database/sql: do not bypass the driver locks with Context methods

When context methods were initially added it was attempted to unify
behavior between drivers without Context methods and those with
Context methods to always return right away when the Context expired.
However in doing so the driver call could be executed outside of the
scope of the driver connection lock and thus bypassing thread safety.

The new behavior waits until the driver operation is complete. It then
checks to see if the context has expired and if so returns that error.

Change-Id: I4a5c7c3263420c57778f36a5ed6fa0ef8cb32b20
Reviewed-on: https://go-review.googlesource.com/32422
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 38 insertions, 8 deletions

diff --git a/src/database/sql/driver/driver.go b/src/database/sql/driver/driver.go
index e2ee7a9b28..c8cbbf0696 100644
--- a/src/database/sql/driver/driver.go
+++ b/src/database/sql/driver/driver.go
@@ -87,12 +87,21 @@ type Pinger interface {
 // statement.
 //
 // Exec may return ErrSkip.
+//
+// Deprecated: Drivers should implement ExecerContext instead (or additionally).
 type Execer interface {
 	Exec(query string, args []Value) (Result, error)
 }
 
-// ExecerContext is like execer, but must honor the context timeout and return
-// when the context is cancelled.
+// ExecerContext is an optional interface that may be implemented by a Conn.
+//
+// If a Conn does not implement ExecerContext, the sql package's DB.Exec will
+// first prepare a query, execute the statement, and then close the
+// statement.
+//
+// ExecerContext may return ErrSkip.
+//
+// ExecerContext must honor the context timeout and return when the context is canceled.
 type ExecerContext interface {
 	ExecContext(ctx context.Context, query string, args []NamedValue) (Result, error)
 }
@@ -104,12 +113,21 @@ type ExecerContext interface {
 // statement.
 //
 // Query may return ErrSkip.
+//
+// Deprecated: Drivers should implement QueryerContext instead (or additionally).
 type Queryer interface {
 	Query(query string, args []Value) (Rows, error)
 }
 
-// QueryerContext is like Queryer, but most honor the context timeout and return
-// when the context is cancelled.
+// QueryerContext is an optional interface that may be implemented by a Conn.
+//
+// If a Conn does not implement QueryerContext, the sql package's DB.Query will
+// first prepare a query, execute the statement, and then close the
+// statement.
+//
+// QueryerContext may return ErrSkip.
+//
+// QueryerContext must honor the context timeout and return when the context is canceled.
 type QueryerContext interface {
 	QueryContext(ctx context.Context, query string, args []NamedValue) (Rows, error)
 }
@@ -133,6 +151,8 @@ type Conn interface {
 	Close() error
 
 	// Begin starts and returns a new transaction.
+	//
+	// Deprecated: Drivers should implement ConnBeginContext instead (or additionally).
 	Begin() (Tx, error)
 }
 
@@ -167,8 +187,8 @@ func ReadOnlyFromContext(ctx context.Context) (readonly bool) {
 // ConnBeginContext enhances the Conn interface with context.
 type ConnBeginContext interface {
 	// BeginContext starts and returns a new transaction.
-	// The provided context should be used to roll the transaction back
-	// if it is cancelled.
+	// If the context is canceled by the user the sql package will
+	// call Tx.Rollback before discarding and closing the connection.
 	//
 	// This must call IsolationFromContext to determine if there is a set
 	// isolation level. If the driver does not support setting the isolation
@@ -215,22 +235,32 @@ type Stmt interface {
 
 	// Exec executes a query that doesn't return rows, such
 	// as an INSERT or UPDATE.
+	//
+	// Deprecated: Drivers should implement StmtExecContext instead (or additionally).
 	Exec(args []Value) (Result, error)
 
 	// Query executes a query that may return rows, such as a
 	// SELECT.
+	//
+	// Deprecated: Drivers should implement StmtQueryContext instead (or additionally).
 	Query(args []Value) (Rows, error)
 }
 
 // StmtExecContext enhances the Stmt interface by providing Exec with context.
 type StmtExecContext interface {
-	// ExecContext must honor the context timeout and return when it is cancelled.
+	// ExecContext executes a query that doesn't return rows, such
+	// as an INSERT or UPDATE.
+	//
+	// ExecContext must honor the context timeout and return when it is canceled.
 	ExecContext(ctx context.Context, args []NamedValue) (Result, error)
 }
 
 // StmtQueryContext enhances the Stmt interface by providing Query with context.
 type StmtQueryContext interface {
-	// QueryContext must honor the context timeout and return when it is cancelled.
+	// QueryContext executes a query that may return rows, such as a
+	// SELECT.
+	//
+	// QueryContext must honor the context timeout and return when it is canceled.
 	QueryContext(ctx context.Context, args []NamedValue) (Rows, error)
 }