Re: [Qemu-devel] [PATCH v3 1/5] qemu-nbd: Add --pid-file option

[Max Reitz]

On 24.05.19 17:07, Eric Blake wrote:
> On 5/8/19 4:18 PM, Max Reitz wrote:
>> --fork is a bit boring if there is no way to get the child's PID.  This
>> option helps.
>>
>> Signed-off-by: Max Reitz <address@hidden>
>> ---
>>  qemu-nbd.c    | 11 +++++++++++
>>  qemu-nbd.texi |  2 ++
>>  2 files changed, 13 insertions(+)
>>
>> diff --git a/qemu-nbd.c b/qemu-nbd.c
>> index dca9e72cee..edb5195208 100644
>> --- a/qemu-nbd.c
>> +++ b/qemu-nbd.c
>> @@ -59,6 +59,7 @@
>>  #define QEMU_NBD_OPT_IMAGE_OPTS    262
>>  #define QEMU_NBD_OPT_FORK          263
>>  #define QEMU_NBD_OPT_TLSAUTHZ      264
>> +#define QEMU_NBD_OPT_PID_FILE      265
>>  
>>  #define MBR_SIZE 512
>>  
>> @@ -111,6 +112,7 @@ static void usage(const char *name)
>>  "                            specify tracing options\n"
>>  "  --fork                    fork off the server process and exit the 
>> parent\n"
>>  "                            once the server is running\n"
>> +"  --pid-file=PATH           store the server's process ID in the given 
>> file\n"
> 
> Are we guaranteed that the pid file does not appear until after the
> socket has been created?
> 
> /me rereads full file...
> 
>> @@ -876,6 +880,9 @@ int main(int argc, char **argv)
>>          case 'L':
>>              list = true;
>>              break;
>> +        case QEMU_NBD_OPT_PID_FILE:
>> +            pid_file_name = optarg;
>> +            break;
>>          }
>>      }
>>  
> ...
>     socket_activation = check_socket_activation();
>     if (socket_activation == 0) {
>         setup_address_and_port(&bindto, &port);
>     } else {
>         /* Using socket activation - check user didn't use -p etc. */
> ...
> 
>> @@ -1196,6 +1203,10 @@ int main(int argc, char **argv)
>>  
>>      nbd_update_server_watch();
>>  
>> +    if (pid_file_name) {
>> +        qemu_write_pidfile(pid_file_name, &error_fatal);
>> +    }
> 
> Yes, we are.  So,
> 
>> +
>>      /* now when the initialization is (almost) complete, chdir("/")
>>       * to free any busy filesystems */
>>      if (chdir("/") < 0) {
>> diff --git a/qemu-nbd.texi b/qemu-nbd.texi
>> index de342c76b8..7f55657722 100644
>> --- a/qemu-nbd.texi
>> +++ b/qemu-nbd.texi
>> @@ -117,6 +117,8 @@ option; or provide the credentials needed for connecting 
>> as a client
>>  in list mode.
>>  @item --fork
>>  Fork off the server process and exit the parent once the server is running.
>> address@hidden --pid-file=PATH
>> +Store the server's process ID in the given file.
> 
> since that is a useful trick to know (the pid file intentionally does
> NOT appear until after the socket is ready to go), we should mention it
> in the documentation.
> 
> I can make that tweak while queueing, if you'd like.

I don’t mind, but I personally find it more useful to just use --fork
and wait for the main process to exit.

Max

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