database/sql: Use all connections in pool

The last connection in the pool was not being handed out correctly.

R=golang-codereviews, gobot, bradfitz
CC=golang-codereviews
https://golang.org/cl/40410043

src/pkg/database/sql/sql.go

@@ -620,8 +620,8 @@ func (db *DB) conn() (*driverConn, error) {
 	}
 
 	// If db.maxOpen > 0 and the number of open connections is over the limit
-	// or there are no free connection, then make a request and wait.
-	if db.maxOpen > 0 && (db.numOpen >= db.maxOpen || db.freeConn.Len() == 0) {
+	// and there are no free connection, make a request and wait.
+	if db.maxOpen > 0 && db.numOpen >= db.maxOpen && db.freeConn.Len() == 0 {
 		// Make the connRequest channel. It's buffered so that the
 		// connectionOpener doesn't block while waiting for the req to be read.
 		ch := make(chan interface{}, 1)

src/pkg/database/sql/sql_test.go

@@ -1033,6 +1033,29 @@ func TestMaxOpenConns(t *testing.T) {
 	}
 }
 
+func TestSingleOpenConn(t *testing.T) {
+	db := newTestDB(t, "people")
+	defer closeDB(t, db)
+
+	db.SetMaxOpenConns(1)
+
+	rows, err := db.Query("SELECT|people|name|")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err = rows.Close(); err != nil {
+		t.Fatal(err)
+	}
+	// shouldn't deadlock
+	rows, err = db.Query("SELECT|people|name|")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err = rows.Close(); err != nil {
+		t.Fatal(err)
+	}
+}
+
 // golang.org/issue/5323
 func TestStmtCloseDeps(t *testing.T) {
 	if testing.Short() {