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>

src/database/sql/fakedb_test.go

@@ -39,6 +39,9 @@ var _ = log.Printf
 // Any of these can be preceded by PANIC|<method>|, to cause the
 // named method on fakeStmt to panic.
 //
+// Any of these can be proceeded by WAIT|<duration>|, to cause the
+// named method on fakeStmt to sleep for the specified duration.
+//
 // Multiple of these can be combined when separated with a semicolon.
 //
 // When opening a fakeDriver's database, it starts empty with no
@@ -119,6 +122,7 @@ type fakeStmt struct {
 	cmd   string
 	table string
 	panic string
+	wait  time.Duration
 
 	next *fakeStmt // used for returning multiple results.
 
@@ -526,14 +530,28 @@ func (c *fakeConn) Prepare(query string) (driver.Stmt, error) {
 		if firstStmt == nil {
 			firstStmt = stmt
 		}
-		if len(parts) >= 3 && parts[0] == "PANIC" {
-			stmt.panic = parts[1]
-			parts = parts[2:]
+		if len(parts) >= 3 {
+			switch parts[0] {
+			case "PANIC":
+				stmt.panic = parts[1]
+				parts = parts[2:]
+			case "WAIT":
+				wait, err := time.ParseDuration(parts[1])
+				if err != nil {
+					return nil, errf("expected section after WAIT to be a duration, got %q %v", parts[1], err)
+				}
+				parts = parts[2:]
+				stmt.wait = wait
+			}
 		}
 		cmd := parts[0]
 		stmt.cmd = cmd
 		parts = parts[1:]
 
+		if stmt.wait > 0 {
+			time.Sleep(stmt.wait)
+		}
+
 		c.incrStat(&c.stmtsMade)
 		var err error
 		switch cmd {
@@ -619,6 +637,16 @@ func (s *fakeStmt) ExecContext(ctx context.Context, args []driver.NamedValue) (d
 		return nil, err
 	}
 
+	if s.wait > 0 {
+		time.Sleep(s.wait)
+	}
+
+	select {
+	default:
+	case <-ctx.Done():
+		return nil, ctx.Err()
+	}
+
 	db := s.c.db
 	switch s.cmd {
 	case "WIPE":