[Postgres-xc-developers] Yet another analysis of "GTM error, could not obtain snapshot"

[Masataka S. <pg...@gm...>]

Hi,

I got a report that an error message "GTM error, could not obtain
snapshot" still produced. I looked deeply in the proxy, then I found
an another bug and misc errors.

I explain the summary of them and attached patches.
All suggestions are welcome.

01_fix_proxy_connid.patch:
 A proxy assigns an connection ID to the connection from back-end. A
connection ID is sent to the GTM when back-end requires a new
transaction ID, back-end disconnected and so on. If a back-end
disconnected, the GTM releases transactions related on the back-end.
  On the other hand, a proxy renumbers the connection ID to fill a
gapping entry in the connection array. But it is not notified to the
GTM, and it causes the release of a transaction that is not related to
disconnected back-end.
 The patch makes the connection ID static by separating the connection
ID from the index of the connection array.

01_fix_proxy_ereport_again.patch:
 I fixed the issue before -- when GTM proxy received an error response
to a command, GTM proxy reported error using an unrelated connection
to the command -- but the patch had mistake. This is the patch that
fixes it in right way.

01_fix_proxy_redundancy.patch:
 A GTM proxy has some redundant code. This patch eliminates them.

Regards.