database/sql: fix tx stmt deadlock when rollback

Tx acquires tx.closemu W-lock and then acquires stmt.closemu.W-lock
to fully close the transaction and associated prepared statement.
Stmt query and execution run in reverse ways - acquires
stmt.closemu.R-lock and then acquires tx.closemu.R-lock to grab tx
connection, which may cause deadlock.

Prevent the lock is held around tx.closePrepared to ensure no
deadlock happens.

Fixes #40985

Change-Id: If53909822b87bce11861a6e3035ecb9476d2cd17
Reviewed-on: https://go-review.googlesource.com/c/go/+/250178
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Daniel Theophanes <kardianos@gmail.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>

src/database/sql/sql_test.go

@@ -2810,6 +2810,36 @@ func TestTxCannotCommitAfterRollback(t *testing.T) {
 	}
 }
 
+// Issue 40985 transaction statement deadlock while context cancel.
+func TestTxStmtDeadlock(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Millisecond)
+	defer cancel()
+	tx, err := db.BeginTx(ctx, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	stmt, err := tx.Prepare("SELECT|people|name,age|age=?")
+	if err != nil {
+		t.Fatal(err)
+	}
+	// Run number of stmt queries to reproduce deadlock from context cancel
+	for i := 0; i < 1e3; i++ {
+		_, err = stmt.Query(1)
+		if err != nil {
+			// Encounter ErrTxDone here is expected due to context cancel
+			if err != ErrTxDone {
+				t.Fatalf("unexpected error while executing stmt, err: %v", err)
+			}
+			break
+		}
+	}
+	_ = tx.Rollback()
+}
+
 // Issue32530 encounters an issue where a connection may
 // expire right after it comes out of a used connection pool
 // even when a new connection is requested.