TCP has an "urgent data" feature that might have been used for this kind of thing, used for Ctrl-C in telnet, etc. It can be used to bypass any pending send buffer and received by the server ahead of any unread data.
I think I can understand why this wasn’t addressed for so long: in the vast majority of cases if your db is exposed on a network level to untrusted sources, then you probably have far bigger problems?
That's the kind of hand-wave that turns into a CVE later. Network exposure is one thing, but weird signal handling in local tooling can still become a cross-session bug or a nasty security footgun on shared infra, terminals, or jump boxes.
If you have shared psql sessions in tmux or on a jump box one bad cancel can trash someone else's work. 'Just firewall it' is how you end up owned by the intern with shell access.
it's also very tricky to do given the current architecture on the server side where one single-threaded process handles the connection and uses (for all intents and purposes) sync io.
In such a scenario, listening (and acting) on cancellation requests on the same connection becomes very hard, so fixing this goes way beyond "just".
From the title I was hoping for this being hacky on the server application side, like how it aborts and clears the memory for a running query.
Still an interesting read. Just wondering, why can't the TCP connection of the query not be used to send a cancellation request? Why does it have the be out of band?
Because Postgres is a very old codebase and was written in a style that assumes there are no threads, and thus there's nothing to listen for a cancellation packet whilst work is getting done. A lot of UNIXes had very poor support for threads for a long time and so this kind of multi-process architecture is common in old codebases.
The TCP URG bit came out of this kind of problem. It triggers a SIGURG signal on UNIX which interrupts the process. Oracle works this way.
These days you'd implement cancellation by having one thread handle inbound messages and another thread do the actual work with shared memory to implement a cooperative cancellation mechanic.
But we should in general have sympathy here. Very little software and very few protocols properly implements any form of cancellation. HTTP hardly does for normal requests, and even if it did, how many web servers abort request processing if the connection drops?
I don't know much about postgres, but as I understand it, it's a pretty standard server application. Read a request from the client, work on the request, send the result, read the next request.
Changing that to poll for a cancellation while working is a big change. Also, the server would need to buffer any pipelined requests while looking for a cancellation request. A second connection is not without wrinkles, but it avoids a lot of network complexity.
Because then the cancellation request would get queued behind any other data that's in flight from the client to the server. In the worst case the TCP buffers are full, and the client cannot even send the request until the server processes some of the existing data that's in-flight.
It's basically got a thread per connection, while it's working on a query that thread isn't listening to incoming traffic on the network socket any more.
The downside is that sometimes connections are proxied in ways that lose these unusual packets. Looking at you, Docker...
If you have shared psql sessions in tmux or on a jump box one bad cancel can trash someone else's work. 'Just firewall it' is how you end up owned by the intern with shell access.
In such a scenario, listening (and acting) on cancellation requests on the same connection becomes very hard, so fixing this goes way beyond "just".
Still an interesting read. Just wondering, why can't the TCP connection of the query not be used to send a cancellation request? Why does it have the be out of band?
The TCP URG bit came out of this kind of problem. It triggers a SIGURG signal on UNIX which interrupts the process. Oracle works this way.
These days you'd implement cancellation by having one thread handle inbound messages and another thread do the actual work with shared memory to implement a cooperative cancellation mechanic.
But we should in general have sympathy here. Very little software and very few protocols properly implements any form of cancellation. HTTP hardly does for normal requests, and even if it did, how many web servers abort request processing if the connection drops?
Changing that to poll for a cancellation while working is a big change. Also, the server would need to buffer any pipelined requests while looking for a cancellation request. A second connection is not without wrinkles, but it avoids a lot of network complexity.
https://learn.microsoft.com/en-us/openspecs/windows_protocol...