Re: Integrating C++ singletons into postgresql extensions???

On 2014-10-17 19:59:54 -0400, Stephen Woodbridge wrote:
> Hi,
>
> I've been writing C++ code that needs to get wrapped into a postgresql
> extension and it has a few singleton classes like:
>
> 1. Config object that holds some general configuration information.
> 2. Stats object for global stats collection.
> 3. Logger object for enabling/disabling debug logging to a file.
> 4. curlpp class for making outbound calls to an OSRM web service from the
> c++ code
>
> I can probably do without 1-3 if I had to when the code is integrated, but
> they are handy to keep around in the code for when we are developing and
> working outside the postgresql database. I could #ifdef then all, but I
> would like to avoid cluttering the code with tons of #ifdef.
>
> Item 4 is a little more problematic and needs to be kept around or something
> else implemented to take its place. I have looked at the curl extension and
> actual built another module in C that uses those ideas., but the curlpp is a
> nice OO class that encapsules and hides all the curl stuff from the C++
> code.
>
> So my question(s) are:
>
> 1. Any insights into singletons working/not working within the postgresql
> server extensions? My understanding is that these are implemented as global
> static classes with a lifetime of the process and they have no destructor,
> so I'm a little worried about memory leaks or multiple queries sharing the
> singleton.

There's no general problem that I can see. Postgres uses processes, not
threads, for individual connections. So any process level leak doesn't
persist for longer than the connection. And if you create the singleton
inside the individual backend, instead of in the postmaster (the parent
server) they won't be shared either.

> 2. Is there another way implement the equivalent of a singleton for C++ code
> that has a lifetime of the query in the server?

Hm. Now it needs to be query lifetime, not connection lifetime? That's
slightly harder. If that's what you need probably the best way to go
about it would be to register a callback with
RegisterResourceReleaseCallback().

> 3. What do for logging classes when integrating C++ code into postgresql?

I don't really know what you want to do here. My guess is that it'd be
most appropriate to map the logging onto postgres' internal
logging. That should easily be possible.

The most important thing to be aware of here is that you need to take
*great* care that exception, *NEVER* permeate through postgres
code. Doing so will cause bad things.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services