Golang bad file descriptor - go

Question

I am getting a bad file descriptor when trying to append to a logging file within my go routine.
write ./log.log: bad file descriptor
The file exists and has 666 for permissions. At first I thought well maybe it is because each one of them is trying to open the file at the same time. I implemented a mutex to try and avoid that but got the same issue so I removed it.
logCh := make(chan string, 150)
go func() {
for {
msg, ok := <-logCh
if ok {
if f, err := os.OpenFile("./log.log", os.O_APPEND, os.ModeAppend); err != nil {
panic(err)
} else {
logTime := time.Now().Format(time.RFC3339)
if _, err := f.WriteString(logTime + " - " + msg); err != nil {
fmt.Print(err)
}
f.Close()
}
} else {
fmt.Print("Channel closed! \n")
break
}
}
}()

Answer 1

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.

Answer 2

it used for me
the code before:
os.OpenFile(fileName, os.O_CREATE|os.O_APPEND, os.ModePerm)
and it occured the error: bad file descriptor,
then i add the os.O_WRONLY into the function
the code after:
os.OpenFile(fileName, os.O_CREATE|os.O_WRONLY|os.O_APPEND, os.ModePerm)
and it did't occured the problem.

Answer 3

This appears to be a discrepancy between windows and linux.
On windows os.O_APPEND implies write access, as can be seen in the code in syscall_windows.go.
if mode&O_APPEND != 0 {
access &^= GENERIC_WRITE
access |= FILE_APPEND_DATA
}
in linux the openflags are passed as-is to a linux syscall
So in DOS/WINDOWS you do not need to explicitly add a write flag with an append flag, as it is implied. (this has been default behaviour since the DOS days)
Go in linux does need the extra flag added to work.
(...but imo it should not need this flag, as appending implicitly implies that I want to write. Should I bugreport this to golang ?)