database/sql: prevent race in driver by locking dc in Next

Database drivers should be called from a single goroutine to ease
driver's design. If a driver chooses to handle context
cancels internally it may do so.

The sql package violated this agreement when calling Next or
NextResultSet. It was possible for a concurrent rollback
triggered from a context cancel to call a Tx.Rollback (which
takes a driver connection lock) while a Rows.Next is in progress
(which does not tack the driver connection lock).

The current internal design of the sql package is each call takes
roughly two locks: a closemu lock which prevents an disposing of
internal resources (assigning nil or removing from lists)
and a driver connection lock that prevents calling driver code from
multiple goroutines.

Fixes #21117

Change-Id: Ie340dc752a503089c27f57ffd43e191534829360
Reviewed-on: https://go-review.googlesource.com/65731
Reviewed-by: Ian Lance Taylor <iant@golang.org>

1 files changed, 12 insertions, 0 deletions

diff --git a/src/database/sql/sql.go b/src/database/sql/sql.go
index 17910904f6..9a3957b267 100644
--- a/src/database/sql/sql.go
+++ b/src/database/sql/sql.go
@@ -2491,6 +2491,12 @@ func (rs *Rows) nextLocked() (doClose, ok bool) {
 	if rs.lastcols == nil {
 		rs.lastcols = make([]driver.Value, len(rs.rowsi.Columns()))
 	}
+
+	// Lock the driver connection before calling the driver interface
+	// rowsi to prevent a Tx from rolling back the connection at the same time.
+	rs.dc.Lock()
+	defer rs.dc.Unlock()
+
 	rs.lasterr = rs.rowsi.Next(rs.lastcols)
 	if rs.lasterr != nil {
 		// Close the connection if there is a driver error.
@@ -2540,6 +2546,12 @@ func (rs *Rows) NextResultSet() bool {
 		doClose = true
 		return false
 	}
+
+	// Lock the driver connection before calling the driver interface
+	// rowsi to prevent a Tx from rolling back the connection at the same time.
+	rs.dc.Lock()
+	defer rs.dc.Unlock()
+
 	rs.lasterr = nextResultSet.NextResultSet()
 	if rs.lasterr != nil {
 		doClose = true