Golang bad file descriptor

You need to add the O_WRONLY flag :

if f, err := os.OpenFile("./log.log", os.O_APPEND|os.O_WRONLY, os.ModeAppend); err != nil { /*[...]*/ }

To explain, here is the linux documentation for open: http://man7.org/linux/man-pages/man2/openat.2.html :

The argument flags must include one of the following access modes:
O_RDONLY, O_WRONLY, or O_RDWR. These request opening the file read-
only, write-only, or read/write, respectively.

If you check /usr/local/go/src/syscall/zerrors_linux_amd64.go:660, you can see that:

O_RDONLY                         = 0x0
O_RDWR                           = 0x2
O_WRONLY                         = 0x1

So by default you get a read-only file descriptor.