Go version 1.8.3
In DB.QueryContext(), Go will close long live connection which exceeded the
duration setted by DB.SetConnMaxLifetime(), the code is below
database/sql/sql.go
895 func (db *DB) conn(ctx context.Context, strategy connReuseStrategy)
(*driverConn, error) {
896 db.mu.Lock()
897 if db.closed {
898 db.mu.Unlock()
899 return nil, errDBClosed
900 }
901 // Check if the context is expired.
902 select {
903 default:
904 case <-ctx.Done():
905 db.mu.Unlock()
906 return nil, ctx.Err()
907 }
908 lifetime := db.maxLifetime
909
910 // Prefer a free connection, if possible.
911 numFree := len(db.freeConn)
912 if strategy == cachedOrNewConn && numFree > 0 {
913 conn := db.freeConn[0]
914 copy(db.freeConn, db.freeConn[1:])
915 db.freeConn = db.freeConn[:numFree-1]
916 conn.inUse = true
917 db.mu.Unlock()
* 918 if conn.expired(lifetime) {*
* 919 conn.Close()*
920 return nil, driver.ErrBadConn
921 }
922 return conn, nil
923 }
The problem is if driver.Close() has some handshake to tear down the
connection, this may introduce unexpected latency. But the driver.Conn
interface not mention this.
database/sql/driver/driver.go
143 type Conn interface {
144 // Prepare returns a prepared statement, bound to this connection.
145 Prepare(query string) (Stmt, error)
146
147 // Close invalidates and potentially stops any current
148 // prepared statements and transactions, marking this
149 // connection as no longer in use.
150 //
151 // Because the sql package maintains a free pool of
152 // connections and only calls Close when there's a surplus of
153 // idle connections, it shouldn't be necessary for drivers to
154 // do their own connection caching.
155 Close() error
156
157 // Begin starts and returns a new transaction.
158 //
159 // Deprecated: Drivers should implement ConnBeginTx instead (or
additionally).
160 Begin() (Tx, error)
161 }
There is already a goroutine to clean the connections, why still need a
synchronous check?
database/sql/sql.go
750 func (db *DB) connectionCleaner(d time.Duration) {
751 const minInterval = time.Second
752
753 if d < minInterval {
754 d = minInterval
755 }
756 t := time.NewTimer(d)
757
758 for {
759 select {
760 case <-t.C:
761 case <-db.cleanerCh: // maxLifetime was changed or db was
closed.
762 }
763
764 db.mu.Lock()
765 d = db.maxLifetime
766 if db.closed || db.numOpen == 0 || d <= 0 {
767 db.cleanerCh = nil
768 db.mu.Unlock()
769 return
770 }
771
772 expiredSince := nowFunc().Add(-d)
773 var closing []*driverConn
774 for i := 0; i < len(db.freeConn); i++ {
775 c := db.freeConn[i]
776 if c.createdAt.Before(expiredSince) {
777 closing = append(closing, c)
778 last := len(db.freeConn) - 1
779 db.freeConn[i] = db.freeConn[last]
780 db.freeConn[last] = nil
781 db.freeConn = db.freeConn[:last]
782 i--
783 }
784 }
Is this by design or need to be fixed?
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