log_files: add blocking download_file; add erase log file function

[dayjaby]

This PR adds two new functions to the log_files plugin:

• a blocking download_file
• an erase function

Note, that for the erase function, LOG_ERASE is being used. This mavlink message does not prescribe any feedback. To check whether the logs got really deleted, you can verify with get_entries.

Running the example looks like:

./MAVSDK/examples/logfile_download$ cmake -Bbuild -H. && cmake --build build -j4 && ./build/logfile_download udp://:14540
-- Configuring done
-- Generating done
-- Build files have been written to: ./MAVSDK/examples/logfile_download/build
[100%] Built target logfile_download
[01:09:06|Info ] MAVSDK version: v0.50.1 (mavsdk_impl.cpp:28)
Waiting to discover system...
[01:09:06|Info ] New system on: 127.0.0.1:58354 (with sysid: 1) (udp_connection.cpp:194)
[01:09:06|Debug] New: System ID: 1 Comp ID: 1 (mavsdk_impl.cpp:484)
[01:09:06|Debug] Component Autopilot (1) added. (system_impl.cpp:361)
[01:09:06|Debug] Component Unsupported component (240) added. (system_impl.cpp:361)
[01:09:07|Debug] Discovered 2 component(s) (system_impl.cpp:529)
[01:09:07|Debug] Received all entries (log_files_impl.cpp:169)
Got log file with ID 0 and date 2021-12-08T16:28:48Z
[01:09:07|Debug] 46080 B of 353765 B (320.16 kiB/s) (log_files_impl.cpp:418)
[01:09:07|Debug] 92160 B of 353765 B (237.55 kiB/s) (log_files_impl.cpp:418)
[01:09:07|Debug] 138240 B of 353765 B (333.791 kiB/s) (log_files_impl.cpp:418)
[01:09:08|Debug] 184320 B of 353765 B (328.368 kiB/s) (log_files_impl.cpp:418)
[01:09:08|Debug] 230400 B of 353765 B (326.467 kiB/s) (log_files_impl.cpp:418)
[01:09:08|Debug] 276480 B of 353765 B (319.142 kiB/s) (log_files_impl.cpp:418)
[01:09:08|Debug] 322560 B of 353765 B (317.143 kiB/s) (log_files_impl.cpp:418)
[01:09:08|Error] Ignoring wrong offset (log_files_impl.cpp:359)
[01:09:08|Error] Ignoring wrong offset (log_files_impl.cpp:359)
[01:09:08|Error] Ignoring wrong offset (log_files_impl.cpp:359)
[01:09:08|Error] Ignoring wrong offset (log_files_impl.cpp:359)
[01:09:08|Error] Ignoring wrong offset (log_files_impl.cpp:359)
[01:09:08|Error] Ignoring wrong offset (log_files_impl.cpp:359)
[01:09:08|Error] Ignoring wrong offset (log_files_impl.cpp:359)
[01:09:08|Debug] 353765 B of 353765 B (306.913 kiB/s) (log_files_impl.cpp:418)

[julianoes]

Thanks for the PR @dayjaby!

[dayjaby]

@julianoes thanks for the review! One more question: should this erase log files be a "fire and forget"-function, like in native MAVLink...or does it make sense to call get_entries after a short amount of time and check whether they got deleted or not? And provide some feedback with either LogFiles::Success or LogFiles::Failure?

[julianoes]

Yer, that's the question. I'm not sure, mostly because the list function takes quite a bit of time. You could add a comment that says "if you want to be sure the log has been deleted, you have to list it again and check".

[julianoes]

@dayjaby I think this would still be worthwhile. @dayjaby would you mind fixing the conflicts and adding the comment about the fact that that you need to check the logfiles again after deleting to make sure it worked.

[julianoes]

@dayjaby I've rebased this, fixed it up, and force pushed it. Let's hope it's now mergeable.

[julianoes]

CI failure probably unrelated.