RE: Reconnect a single connection used by multiple threads in embedded SQL in C application causes error. - Mailing list pgsql-bugs

Hi Noah,

Thank you for replying.

> > And, is there limitation that we can't CONNECT or DISCONNECT the DEFAULT connection inside a thread in
multithreadedapplication? 
>
> For now, yes.  This is a bug.  The documentation you quoted is out of date
> with respect to the code.  That documentation comes from commit 8f61184
> (2003-12).  The behavior of connections shared across threads changed in
> commit 757fb0e (2004-03).  Since that change, there's a per-thread record of
> the default connection, in addition to a global notion of the default
> connection.  I suspect a few use cases work well today:
>
> - One thread does all the CONNECT and DISCONNECT commands.  Other threads just
>   use the default connection, with mutual exclusion.
> - Each thread does its own CONNECT, DISCONNECT, and other commands.  Each
>   thread has its own default connection.  No sharing at all.

The documentation from commit 8f61184 seems to have been updated by the commit 0f33ee0 (2017-06).
However, there is still lacking of information about the DEFAULT connection on per-thread, I understood.


> Your example does the CONNECT to establish the default connection in one
> thread, and it does the DISCONNECT of the default connection in another
> thread.  That's buggy today.
>
> Another behavior that may qualify as a bug: ECPGsetconn() updates only the
> thread-specific connection, while ECPGconnect() updates both the
> thread-specific and global connections.

I also understood that my example is impossible by the implementation bugs.

I think it would take some time to fix these bugs.
If it takes long time to fix it, will this limitation(*) be added to the documentation?

[*] Based on your explanation:
    "If we use the default connection inside a thread in multithreaded application,
     CONNECT and DISCONNECT commands are allowed only following cases.
     - One thread does all the CONNECT and DISCONNECT commands.
       Other threads just use the default connection, with mutual exclusion.
     - Each thread does its own CONNECT, DISCONNECT, and other commands.
       Each thread has its own default connection. No sharing at all."

----

An additional report:
I also ran my reproduction application built with rh-postgresql12 libraries (installed from Red Hat Software
Collections), 
and it crashed instead of an error in step 5 in my first email.

* Actually, the error message in my first email came from postgresql built by myself (./configure --prefix=xxx).

The backtrace is following.
(gdb) bt
#0  0x00007fa11a4ddd0f in ecpg_find_prepared_statement () from
/opt/rh/rh-postgresql12/root/usr//lib64/libecpg.so.rh-postgresql12-6
#1  0x00007fa11a4de11b in ecpg_prepared () from /opt/rh/rh-postgresql12/root/usr//lib64/libecpg.so.rh-postgresql12-6
#2  0x00007fa11a4d8b65 in ecpg_do_prologue () from /opt/rh/rh-postgresql12/root/usr//lib64/libecpg.so.rh-postgresql12-6
#3  0x00007fa11a4d8c33 in ecpg_do () from /opt/rh/rh-postgresql12/root/usr//lib64/libecpg.so.rh-postgresql12-6
#4  0x00007fa11a4d8d5b in ECPGdo () from /opt/rh/rh-postgresql12/root/usr//lib64/libecpg.so.rh-postgresql12-6
#5  0x0000000000401143 in thread_main2 ()
#6  0x00007fa11a2bdea5 in start_thread () from /lib64/libpthread.so.0
#7  0x00007fa119fe68cd in clone () from /lib64/libc.so.6

I think this happened because the PGconn referenced by Thread#1 was destroyed in step 4 and then overwritten with
invaliddata. 

Regards,
Yusuke Egashira