Re: libpq is not thread safe

Zdenek Kotala wrote:
>
> Tom Lane p??e v ne 03. 05. 2009 v 16:39 -0400:
> > Zdenek Kotala <Zdenek.Kotala@Sun.COM> writes:
> > > When postgreSQL is compiled with --thread-safe that libpq should be
> > > thread safe. But it is not true when somebody call fork(). The problem
> > > is that fork() forks only active threads and some mutex can stay locked
> > > by another thread. We use ssl_config mutex which is global.
> >
> > fork() without exec() when there are open libpq connections is
> > unbelievably dangerous anyway --- you will have multiple processes
> > that all think they own the same database connection.
>
> The difference is that developer can close connection, but he is not
> able to unlock a lock after fork. OK libpq does not offer any PQFinish
> variant which frees only resources and close connection without
> terminate message, but he can do it with dirty hacking.
>
> Another advantage of atfork handler is that you can close all open
> connection and clean resource (similar to what pkcs11 library does). But
> at this moment libpq does not keep list of open connections and atfork
> handler works only with pthreads.
>
> > I think writing code to deal with this for the ssl_config mutex is entirely a waste
> > of time.
>
> yeah, I prefer to document it together how to deal with fork() and
> sessions.

Done, patch attached and applied.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + If your life is a hard drive, Christ can be your backup. +
Index: doc/src/sgml/libpq.sgml
===================================================================
RCS file: /cvsroot/pgsql/doc/src/sgml/libpq.sgml,v
retrieving revision 1.288
diff -c -c -r1.288 libpq.sgml
*** doc/src/sgml/libpq.sgml    27 Apr 2009 16:27:36 -0000    1.288
--- doc/src/sgml/libpq.sgml    28 May 2009 20:01:37 -0000
***************
*** 64,69 ****
--- 64,79 ----
     whether a connection was successfully made before queries are sent
     via the connection object.

+    <warning>
+     <para>
+      On Unix, forking a process with open libpq connections can lead to
+      unpredictable results because the parent and child processes share
+      the same sockets and operating system resources.  For this reason,
+      such usage is not recommended, though doing an <function>exec</> from
+      the child process to load a new executable is safe.
+     </para>
+    </warning>
+
     <note>
      <para>
       On Windows, there is a way to improve performance if a single