidator = (*fakeConn)(nil)
+
+func (c *fakeConn) ValidConnection() bool {
+	return !c.isBad()
+}
+
 func (c *fakeConn) Close() (err error) {
 	drv := fdriver.(*fakeDriver)
 	defer func() {

src/database/sql/sql.go

@@ -421,7 +421,6 @@ type DB struct {
 	// It is closed during db.Close(). The close tells the connectionOpener
 	// goroutine to exit.
 	openerCh          chan struct{}
-	resetterCh        chan *driverConn
 	closed            bool
 	dep               map[finalCloser]depSet
 	lastPut           map[*driverConn]string // stacktrace of last conn's put; debug only
@@ -460,10 +459,10 @@ type driverConn struct {
 
 	sync.Mutex  // guards following
 	ci          driver.Conn
+	needReset   bool // The connection session should be reset before use if true.
 	closed      bool
 	finalClosed bool // ci.Close has been called
 	openStmt    map[*driverStmt]bool
-	lastErr     error // lastError captures the result of the session resetter.
 
 	// guarded by db.mu
 	inUse      bool
@@ -489,6 +488,36 @@ func (dc *driverConn) expired(timeout time.Duration) bool {
 	return dc.createdAt.Add(timeout).Before(nowFunc())
 }
 
+// resetSession checks if the driver connection needs the
+// session to be reset and if required, resets it.
+func (dc *driverConn) resetSession(ctx context.Context) error {
+	dc.Lock()
+	defer dc.Unlock()
+
+	if !dc.needReset {
+		return nil
+	}
+	if cr, ok := dc.ci.(driver.SessionResetter); ok {
+		return cr.ResetSession(ctx)
+	}
+	return nil
+}
+
+// validateConnection checks if the connection is valid and can
+// still be used. It also marks the session for reset if required.
+func (dc *driverConn) validateConnection(needsReset bool) bool {
+	dc.Lock()
+	defer dc.Unlock()
+
+	if needsReset {
+		dc.needReset = true
+	}
+	if cv, ok := dc.ci.(driver.ConnectionValidator); ok {
+		return cv.ValidConnection()
+	}
+	return true
+}
+
 // prepareLocked prepares the query on dc. When cg == nil the dc must keep track of
 // the prepared statements in a pool.
 func (dc *driverConn) prepareLocked(ctx context.Context, cg stmtConnGrabber, query string) (*driverStmt, error) {
@@ -514,19 +543,6 @@ func (dc *driverConn) prepareLocked(ctx context.Context, cg stmtConnGrabber, que
 	return ds, nil
 }
 
-// resetSession resets the connection session and sets the lastErr
-// that is checked before returning the connection to another query.
-//
-// resetSession assumes that the embedded mutex is locked when the connection
-// was returned to the pool. This unlocks the mutex.
-func (dc *driverConn) resetSession(ctx context.Context) {
-	defer dc.Unlock() // In case of panic.
-	if dc.closed {    // Check if the database has been closed.
-		return
-	}
-	dc.lastErr = dc.ci.(driver.SessionResetter).ResetSession(ctx)
-}
-
 // the dc.db's Mutex is held.
 func (dc *driverConn) closeDBLocked() func() error {
 	dc.Lock()
@@ -716,14 +732,12 @@ func OpenDB(c driver.Connector) *DB {
 	db := &DB{
 		connector:    c,
 		openerCh:     make(chan struct{}, connectionRequestQueueSize),
-		resetterCh:   make(chan *driverConn, 50),
 		lastPut:      make(map[*driverConn]string),
 		connRequests: make(map[uint64]chan connRequest),
 		stop:         cancel,
 	}
 
 	go db.connectionOpener(ctx)
-	go db.connectionResetter(ctx)
 
 	return db
 }
@@ -1118,23 +1132,6 @@ func (db *DB) connectionOpener(ctx context.Context) {
 	}
 }
 
-// connectionResetter runs in a separate goroutine to reset connections async
-// to exported API.
-func (db *DB) connectionResetter(ctx context.Context) {
-	for {
-		select {
-		case <-ctx.Done():
-			close(db.resetterCh)
-			for dc := range db.resetterCh {
-				dc.Unlock()
-			}
-			return
-		case dc := <-db.resetterCh:
-			dc.resetSession(ctx)
-		}
-	}
-}
-
 // Open one new connection
 func (db *DB) openNewConnection(ctx context.Context) {
 	// maybeOpenNewConnctions has already executed db.numOpen++ before it sent
@@ -1216,14 +1213,13 @@ func (db *DB) conn(ctx context.Context, strategy connReuseStrategy) (*driverConn
 			conn.Close()
 			return nil, driver.ErrBadConn
 		}
-		// Lock around reading lastErr to ensure the session resetter finished.
-		conn.Lock()
-		err := conn.lastErr
-		conn.Unlock()
-		if err == driver.ErrBadConn {
+
+		// Reset the session if required.
+		if err := conn.resetSession(ctx); err == driver.ErrBadConn {
 			conn.Close()
 			return nil, driver.ErrBadConn
 		}
+
 		return conn, nil
 	}
 
@@ -1272,11 +1268,9 @@ func (db *DB) conn(ctx context.Context, strategy connReuseStrategy) (*driverConn
 			if ret.conn == nil {
 				return nil, ret.err
 			}
-			// Lock around reading lastErr to ensure the session resetter finished.
-			ret.conn.Lock()
-			err := ret.conn.lastErr
-			ret.conn.Unlock()
-			if err == driver.ErrBadConn {
+
+			// Reset the session if required.
+			if err := ret.conn.resetSession(ctx); err == driver.ErrBadConn {
 				ret.conn.Close()
 				return nil, driver.ErrBadConn
 			}
@@ -1337,6 +1331,11 @@ const debugGetPut = false
 // putConn adds a connection to the db's free pool.
 // err is optionally the last error that occurred on this connection.
 func (db *DB) putConn(dc *driverConn, err error, resetSession bool) {
+	if err != driver.ErrBadConn {
+		if !dc.validateConnection(resetSession) {
+			err = driver.ErrBadConn
+		}
+	}
 	db.mu.Lock()
 	if !dc.inUse {
 		if debugGetPut {
@@ -1368,41 +1367,13 @@ func (db *DB) putConn(dc *driverConn, err error, resetSession bool) {
 	if putConnHook != nil {
 		putConnHook(db, dc)
 	}
-	if db.closed {
-		// Connections do not need to be reset if they will be closed.
-		// Prevents writing to resetterCh after the DB has closed.
-		resetSession = false
-	}
-	if resetSession {
-		if _, resetSession = dc.ci.(driver.SessionResetter); resetSession {
-			// Lock the driverConn here so it isn't released until
-			// the connection is reset.
-			// The lock must be taken before the connection is put into
-			// the pool to prevent it from being taken out before it is reset.
-			dc.Lock()
-		}
-	}
 	added := db.putConnDBLocked(dc, nil)
 	db.mu.Unlock()
 
 	if !added {
-		if resetSession {
-			dc.Unlock()
-		}
 		dc.Close()
 		return
 	}
-	if !resetSession {
-		return
-	}
-	select {
-	default:
-		// If the resetterCh is blocking then mark the connection
-		// as bad and continue on.
-		dc.lastErr = driver.ErrBadConn
-		dc.Unlock()
-	case db.resetterCh <- dc:
-	}
 }
 
 // Satisfy a connRequest or put the driverConn in the idle pool and return true