database/sql: add driver.ResetSessioner and add pool support

A single database connection ususally maps to a single session.
A connection pool is logically also a session pool. Most
sessions have a way to reset the session state which is desirable
to prevent one bad query from poisoning another later query with
temp table name conflicts or other persistent session resources.

It also lets drivers provide users with better error messages from
queryies when the underlying transport or query method fails.
Internally the driver connection should now be marked as bad, but
return the actual connection. When ResetSession is called on the
connection it should return driver.ErrBadConn to remove it from
the connection pool. Previously drivers had to choose between
meaningful error messages or poisoning the connection pool.

Lastly update TestPoolExhaustOnCancel from relying on a
WAIT query fixing a flaky timeout issue exposed by this
change.

Fixes #22049
Fixes #20807

Change-Id: Idffa1a7ca9ccfe633257c4a3ae299b864f46c5b6
Reviewed-on: https://go-review.googlesource.com/67630
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/database/sql/driver/driver.go b/src/database/sql/driver/driver.go
index f5a2e7c16c..6113af79c5 100644
--- a/src/database/sql/driver/driver.go
+++ b/src/database/sql/driver/driver.go
@@ -222,6 +222,18 @@ type ConnBeginTx interface {
 	BeginTx(ctx context.Context, opts TxOptions) (Tx, error)
 }
 
+// ResetSessioner may be implemented by Conn to allow drivers to reset the
+// session state associated with the connection and to signal a bad connection.
+type ResetSessioner 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(ctx context.Context) error
+}
+
 // Result is the result of a query execution.
 type Result interface {
 	// LastInsertId returns the database's auto-generated ID