[PATCH]: unlink legacy unix socket when unit starts

[shanlei at asiainfo.com]

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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