test: make sure that underlying connection is closed

[romanhabibov]

Closes #31

[LeonidVas]

HI! Thank you for the patch.

I suggest using in title something like:" test: add test for ...".

Add Closes #31

AFAIU, you save the descriptor, close it and expect the same descriptor will be used for a connection. This is a good hypothesis, but please add a comment to your hypothesis. Also, I'm expecting some links to prove your hypothesis (if possible).
Is this only true for linux or other OS too? Will such test work on MacOS, WSL, WSL2? Maybe run this test on Linux only?

[romanhabibov]

HI! Thank you for the patch.

I suggest using in title something like:" test: add test for ...".

Add Closes #31

AFAIU, you save the descriptor, close it and expect the same descriptor will be used for a connection. This is a good hypothesis, but please add a comment to your hypothesis. Also, I'm expecting some links to prove your hypothesis (if possible).
Is this only true for linux or other OS too? Will such test work on MacOS, WSL, WSL2? Maybe run this test on Linux only?

I couldn't find any directly obvious proof, but let me try. Basing on the statement, that file descriptors are recycled in ascending order. It means, that if we call open() and immediately close(), we get the first vacant index for a new resource in the file descriptor table. It is important not to call any procedures between open() and close() that may affect the file descriptor table. After that, we immediately create connection, which creates a socket. Socket is a resource, which occupies the first vacant index in the file descriptor table.

https://sites.ualberta.ca/dept/chemeng/AIX-43/share/man/info/C/a_doc_lib/aixprggd/genprogc/fdescript.htm

[Totktonada]

I couldn't find any directly obvious proof <...>

The kernel code:

https://github.com/torvalds/linux/blob/585e5b17b92dead8a3aca4e3c9876fbca5f7e0ba/fs/file.c#L503-L510
https://github.com/torvalds/linux/blob/585e5b17b92dead8a3aca4e3c9876fbca5f7e0ba/fs/file.c#L531-L532

The simplified description of this code:

https://www.win.tue.nl/~aeb/linux/vfs/trail-2.html

Another description:

https://stackoverflow.com/a/33502563/1598057

[LeonidVas]

HI! Thank you for the patch.
I suggest using in title something like:" test: add test for ...".
Add Closes #31
AFAIU, you save the descriptor, close it and expect the same descriptor will be used for a connection. This is a good hypothesis, but please add a comment to your hypothesis. Also, I'm expecting some links to prove your hypothesis (if possible).
Is this only true for linux or other OS too? Will such test work on MacOS, WSL, WSL2? Maybe run this test on Linux only?

I couldn't find any directly obvious proof, but let me try. Basing on the statement, that file descriptors are recycled in ascending order. It means, that if we call open() and immediately close(), we get the first vacant index for a new resource in the file descriptor table. It is important not to call any procedures between open() and close() that may affect the file descriptor table. After that, we immediately create connection, which creates a socket. Socket is a resource, which occupies the first vacant index in the file descriptor table.

https://sites.ualberta.ca/dept/chemeng/AIX-43/share/man/info/C/a_doc_lib/aixprggd/genprogc/fdescript.htm

Alexander found confirmation. Please add this as a comment to your code with describing how the test works.

[romanhabibov]

HI! Thank you for the patch.
I suggest using in title something like:" test: add test for ...".
Add Closes #31
AFAIU, you save the descriptor, close it and expect the same descriptor will be used for a connection. This is a good hypothesis, but please add a comment to your hypothesis. Also, I'm expecting some links to prove your hypothesis (if possible).
Is this only true for linux or other OS too? Will such test work on MacOS, WSL, WSL2? Maybe run this test on Linux only?

I couldn't find any directly obvious proof, but let me try. Basing on the statement, that file descriptors are recycled in ascending order. It means, that if we call open() and immediately close(), we get the first vacant index for a new resource in the file descriptor table. It is important not to call any procedures between open() and close() that may affect the file descriptor table. After that, we immediately create connection, which creates a socket. Socket is a resource, which occupies the first vacant index in the file descriptor table.
https://sites.ualberta.ca/dept/chemeng/AIX-43/share/man/info/C/a_doc_lib/aixprggd/genprogc/fdescript.htm

Alexander found confirmation. Please add this as a comment to your code with describing how the test works.

Done.

[LeonidVas]

LGTM.

[Totktonada]

LGTM.