database/sql: ensure releaseConn is defined before a possible close

When running a Query on Stmt a dependency is added to the stmt and
rows. To do that it needs a reference to Rows, so the releaseConn
function is defined after the definition. However the
rows.initContextClose was set to run before the releaseConn was
set on rows, setting up a situation where the connection could
be canceled before the releaseConn was set and resulting in
a segfault.

Fixes #20160

Change-Id: I5592e7db2cf653dfc48d42cbc2b03ca20501b1a0
Reviewed-on: https://go-review.googlesource.com/42139
Run-TryBot: Daniel Theophanes <kardianos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/database/sql/sql.go

@@ -2183,12 +2183,17 @@ func (s *Stmt) QueryContext(ctx context.Context, args ...interface{}) (*Rows, er
 				rowsi: rowsi,
 				// releaseConn set below
 			}
-			rows.initContextClose(ctx)
+			// addDep must be added before initContextClose or it could attempt
+			// to removeDep before it has been added.
 			s.db.addDep(s, rows)
+
+			// releaseConn must be set before initContextClose or it could
+			// release the connection before it is set.
 			rows.releaseConn = func(err error) {
 				releaseConn(err)
 				s.db.removeDep(s, rows)
 			}
+			rows.initContextClose(ctx)
 			return rows, nil
 		}
 

src/database/sql/sql_test.go

@@ -3044,6 +3044,46 @@ func TestIssue18429(t *testing.T) {
 	wg.Wait()
 }
 
+// TestIssue20160 attempts to test a short context life on a stmt Query.
+func TestIssue20160(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	ctx := context.Background()
+	sem := make(chan bool, 20)
+	var wg sync.WaitGroup
+
+	const milliWait = 30
+
+	stmt, err := db.PrepareContext(ctx, "SELECT|people|name|")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer stmt.Close()
+
+	for i := 0; i < 100; i++ {
+		sem <- true
+		wg.Add(1)
+		go func() {
+			defer func() {
+				<-sem
+				wg.Done()
+			}()
+			ctx, cancel := context.WithTimeout(ctx, time.Duration(rand.Intn(milliWait))*time.Millisecond)
+			defer cancel()
+
+			// This is expected to give a cancel error most, but not all the time.
+			// Test failure will happen with a panic or other race condition being
+			// reported.
+			rows, _ := stmt.QueryContext(ctx)
+			if rows != nil {
+				rows.Close()
+			}
+		}()
+	}
+	wg.Wait()
+}
+
 // TestIssue18719 closes the context right before use. The sql.driverConn
 // will nil out the ci on close in a lock, but if another process uses it right after
 // it will panic with on the nil ref.