fix(ftp): List empty directory

[plichard]

Solved Problem

When trying to list an empty directory, I found that the mavlink ftp implementation does not return EOF.

Solution

payload->offset == 0 should be checked as well for EOF.
This also changes the behavior to return kErrFailErrno when retrieving a dir entry fails, instead of failing silently (while removing duplicated payload->size and closedir logic).

Changelog Entry

For release notes:

Bugfix mavlink ftp not returning EOF on empty directory list.

[dakejahl]

what issue is this fixing? What are the steps to reproduce?

[plichard]

If I remember correctly, was a while ago, the directory listing of an empty directory would time-out with certain mavlink ftp clients, because they were waiting for the NAK (EOF) as described in point 4 of https://mavlink.io/en/services/ftp.html#list_directory, which the current implementation would never send (for an empty directory).

I can give you more details when I come back from holidays, I don't have access to the setup right now.

[dakejahl]

okay I see. Tested with mavsdk and I see the ACK now