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, 1 insertions, 0 deletions

diff --git a/src/database/sql/fakedb_test.go b/src/database/sql/fakedb_test.go
index 4dcd096ca4..8e77df4ace 100644
--- a/src/database/sql/fakedb_test.go
+++ b/src/database/sql/fakedb_test.go
@@ -943,6 +943,7 @@ type rowsCursor struct {
 }
 
 func (rc *rowsCursor) touchMem() {
+	rc.parentMem.touchMem()
 	rc.line++
 }