How do I check for specific types of error among those returned by ioutil.ReadFile?

Question

When I use ioutil to read a file, it may return an error. But if I want to filter some error code, what should I do?

res, err := ioutil.ReadFile("xxx")
if err != nil {
    fmt.Println(err.Error())  
}
...

In the code snippet above, when a file has no permission, fmt.Println(err.Error()) will print "open xxxxx: permission denied. If I want to capture this kind error, how can I know that file read failed because permission was denied?

Should I search err.Error() for the string permission denied - this looks ungraceful. Is there any better way?

Thanks in advance.

Update

After trying @Intermernet solution, I found that it will not hit case os.ErrPermission, it will hit default and print "open xxx: Permission denied".

@Aedolon solution is ok, os.IsPermission(err) can tell that a file has failed with permission deny.

Answer 1

According to the current API, ioutil.ReadFile doesn't guarantee any specific behaviour except that it will return err == nil when successful. Even the syscall package doesn't actually guarantee a specific error.

The current implementation of ioutil.ReadFile uses os.Open, which will return *os.PathError when failing to open a file, not os.ErrPermission or anything else. os.PathError contains a field Err which is also an error - in this case, a syscall.Errno. The string "permission denied" is produced from a private table of error messages and it's both architecture and implementation-specific. In my Windows machine it says "Access is denied" instead.

AFAIK, the correct method is to use os.IsPermission(err), which will return true in case of lacking permissions.

Answer 2

Per documentation, errors returned should be tested against these errors using errors.Is

var (
// ErrInvalid indicates an invalid argument.
// Methods on File will return this error when the receiver is nil.
ErrInvalid = errInvalid() // "invalid argument"

ErrPermission = errPermission() // "permission denied"
ErrExist      = errExist()      // "file already exists"
ErrNotExist   = errNotExist()   // "file does not exist"
ErrClosed     = errClosed()     // "file already closed"
ErrNoDeadline = errNoDeadline() // "file type does not support deadline"
)

As an example:

f, err := ioutil.ReadFile("your_file")

if err != nil {
    if errors.Is(err, os.ErrNotExist) {
        // your code
    } else if errors.Is(err, os.ErrPermission) {
        // your code
    } else {
        panic(err)
    }
}