Readable error when toolchain paths not set.

[sarahmarshy]

Fixes #2360.

New error:
[Error] Toolchain path does not exist for IAR.
Current value: /default/path/that/doesnt/exist
(System exit before any build system calls)

@0xc0170 @bridadan

[theotherjimmy]

I disagree with adding it to the toolchains' constructor for several reasons:

• It allows a constructor to fail
• Many uses of the toolchain classes do not invoke a compiler
  • the online export system, for example, would not be able to export to any non-armc5 export. That would be a BAD THING (TM)

Maybe we should check for the binaries at the moment we know that the mbedToolchain instance is going to need the executable. I'm thinking in the compile_sources method, the build_library method, and the link_program method. Personally (for reuse purposes) I would make it a decorator, and simply add the decorator to all the mentoined methods.

Further, could we throw an exception, or maybe do sys.exit with a non-zero return code?

[sarahmarshy]

@theotherjimmy one consideration: at the time of those functions, resource scan has already happened. It would just take some extra time to fail, but it is marginal. I think the things mentioned should supersede the timing consideration.

[theotherjimmy]

I agree that it would delay the failure until after the toolchains perform the scan, but the current implementation is simply incorrect for the aforementioned reasons.

[theotherjimmy]

❤️ the docstring!

[sarahmarshy]

I have seen the light! Going to try to add them to everything I modify in the future.

[bridadan]

Do we really want to exit inside of this function? It seems like we'd just want to "check_toolchain_path" here, return True or False, and delegate the appropriate action farther up the call stack. For instance, the toolchains could call this, check the return value, and throw an exception. That could be caught by the CLI scripts (build.py, make.py, test.py, etc) and they could exit gracefully.

[sarahmarshy]

It should exit gracefully with the error? Unless you need some exit value. Check the new PR.

[mazimkhan]

@bridadan do you think it is breaking change. I see that the build has failed while trying to get GCC_ARM toolchain path. I see

17:13:53 + mbed-cli test --compile --test-spec test_spec.json -vv -t GCC_ARM -m K64F
17:14:19 [ERROR] Toolchain path does not exist for GCC_ARM.

in http://e108747.cambridge.arm.com:8080/job/mbed-os/job/mbed-os-pr-build-parameterized/612/console

[bridadan]

@sarahmarshy It does exit gracefully with an error, but I think we typically try to stay away from calling exit within API functions (like toolchains or build_api). Usually I think we try to return error codes or throw exceptions in API functions and then let the top level CLI script that's calling them handle the exception and exit. Thoughts on this @theotherjimmy ?

@mazimkhan You're right I think this might cause issues with GCC_ARM because the toolchains can find it in the system PATH. I think this is only for GCC_ARM though unfortunately :/ @sarahmarshy sounds like another corner case.

[sarahmarshy]

Let's try this again. Moved to #2418