[mig modules] automatically decompress files on the fly for content inspect in `file` module (Bugzilla #1116681)

[mozilla]

[mig modules] automatically decompress files on the fly for content inspect in `file` module (Bugzilla #1116681)

[jvehent]

I actually had to do that in another Go program, and it's trivial to do.

fd, err := os.Open(file)
if err != nil {
        fmt.Fprintf(os.Stderr, "failed to open %s: %v\n", file, err)
}
defer fd.Close()
magic := make([]byte, 2)
n, err := fd.Read(magic)
if err != nil {
        fmt.Fprintf(os.Stderr, "failed to read magic number from %s: %v\n", file, err)
}
if n != 2 {
        fmt.Fprintf(os.Stderr, "read wrong number of bytes for magic number from %s: %v\n", file, err)
}
// rewind
_, err = fd.Seek(0, 0)
if err != nil {
        fmt.Fprintf(os.Stderr, "failed to seek(0,0) on %s: %v\n", file, err)
}
if magic[0] == 0x1f && magic[1] == 0x8b {
        // this is gzip, gunzip it
        gzip, err := gzip.NewReader(fd)
        if err != nil {
                fmt.Fprintf(os.Stderr, "failed to gunzip %s: %v\n", file, err)
        }
        defer gzip.Close()
        go processFile(gzip, stats, next)
} else {
        // file doesn't need decompression
        go processFile(fd, stats, next)
}

[jvehent]

FYI: https://github.com/dsnet/compress