database/sql: deflake TestPendingConnsAfterErr and fix races, panics

TestPendingConnsAfterErr only cared that things didn't deadlock, so 5
seconds is a sufficient timer. We don't need 100 milliseconds.

I was able to reproduce with a tiny (5 nanosecond) timeout value,
instead of 100 milliseconds. In the process of testing with -race and
a high -count= value, I noticed several data races and panics
(sendings on a closed channel) which are also fixed in this change.

Fixes #15684

Change-Id: Ib4605fcc0f296e658cb948352ed642b801cb578c
Reviewed-on: https://go-review.googlesource.com/24550Reviewed-by: Marko Tiikkaja <marko@joh.to>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/database/sql/sql.go

@@ -718,6 +718,9 @@ func (db *DB) maybeOpenNewConnections() {
 	for numRequests > 0 {
 		db.numOpen++ // optimistically
 		numRequests--
+		if db.closed {
+			return
+		}
 		db.openerCh <- struct{}{}
 	}
 }
@@ -915,6 +918,9 @@ func (db *DB) putConn(dc *driverConn, err error) {
 // If a connRequest was fulfilled or the *driverConn was placed in the
 // freeConn list, then true is returned, otherwise false is returned.
 func (db *DB) putConnDBLocked(dc *driverConn, err error) bool {
+	if db.closed {
+		return false
+	}
 	if db.maxOpen > 0 && db.numOpen > db.maxOpen {
 		return false
 	}

src/database/sql/sql_test.go

@@ -144,7 +144,7 @@ func closeDB(t testing.TB, db *DB) {
 	count := db.numOpen
 	db.mu.Unlock()
 	if count != 0 {
-		t.Fatalf("%d connections still open after closing DB", db.numOpen)
+		t.Fatalf("%d connections still open after closing DB", count)
 	}
 }
 
@@ -1239,7 +1239,7 @@ func TestPendingConnsAfterErr(t *testing.T) {
 	time.Sleep(10 * time.Millisecond) // make extra sure all workers are blocked
 	close(unblock)                    // let all workers proceed
 
-	const timeout = 100 * time.Millisecond
+	const timeout = 5 * time.Second
 	to := time.NewTimer(timeout)
 	defer to.Stop()
 
@@ -1615,6 +1615,8 @@ func TestManyErrBadConn(t *testing.T) {
 			}
 		}()
 
+		db.mu.Lock()
+		defer db.mu.Unlock()
 		if db.numOpen != nconn {
 			t.Fatalf("unexpected numOpen %d (was expecting %d)", db.numOpen, nconn)
 		} else if len(db.freeConn) != nconn {