[Pdns-users] [EXT] Re: remote backend

[Brian Candler]

That code is incomplete and not runnable. What is "[0:netip]" for 
example? More importantly, what is "c" when you do c.Write(data) ?

However, there is also an unstated question here, which is "how are the 
requests and responses delimited when PowerDNS using the unixsock remote 
backend?"  You're just assuming that a Read() and a Write() are 
sufficient to delimit messages, and that seems pretty unlikely to me.

Now, looking in the Ruby code in modules/remotebackend/unittest_pipe.rb, 
it appears that the format is newline-delimited, i.e. this is JSON-Lines 
<https://jsonlines.org/>.

This appears to be confirmed in modules/remotebackend/pipeconnector.cc

int PipeConnector::send_message(const Json& input)
{
   auto line = input.dump();
   launch();

*  line.append(1, '\n');*

...

     if (!stringfgets(d_fp.get(), receive)) {
       throw PDNSException("Child closed pipe");
     }

// stringfgets is in pdns/misc.cc, and it reads up to a newline:

   do {
     if(!fgets(buffer, sizeof(buffer), fp))
       return !line.empty();

     line.append(buffer);
*  } while(!strchr(buffer, '\n'));*
   return true;

So the main issue seems to be: you need to write a newline after your 
JSON response.  (And make sure your response *isn't* pretty-printed, and 
any embedded newlines in strings are represented by "\n", so that each 
JSON object is exactly one line)

Then instead of Read(1024), you need to either read up to a newline 
(e.g. using bufio.Scanner or bufio.Reader) - or you need to use a 
streaming JSON decoder which will use the end of each JSON object as the 
delimiter, and ignores the newlines in between them.  (Go's 
encoding/json can quite happily do that)

HTH,

Brian.
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