Improve the code style in HTTP server

[markccchiang]

Description

• What is implemented or fixed? Mention the linked issue(s), if any. Closes #1260.

• How does this PR solve the issue? Give a brief summary. To avoid the magic hardcoded strings, I think replacing them with global const string variables is good enough. Applying new enum values, we may need an additional map that transfers the enum to string, which is inefficient. If my understanding is correct (?)

• Are there any companion PRs (frontend, protobuf)? No.

• Is there anything else that testers should know (e.g. exactly how to reproduce the issue)? All tests should not be affected.

Checklist

• [x] changelog updated ~/ no changelog update needed~
• [x] e2e test passing / ~corresponding fix added~ / ~new e2e test created~
• [x] ~protobuf updated to the latest dev commit /~ no protobuf update needed
• [ ] protobuf version bumped / protobuf version not bumped
• [ ] added reviewers and assignee
• [ ] added ZenHub estimate, milestone, and release

[confluence]

When I made the enum suggestion I didn't realise that C++ did not have an enum type with string values -- I'm happy to drop the enum idea and continue using strings. However, I have more specific remarks about how this code should be modified.

[confluence]

Looks good to me. Added @kswang1029 as a reviewer, for testing.

[markccchiang]

Looks like we can not register the routes using a loop, otherwise, it will fail when executing the command /carta_backend --frontend_folder /Users/path/to/carta-frontend/build --port 3002 --omp_threads 8 --debug_no_auth --no_browser --verbosity 5. I am wondering if it is related to the life circle of the handler functions. @confluence do you have any ideas?

[github-actions[bot]]

Package	Line Rate	Health
src.Cache	66%	➖
src.DataStream	52%	➖
src.FileList	67%	➖
src.Frame	50%	➖
src.HttpServer	42%	➖
src.ImageData	28%	❌
src.ImageFitter	83%	✔
src.ImageGenerators	43%	➖
src.ImageStats	76%	✔
src.Logger	38%	➖
src.Main	52%	➖
src.Region	18%	❌
src.Session	30%	❌
src.Table	52%	➖
src.ThreadingManager	87%	✔
src.Timer	85%	✔
src.Util	48%	➖
Summary	38% (7087 / 18728)	❌

[markccchiang]

Looks like we can not register the routes using a loop, otherwise, it will fail when executing the command /carta_backend --frontend_folder /Users/path/to/carta-frontend/build --port 3002 --omp_threads 8 --debug_no_auth --no_browser --verbosity 5. I am wondering if it is related to the life circle of the handler functions. @confluence do you have any ideas?

I guess this may be due to the interpretation of the code during the compiling time. The argument variable in the handler function has to be global. Otherwise, the registered handler function will not be valid and fail to open the frontend. @kswang1029 could you please check again if the problem is solved? Thank you!

[confluence]

@markccchiang what error did you get with the previous version of the code? I can't reproduce it here (and valgrind isn't reporting any issues either). This could be a difference between compilers.

I agree that it has to do with the lifetime of the object_type variable. The lambdas are executed after the RegisterRoutes function has exited, so any objects defined in the function's scope have been destroyed. Your original object_type was a reference to a value in a vector defined in the function scope. This variable doesn't have to be global, but it does have to outlive the execution of the function. I believe that you could also have fixed this by explicitly capturing the variable by copy in all of the lambdas, but I think that the current code is fine -- using the existing map keys instead of repeating the strings is cleaner.

[markccchiang]

@markccchiang what error did you get with the previous version of the code? I can't reproduce it here (and valgrind isn't reporting any issues either). This could be a difference between compilers.

@confluence with my Mac compiler I got an error message when opening the frontend by the command /carta_backend --frontend_folder /Users/path/to/carta-frontend/build ... like this:

This was first found by @kswang1029. It was also found on a specific computer when opening the CARTA v4.0 electron app (with the dev branch). Not sure if it is related to the same root cause of the problem.