postgres_fdw: Allow cancellation of transaction control commands. Commit f039eaac7131ef2a4cf63a10cf98486f8bcd09d2, later back-patched with commit 1b812afb0eafe125b820cc3b95e7ca03821aa675, allowed many of the queries issued by postgres_fdw to fetch remote data to respond to cancel interrupts in a timely fashion. However, it didn't do anything about the transaction control commands, which remained noninterruptible.
Improve the situation by changing do_sql_command() to retrieve query results using pgfdw_get_result(), which uses the asynchronous interface to libpq so that it can check for interrupts every time libpq returns control. Since this might result in a situation where we can no longer be sure that the remote transaction state matches the local transaction state, add a facility to force all levels of the local transaction to abort if we've lost track of the remote state; without this, an apparently-successful commit of the local transaction might fail to commit changes made on the remote side. Also, add a 60-second timeout for queries issue during transaction abort; if that expires, give up and mark the state of the connection as unknown. Drop all such connections when we exit the local transaction. Together, these changes mean that if we're aborting the local toplevel transaction anyway, we can just drop the remote connection in lieu of waiting (possibly for a very long time) for it to complete an abort. This still leaves quite a bit of room for improvement. PQcancel() has no asynchronous interface, so if we get stuck sending the cancel request we'll still hang. Also, PQsetnonblocking() is not used, which means we could block uninterruptibly when sending a query. There might be some other optimizations possible as well. Nonetheless, this allows us to escape a wait for an unresponsive remote server quickly in many more cases than previously. Report by Suraj Kharage. Patch by me and Rafia Sabih. Review and testing by Amit Kapila and Tushar Ahuja. Discussion: http://postgr.es/m/caf1dzpu8kx+fmxebfop289xtm3bz3t+zfxhmkavr98bh-c0...@mail.gmail.com Branch ------ REL9_4_STABLE Details ------- https://git.postgresql.org/pg/commitdiff/c02c450cfc0ff2fa85f7bbef61b850ec2be61dd5 Modified Files -------------- contrib/postgres_fdw/connection.c | 353 ++++++++++++++++++++++++++++++++------ 1 file changed, 300 insertions(+), 53 deletions(-) -- Sent via pgsql-committers mailing list (pgsql-committers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-committers