Hi, On 2025-01-22 07:57:52 -0800, Paul Ramsey wrote: > I just ran across this change when a bunch of CI’s I run barfed. > > https://github.com/postgres/postgres/commit/d4a43b283751b23d32bbfa1ecc2cad2d16e3dde9 > > The problem we are having in extension land is that we often run functions > in external libraries that take a long time to return, and we would like to > ensure that PgSQL users can cancel their queries, even when control has > passed into those functions. > > The way we have done it, historically, has been to take the return value of > pqsignal(SIGINT, extension_signint_handler) and remember it, and then, > inside extension_signint_handler, call the pgsql handler once we have done > our own business.
I think that's a really bad idea. For one, postgres might use signals in different process types for different things. We are also working on not using plain signals in a bunch of places. It's also really hard to write correct signal handlers. E.g.: > https://github.com/pramsey/pgsql-http/blob/master/http.c#L345 That signal handler is profoundly unsafe: http_interrupt_handler(int sig) { /* Handle the signal here */ elog(DEBUG2, "http_interrupt_handler: sig=%d", sig); http_interrupt_requested = sig; pgsql_interrupt_handler(sig); return; } You're definitely not allowed to elog in a signal handler unless you take *extraordinary* precautions (basically converting the normal execution path to only perform async-signal-safe work). This signal handler can e.g. corrupt the memory context used by elog.c, deadlock in the libc memory allocator, break the FE/BE protocol if any client ever set client_min_messages=debug2, jump out of a signal handler in case of an allocation failure and many other things. > It is possible we have been Doing It Wrong all this time, and would love some > pointers on the right way to do this. All your interrupt handler is doing "for you" is setting http_interrupt_requested, right? Why do you need that variable? Seems you could just check postgres' QueryCancelPending? And perhaps ProcDiePending, if you want to handle termination too. Greetings, Andres Freund
