Reading files with a BOM in Go

I need to read Unicode files that may or may not contain a byte-order mark. I could of course check the first few bytes of the file myself, and discard a BOM if I find one. But before I do, is there any standard way of doing this, either in the core libraries or a third party?

No standard way, IIRC (and the standard library would really be a wrong layer to implement such a check in) so here are two examples of how you could deal with it yourself.

One is to use a buffered reader above your data stream:

import (
    "bufio"
    "os"
    "log"
)

func main() {
    fd, err := os.Open("filename")
    if err != nil {
        log.Fatal(err)
    }
    defer closeOrDie(fd)
    br := bufio.NewReader(fd)
    r, _, err := br.ReadRune()
    if err != nil {
        log.Fatal(err)
    }
    if r != '\uFEFF' {
        br.UnreadRune() // Not a BOM -- put the rune back
    }
    // Now work with br as you would do with fd
    // ...
}

Another approach, which works with objects implementing the io.Seeker interface, is to read the first three bytes and if they're not BOM, io.Seek() back to the beginning, like in:

import (
    "os"
    "log"
)

func main() {
    fd, err := os.Open("filename")
    if err != nil {
        log.Fatal(err)
    }
    defer closeOrDie(fd)
    bom := [3]byte
    _, err = io.ReadFull(fd, bom[:])
    if err != nil {
        log.Fatal(err)
    }
    if bom[0] != 0xef || bom[1] != 0xbb || bom[2] != 0xbf {
        _, err = fd.Seek(0, 0) // Not a BOM -- seek back to the beginning
        if err != nil {
            log.Fatal(err)
        }
    }
    // The next read operation on fd will read real data
    // ...
}

This is possible since instances of *os.File (what os.Open() returns) support seeking and hence implement io.Seeker. Note that that's not the case for, say, Body reader of HTTP responses since you can't "rewind" it. bufio.Buffer works around this feature of non-seekable streams by performing some buffering (obviously) — that's what allows you yo UnreadRune() on it.

Note that both examples assume the file we're dealing with is encoded in UTF-8. If you need to deal with other (or unknown) encoding, things get more complicated.