How to close the file description when a method return it ?

[writePerfectCode]

If a method of a service return a file description to the caller, where and how to close the file description ?
I have a try to close it in caller, it will a coredump in caller.
If have to close it in service, how ?

[williamspatrick]

It should be closed by the recipient of the file descriptor (the caller?). I believe there are examples of using the file descriptor calls in bmcweb. Do you have examples of failing code?

[writePerfectCode]

checked bmcweb， seems it just send file description as method param, but not a return value

[williamspatrick]

https://github.com/openbmc/bmcweb/blob/de0c960c4262169ea92a4b852dd5ebbe3810bf00/redfish-core/lib/log_services.hpp#L863
https://github.com/openbmc/bmcweb/blob/de0c960c4262169ea92a4b852dd5ebbe3810bf00/redfish-core/lib/log_services.hpp#L808

I'm not sure how it works, but in this code it seems like the unix_fd is either leaked or somehow released automatically by the sd-bus infrastructure. I'd need to investigate more. There isn't any close call though.

[spinler]

When I did some code for a method that returned an FD with the sdbusplus bindings a few years ago, I think I remember that the FDs would leak if you didn't do anything, so I put in a sd_io_event_add_defer to close the file descriptor when the code got back to the event loop and that worked.

[williamspatrick]

@spinler Any idea why you couldn't close it at the consumption location?

[spinler]

In the client? That's a dup'ed descriptor so the original in the server remained open.

[williamspatrick]

For the server, I would expect:

• FD is open.
• FD is sent as part of the response.
• FD can be immediately closed.

For the client, I would expect:

• FD is open (due to it coming from the sendfd).
• FD can be closed at any point and must be closed before the incoming message is released.

I don't really understand why the dup was ever necessary.

[writePerfectCode]

I find a funny thing, I open a file, and set the fd as param to call a service method, while the servie handle the method call, I can see a new fd in /proc/xxx/fd for the service, after the method return, the caller close the fd, then I find the fd in for service /proc/xxx/fd also disapper

[tianyihah]

For the server, I would expect:

• FD is open.
• FD is sent as part of the response.
• FD can be immediately closed.

For the client, I would expect:

• FD is open (due to it coming from the sendfd).
• FD can be closed at any point and must be closed before the incoming message is released.

I don't really understand why the dup was ever necessary.

Dup copy the fd and not share file descriptor flags (the close-on-exec flag), you can safely close fd(copy instance).