[PATCH]: unlink legacy unix socket when unit starts

[shanlei at asiainfo.com]

I have reconsided the situation:
compare to TCP sockets, if a TCP bind fails, we know other programs are 
using the socket ,the ops team then fix the mis-configured port number, 
change unit or other programs.
the unix socket should behave similarly.
the diffrence is when a tcp server is done, the socket is recycled by 
system, while the unix socket file is left in disk, which block "unit"'s 
next boot.
may the idea patch is to unlink the unix socket file on shutdown, 
instead of startup?


On 2022-04-25 14:38，Scott van Kalken wrote：
> What if it were a command line option to delete on start? This way you
> preserve the existing behaviour while introducing the desired
> behaviour. It’s up to the user / admin to decide if they want to
> “force delete” as an example.
> 
> On Mon, 25 Apr 2022 at 4:33 pm, <shanlei at asiainfo.com> wrote:
> 
>> I know the concern, that if we misconfigure the socket file, other
>> file
>> may be deleted accidentally.
>> But if the program wont delete the file, bind will fail , that means
>> we
>> have to delete the file manually before every start. that not a good
>> 
>> option to maintain a system.
>> 
>> 在 2022-04-24 18:05，Александр Поволоцкий
>> wrote：
>>> Unlink socket that can be in use? I think it's not a good idea.
>>> 
>>> On 24.04.2022 12:25, shanlei at asiainfo.com wrote:
>>>> 
>>>> hello,guys,
>>>> 
>>>> We found if we add a unix socket listener, the file was left in
>> disk
>>>> when unit got shutdown.
>>>> If the unit starts again and load configuration from its state
>> folder,
>>>> it would fail with error msg "file exists",
>>>> so We think the proper process is to "delete the legacy unix
>> socket
>>>> before start", and we present following patch:
>>>> 
>>>> 
>>>> # HG changeset patch
>>>> # User stdanley <shanlei at asiainfo.com>
>>>> # Date 1650792235 -28800
>>>> #      Sun Apr 24 17:23:55 2022 +0800
>>>> # Node ID 29738840c393f12c7d7aa8f3959ed1655b39cd4d
>>>> # Parent  8e06a879600e83af11345d023c53c2ca445cf82c
>>>> unlink legacy unix socket file before binding.
>>>> 
>>>> diff -r 8e06a879600e -r 29738840c393 src/nxt_main_process.c
>>>> --- a/src/nxt_main_process.c    Tue Apr 12 04:16:00 2022 +0100
>>>> +++ b/src/nxt_main_process.c    Sun Apr 24 17:23:55 2022 +0800
>>>> @@ -1142,6 +1142,15 @@
>>>> }
>>>> 
>>>> #endif
>>>> +#if (NXT_HAVE_UNIX_DOMAIN)
>>>> +
>>>> +    if (sa->u.sockaddr.sa_family == AF_UNIX
>>>> +        && sa->type == SOCK_STREAM
>>>> +        && sa->u.sockaddr_un.sun_path[0] != '\0') {
>>>> +
>>>> +        (void) unlink(sa->u.sockaddr_un.sun_path);
>>>> +}
>>>> +#endif
>>>> 
>>>> if (bind(s, &sa->u.sockaddr, sa->socklen) != 0) {
>>>> err = nxt_errno;
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