database/sql: Use Tx.ctx in Tx non-context methods

The Tx methods Query and Exec uses context.Background()
even Tx was created by context.
This patch enables using Tx.ctx in all Tx methods
which do not has context arg.
Backward compatibility:
- If Tx has created without context, nothing changes.
- If Tx has created with context and non-context method is called:
  - If context is expired, the execution fails,
    but it can fail on Commit or Rollback as well,
    so in terms of whole transaction - nothing changes.
  - If context is not expired, nothing changes too.

Fixes #20098
Change-Id: I9570a2deaace5875bb4c5dcf7b3a084a6bcd0d00
Reviewed-on: https://go-review.googlesource.com/44956
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/database/sql/sql.go

@@ -1830,7 +1830,7 @@ func (tx *Tx) PrepareContext(ctx context.Context, query string) (*Stmt, error) {
 //
 // To use an existing prepared statement on this transaction, see Tx.Stmt.
 func (tx *Tx) Prepare(query string) (*Stmt, error) {
-	return tx.PrepareContext(context.Background(), query)
+	return tx.PrepareContext(tx.ctx, query)
 }
 
 // StmtContext returns a transaction-specific prepared statement from
@@ -1928,7 +1928,7 @@ func (tx *Tx) StmtContext(ctx context.Context, stmt *Stmt) *Stmt {
 // The returned statement operates within the transaction and will be closed
 // when the transaction has been committed or rolled back.
 func (tx *Tx) Stmt(stmt *Stmt) *Stmt {
-	return tx.StmtContext(context.Background(), stmt)
+	return tx.StmtContext(tx.ctx, stmt)
 }
 
 // ExecContext executes a query that doesn't return rows.
@@ -1947,7 +1947,7 @@ func (tx *Tx) ExecContext(ctx context.Context, query string, args ...interface{}
 // Exec executes a query that doesn't return rows.
 // For example: an INSERT and UPDATE.
 func (tx *Tx) Exec(query string, args ...interface{}) (Result, error) {
-	return tx.ExecContext(context.Background(), query, args...)
+	return tx.ExecContext(tx.ctx, query, args...)
 }
 
 // QueryContext executes a query that returns rows, typically a SELECT.
@@ -1965,7 +1965,7 @@ func (tx *Tx) QueryContext(ctx context.Context, query string, args ...interface{
 
 // Query executes a query that returns rows, typically a SELECT.
 func (tx *Tx) Query(query string, args ...interface{}) (*Rows, error) {
-	return tx.QueryContext(context.Background(), query, args...)
+	return tx.QueryContext(tx.ctx, query, args...)
 }
 
 // QueryRowContext executes a query that is expected to return at most one row.
@@ -1980,7 +1980,7 @@ func (tx *Tx) QueryRowContext(ctx context.Context, query string, args ...interfa
 // QueryRow always returns a non-nil value. Errors are deferred until
 // Row's Scan method is called.
 func (tx *Tx) QueryRow(query string, args ...interface{}) *Row {
-	return tx.QueryRowContext(context.Background(), query, args...)
+	return tx.QueryRowContext(tx.ctx, query, args...)
 }
 
 // connStmt is a prepared statement on a particular connection.