add function to return all inherited files

[hunts]

This is helpful in the use case of integrating with systemd socket activation.

When there is no parent, the app need to scan or use actication.Files() to
find all the fds and call upgrader.AddFile() to track them. But when there is a
parent, with the help of this new function, we can find the fds from upgrader
instance rather then by scanning fds or calling actication.Files() both of which
creates a set of os.File representing the underlying fds.

Without this change, there would be two set of os.File objects that represent
the same fd set. One set of os.File is hold by the upgrader while the another set
returning from fd scanning (or actication.Files()) is being used at other places.
This raise a risk of closing the same fd twice at different time which is really bad
when the fd number get reused by other files during the two close calls.

[lmb]

Hi Hunts,

If I remember correctly, files are dup()ed before being added to Fds:

tableflip/fds.go

Lines 316 to 319 in 287e7a4

	file, err := dupConn(conn, key)
	if err != nil {
	return fmt.Errorf("can't dup %s (%s %s): %s", kind, network, addr, err)
	}

This means that you can close the file you passed to Upgrader.Fds.AddFile after the call returns without creating a problem. In pseudo code, I'd probably do something like this:

upg, err := ...

if !upg.HasParent() {
  for _, f := range systemdInerited {
    err := upg.AddFile // upg.AddListener, etc.
    f.Close()  
  }
}

// Continue normal start up
ln, err := upg.Listen("tcp", ...)

Does that solve your problem?

[lmb]

(NB: I don't have merge access to this repository anymore!)

[hunts]

The problem is occurring at this step:

// Continue normal start up
ln, err := upg.Listen("tcp", ...)

We don't have named addresses but a list of fd passed from systemd. If you still remember this piece of code:

/etc/systemd/system/<service-name>.socket:
  file.managed:
{%- for addr in ["127.0.0.1", "[::1]"] %}
  {%- for _ in range(0, 128) %}
          - ListenDatagram: "{{ addr }}:53"
  {%- endfor %}
{%- endfor %}

What we can do is either like:

// Continue normal start up
for _, f := range systemdInerited {
    ln, err := net.FileConn(f) 
    ...
}

Or like:

// Continue normal start up
files, err := upg.Files()
for _, f := range files {
    ln, err := net.FileConn(f) 
    ...
}

The former approach has the problem we I've describe in the PR message. The later one requires the Files() methods we are introducing here.